Executive Summary
Current Transformer: A Key Measurement Device for Smart Building Energy Management
Buildings are becoming increasingly intelligent and energy-intensive.
Modern commercial and smart buildings contain large numbers of electrical loads, including:
- HVAC systems
- Lighting systems
- Elevators
- Pumps
- Air compressors
- Data rooms
- EV charging stations
- Office equipment
- Retail equipment
Traditional utility meters can measure total building electricity consumption, but they usually cannot explain where the energy is being consumed.
For building owners and facility managers, the more important questions are:
- How much energy does the HVAC system consume?
- Which floor consumes the most electricity?
- How much energy does each tenant use?
- How much electricity is consumed by EV chargers?
- Which electrical system is responsible for peak demand?
- Where can energy efficiency be improved?
A modern building energy monitoring system therefore requires multiple measurement points.
The basic architecture is:
Utility Grid
↓
Main Distribution Board
↓
Current Transformer
↓
Smart Energy Meter
↓
RS485 / Modbus
↓
EMS / BMS
↓
Energy Analysis
↓
Energy OptimizationCurrent transformers provide the current measurement foundation.
Energy meters convert electrical measurements into usable power and energy data.
EMS and BMS platforms transform that data into actionable building management information.
Introduction
Why Smart Buildings Need Detailed Energy Monitoring
Buildings are no longer passive electrical consumers.
Modern buildings are complex energy systems containing multiple independent electrical subsystems.
A commercial building may have:
Grid Power
│
├── HVAC System
├── Lighting
├── Elevators
├── Pumps
├── Office Loads
├── Data Room
├── EV Charging
└── Tenant LoadsIf only the main utility meter is monitored, facility managers can see total consumption but cannot identify the contribution of each subsystem.
This creates a major energy management problem.
The Difference Between Total Energy Monitoring and Sub-Metering
Traditional Building Monitoring
Utility Meter
↓
Total Building ConsumptionThe building owner knows how much electricity has been consumed.
But the system cannot easily answer:
Which system consumed the electricity?
Smart Building Monitoring
Main Meter
│
├── HVAC Meter
├── Lighting Meter
├── Floor Meter
├── Tenant Meter
├── EV Charger Meter
└── Equipment MeterNow energy consumption can be analyzed by:
- System
- Floor
- Tenant
- Equipment
- Time period
This is the foundation of intelligent building energy management.
Chapter 1 — What Is a Current Transformer for Smart Building Applications?
Definition
A current transformer (CT) is an electrical measurement device used to convert high AC current into a smaller measurable signal that can be safely connected to an energy meter or power monitoring device.
In smart buildings, CTs are commonly installed in:
- Main distribution boards
- Sub-distribution panels
- HVAC feeders
- Lighting circuits
- Tenant feeders
- EV charging circuits
- Equipment panels
How a CT Works in a Building Energy Monitoring System
The measurement process can be simplified as:
Building Electrical Load
↓
Current Transformer
↓
Current Signal
↓
Energy Meter
↓
Electrical Parameter Calculation
↓
RS485 / Modbus
↓
BMS / EMS PlatformThe CT itself primarily provides current measurement.
The energy meter combines current information with voltage measurements to calculate electrical parameters such as:
- Current
- Voltage
- Active power
- Reactive power
- Power factor
- Energy consumption
Chapter 2 — Why Current Transformers Are Important in Smart Buildings
There are six major applications.
2.1 Main Building Power Monitoring
The first measurement level is the building incoming power.
Typical architecture:
Utility Grid
↓
Main Switchboard
↓
Current Transformer
↓
Energy Meter
↓
EMSThe system provides visibility into:
- Total electricity consumption
- Maximum demand
- Load profile
- Power factor
- Energy trends
This data provides the baseline for building energy management.
2.2 HVAC Energy Monitoring
HVAC is often one of the largest energy-consuming systems in commercial buildings.
Typical equipment includes:
- Chillers
- Air handling units
- Cooling towers
- Pumps
- Fans
- Compressors
A CT-based monitoring system can measure the energy consumption of individual HVAC feeders or equipment groups.
Architecture:
HVAC Distribution Panel
↓
CT
↓
Energy Meter
↓
BMS / EMSWhy HVAC Monitoring Matters
Without sub-metering, a facility manager may know:
The building consumed 100,000 kWh this month.
But with HVAC monitoring, the manager may determine:
HVAC consumed 42,000 kWh.
This makes energy optimization much more targeted.
2.3 Lighting Energy Monitoring
Lighting systems can be monitored separately by:
- Floor
- Zone
- Building section
- Commercial area
Architecture:
Lighting Distribution Panel
↓
CT
↓
Energy Meter
↓
EMSThis helps identify:
- Excessive lighting consumption
- Lighting schedules
- After-hours consumption
- Energy-saving opportunities
2.4 Tenant Energy Monitoring
Tenant sub-metering is particularly important in:
- Office buildings
- Shopping centers
- Business parks
- Mixed-use buildings
- Commercial complexes
A typical system is:
Main Distribution
↓
Tenant Feeder
↓
CT
↓
Energy Meter
↓
Tenant Energy DataThis allows building operators to allocate electricity consumption by tenant.
Benefits
- More accurate energy billing
- Transparent tenant consumption
- Reduced billing disputes
- Better facility management
2.5 EV Charging Energy Monitoring
Smart buildings increasingly include EV charging infrastructure.
Typical applications include:
- Office parking
- Shopping malls
- Hotels
- Residential complexes
- Business parks
A CT can monitor the feeder supplying EV chargers.
Architecture:
Building Distribution
↓
EV Charging Distribution Panel
↓
CT
↓
Energy Meter
↓
EMS / EV Management PlatformThis allows operators to understand:
- Charging energy consumption
- Charging load
- Peak demand
- Building-versus-EV energy consumption
2.6 Equipment and Auxiliary Load Monitoring
Other building systems can also be monitored.
Examples:
- Elevators
- Water pumps
- Fire pumps
- Ventilation systems
- Server rooms
- Security systems
This creates a more detailed energy map of the building.
Chapter 3 — Smart Building Energy Monitoring Architecture
A complete smart building monitoring system typically contains four levels.
Level 1 — Building Incoming Power
Measures:
- Total building consumption
- Maximum demand
- Overall load profile
Components:
CT + Energy Meter
Level 2 — Distribution System
Measures:
- Floor distribution
- HVAC distribution
- Lighting distribution
- EV charging distribution
Components:
CT + Energy Meter
Level 3 — Equipment and Tenant Level
Measures:
- Individual equipment
- Tenant feeders
- Equipment groups
Components:
CT + Energy Meter / Multi-Circuit Energy Meter
Level 4 — Management Platform
Data is transmitted to:
- EMS
- BMS
- SCADA
- Cloud platform
Complete Architecture
Utility Grid
│
▼
Main Distribution
│
┌────────┼────────┐
▼ ▼ ▼
CT CT CT
│ │ │
Main HVAC Lighting
Meter Meter Meter
│ │ │
└────────┼────────┘
▼
RS485 Modbus
│
▼
EMS / BMS
│
┌───────────┼───────────┐
▼ ▼ ▼
Dashboard Reports AnalysisChapter 4 — Current Transformer Applications in Commercial Buildings
Different building types have different energy monitoring requirements.
4.1 Office Buildings
Typical monitoring points:
- Main incoming power
- HVAC
- Lighting
- Elevators
- Tenant floors
Primary objectives:
- Reduce operating costs
- Improve HVAC efficiency
- Allocate tenant energy
4.2 Shopping Malls
Major electrical loads include:
- HVAC
- Escalators
- Lighting
- Refrigeration
- Restaurants
- EV charging
A multi-level monitoring system helps identify energy consumption by:
- Retail area
- Common area
- Restaurant
- HVAC system
- EV charging area
4.3 Hotels
Energy-consuming systems include:
- Guest rooms
- HVAC
- Kitchen
- Laundry
- Water heating
- Lighting
CT-based sub-metering can provide energy data for individual systems.
4.4 Hospitals
Hospitals require continuous electrical operation.
Monitoring may include:
- HVAC
- Medical equipment
- Lighting
- Pumps
- Critical power systems
Energy monitoring should be designed without compromising electrical reliability.
4.5 Data-Intensive Commercial Buildings
Buildings increasingly contain:
- Server rooms
- Network equipment
- Edge computing systems
These loads may require dedicated monitoring.
This creates an overlap between:
Smart Building Energy Monitoring
and
Data Center Power Monitoring.
Chapter 5 — CT + Energy Meter + EMS: The Core Smart Building Solution
A current transformer is only one component of the complete monitoring system.
The three key layers are:
Layer 1 — CT
Measures current.
↓
Layer 2 — Energy Meter
Calculates electrical parameters and energy.
↓
Layer 3 — EMS / BMS
Analyzes and manages the data.
Complete architecture:
Electrical Load
↓
Current Transformer
↓
Smart Energy Meter
↓
RS485 Modbus RTU
↓
Gateway
↓
EMS / BMS
↓
Energy DashboardWhat Does the EMS Do With CT Data?
The system can transform raw measurements into useful management information.
For example:
Raw Measurement
Current:
250A
Power:
150kW
Energy:
1,250kWh
Management Information
The EMS can determine:
- Daily consumption
- Monthly consumption
- Peak demand
- Energy trend
- Equipment comparison
This is the fundamental difference between simple electrical measurement and intelligent energy management.
Chapter 6 — Multi-Circuit Energy Monitoring for Smart Buildings
Large commercial buildings may contain dozens or hundreds of electrical circuits.
Installing one independent meter for every circuit can create:
- Higher hardware costs
- Larger installation requirements
- More wiring
- More communication nodes
A multi-circuit energy meter can simplify this architecture.
Typical Application
HVAC Circuit
↓
CT ──┐
Lighting Circuit
↓
CT ──┤
Tenant Circuit
↓
CT ──┤
EV Charging Circuit
↓
CT ──┤
Auxiliary Circuit
↓
CT ──┘
↓
Multi-Circuit Energy Meter
↓
RS485 Modbus
↓
EMS / BMSBenefits of Multi-Circuit Monitoring
Reduce Hardware Requirements
Multiple circuits can be monitored through one measurement device.
Save Panel Space
Useful for compact building distribution cabinets.
Simplify Communication
Multiple measurement points can be integrated through fewer communication nodes.
Improve Energy Visibility
Building managers can monitor different systems from one platform.
YADA Solution Positioning
YADA’s smart building energy monitoring architecture can combine:
Current Measurement
SCT / CTF Current Transformers
↓
Energy Measurement
↓
Multi-Circuit Measurement
DTSD3366D-4P Multi-Circuit Energy Meter
↓
Communication
RS485 Modbus RTU
↓
Management
EMS / BMS
Chapter 7 — Where Are Current Transformers Installed in Smart Buildings?
A smart building can contain hundreds of electrical loads distributed across different floors, systems and tenants.
For effective energy management, CTs should be installed at strategically important measurement points.
The typical monitoring hierarchy is:
Building Level
↓
Distribution Level
↓
System Level
↓
Tenant / Equipment LevelEach level provides a different type of energy information.
7.1 Main Incoming Power Monitoring
The main incoming feeder is the highest-level measurement point.
Typical architecture:
Utility Grid
↓
Main Switchboard
↓
Current Transformer
↓
Smart Energy Meter
↓
EMS / BMSThe measurement data can be used to monitor:
- Total building energy consumption
- Maximum demand
- Load profile
- Power factor
- Peak electricity periods
- Monthly energy trends
Why Main Incoming Monitoring Is Important
Without a reliable baseline, building operators cannot accurately evaluate energy-saving measures.
For example:
Before HVAC optimization:
120,000 kWh/month
After HVAC optimization:
105,000 kWh/month
The main meter provides the overall energy-saving result.
However, it does not explain which system created the savings.
That requires sub-metering.
7.2 HVAC Energy Monitoring
HVAC Is One of the Most Important Building Energy Loads
Commercial buildings frequently operate:
- Chillers
- Cooling towers
- Air handling units
- Pumps
- Fans
- Compressors
These systems can consume substantial amounts of electrical energy.
HVAC CT Monitoring Architecture
Main Distribution
↓
HVAC Feeder
↓
Current Transformer
↓
Energy Meter
↓
EMS / BMSFor larger systems:
Chiller 1 → CT → Meter
Chiller 2 → CT → Meter
Cooling Pump → CT → Meter
AHU → CT → Meter
Fan System → CT → Meter
↓
EMSWhat Can HVAC Energy Monitoring Reveal?
The system can help identify:
- High-energy chillers
- Excessive HVAC operation
- Off-hours consumption
- Seasonal energy trends
- Abnormal equipment loading
Example
Suppose two chillers have similar cooling capacity.
Chiller A
Energy consumption:
8,000 kWh
Chiller B
Energy consumption:
10,500 kWh
The difference may indicate:
- Different operating conditions
- Different efficiency
- Maintenance requirements
- Uneven loading
Energy monitoring provides the data needed for further investigation.
7.3 Lighting Energy Monitoring
Lighting is another important building load.
Large buildings may have separate lighting systems for:
- Offices
- Corridors
- Parking areas
- Retail spaces
- Common areas
- Exterior lighting
Monitoring Architecture
Lighting Distribution Panel
↓
CT
↓
Energy Meter
↓
EMSWhat Can Be Monitored?
- Lighting energy consumption
- Operating schedules
- After-hours usage
- Floor-level consumption
- Common-area consumption
Energy-Saving Application
If a building has unexpectedly high nighttime consumption, the EMS can identify the lighting circuit responsible.
This allows facility managers to investigate:
- Lighting schedules
- Control system configuration
- Manual switching
- Equipment faults
7.4 Tenant Energy Monitoring
Tenant sub-metering is one of the most commercially important applications for smart building energy measurement.
It is common in:
- Office buildings
- Shopping malls
- Business parks
- Commercial complexes
- Mixed-use developments
Tenant Monitoring Architecture
Main Distribution
↓
Tenant Feeder
↓
Current Transformer
↓
Energy Meter
↓
Tenant Energy Data
↓
Billing / EMS PlatformWhy Tenant Energy Monitoring Matters
Building owners may need to determine:
How much electricity did each tenant consume?
The answer should be based on measured energy rather than estimated allocation.
Benefits
Accurate Energy Allocation
Each tenant receives energy data based on an independent measurement point.
Transparent Billing
Reduces disputes between:
- Property managers
- Tenants
- Facility operators
Better Energy Management
Building operators can compare consumption between different tenants or areas.
7.5 Floor-Level Energy Monitoring
Large commercial buildings can also be divided by floor.
Example:
Floor 1 → CT → Energy Meter
Floor 2 → CT → Energy Meter
Floor 3 → CT → Energy Meter
Floor 4 → CT → Energy Meter
↓
EMSThis allows managers to compare:
- Energy consumption by floor
- Energy consumption per square meter
- Operating-hour consumption
7.6 EV Charging Energy Monitoring
EV charging is becoming increasingly common in modern commercial buildings.
Typical locations include:
- Office parking
- Shopping centers
- Hotels
- Residential complexes
- Business parks
EV Charging Monitoring Architecture
Building Distribution
↓
EV Charging Panel
↓
Current Transformer
↓
Energy Meter
↓
EMS / EV Management PlatformThe monitoring system can provide:
- Charging energy
- Charging load
- Peak demand
- Daily charging trends
- Building versus EV energy consumption
Why EV Charging Requires Dedicated Monitoring
EV charging can create significant electrical demand.
If charging occurs during existing building peak periods, it may increase:
- Peak demand
- Transformer loading
- Distribution system stress
CT-based monitoring provides the data required for load management.
7.7 Elevator and Vertical Transportation Monitoring
Large buildings may have:
- Elevators
- Escalators
- Moving walkways
These systems can also be separately monitored.
Architecture:
Elevator Distribution Circuit
↓
CT
↓
Energy Meter
↓
EMSEnergy monitoring can help identify:
- Operating energy
- Standby consumption
- Daily operating patterns
7.8 Data Room and IT Load Monitoring
Smart buildings increasingly contain:
- Network rooms
- Server rooms
- Communication equipment
- Edge computing systems
These loads can be monitored separately.
Typical architecture:
Building Distribution
↓
IT Room Feeder
↓
CT
↓
Energy Meter
↓
EMS / BMSThis creates a useful connection between:
Building Energy Management
and
Data Center Power Monitoring.
Chapter 8 — Current Transformer Applications by Building Type
Different building types have different monitoring priorities.
8.1 Office Buildings
Typical monitoring points:
- Main incoming power
- HVAC
- Lighting
- Elevators
- Tenant feeders
- Server rooms
Primary objectives:
- Reduce operating costs
- Improve HVAC efficiency
- Allocate tenant energy
8.2 Shopping Malls
Major energy consumers include:
- HVAC
- Escalators
- Lighting
- Refrigeration
- Restaurants
- EV charging
Recommended monitoring structure:
Main Power
↓
HVAC
Lighting
Retail
Restaurants
EV Charging
Common Areas
↓
EMS8.3 Hotels
Typical loads:
- Guest rooms
- HVAC
- Kitchen
- Laundry
- Water heating
- Lighting
CT-based sub-metering can separate energy consumption by system.
8.4 Hospitals
Hospitals require reliable and continuous electrical operation.
Typical monitoring points include:
- HVAC
- Medical equipment
- Lighting
- Pumps
- Critical power systems
Monitoring architecture should be designed together with the facility’s electrical reliability requirements.
8.5 Commercial Complexes
Mixed-use buildings may combine:
- Offices
- Retail
- Restaurants
- Hotels
- Parking
- EV charging
These facilities benefit significantly from multi-level energy monitoring.
Chapter 9 — Split Core CT vs Solid Core CT for Smart Buildings
One of the most important CT selection decisions is whether to use:
Solid Core CT
or
Split Core CT
The correct choice depends heavily on whether the project is new construction or retrofit.
9.1 Solid Core Current Transformer
A solid core CT has a closed magnetic core.
The conductor normally needs to pass through the CT during installation.
Advantages
- Stable mechanical construction
- Suitable for permanent installations
- Available in high-accuracy configurations
- Well suited to new electrical panels
Recommended Applications
- New commercial buildings
- New switchboards
- New distribution cabinets
- New construction projects
9.2 Split Core Current Transformer
A split core CT can be opened and installed around an existing conductor.
Installation
Existing Cable
↓
Open CT
↓
Place Around Cable
↓
Close CT
↓
Connect Meter
↓
Start MonitoringAdvantages
Retrofit Friendly
No need to completely remove existing conductors.
Faster Installation
Useful when many measurement points need to be added.
Reduced Operational Disruption
Particularly valuable in:
- Occupied office buildings
- Shopping malls
- Hotels
- Hospitals
- Existing commercial facilities
Split Core CT Comparison
| Feature | Solid Core CT | Split Core CT |
|---|---|---|
| New Construction | Excellent | Good |
| Retrofit Projects | Limited | Excellent |
| Installation Speed | Moderate | Fast |
| Existing Cable Installation | Difficult | Easy |
| Building Shutdown Risk | Higher | Lower |
| Energy Retrofit | Good | Excellent |
| Panel Upgrade | Good | Excellent |
Chapter 10 — How to Select the Correct CT Ratio
CT ratio is one of the most important technical parameters.
A CT ratio defines the relationship between primary current and secondary current.
Example:
500A / 5A
means a primary current of 500A corresponds to a secondary current of 5A.
10.1 Select According to Actual Operating Current
Do not select a CT only according to the maximum theoretical current.
Consider:
- Minimum load
- Normal load
- Maximum load
- Future expansion
Example
Actual HVAC feeder:
Normal current:
180A
Maximum current:
230A
A CT around:
250A rated primary current
may be more appropriate than an unnecessarily oversized CT.
The final selection should always follow the electrical design and meter requirements.
10.2 Building Incoming Feeder
Large commercial buildings may require high-current CTs.
Typical applications:
- Main transformer output
- Main switchboard
- Large distribution feeders
The CT ratio must be selected based on the actual system design.
10.3 HVAC Feeder
HVAC systems may have:
- 100A feeders
- 250A feeders
- 500A feeders
- Larger distribution systems
The CT should match the corresponding feeder current.
10.4 Tenant Feeder
Tenant circuits often have lower current ratings than main building feeders.
Selecting a CT too large for the actual tenant load may reduce useful measurement resolution.
Chapter 11 — CT Accuracy Class for Smart Building Energy Management
Accuracy requirements depend on the application.
Basic Equipment Monitoring
Class 1.0
Suitable for:
- General equipment monitoring
- Load status monitoring
- Basic energy analysis
Building Energy Management
Class 0.5
Suitable for:
- EMS
- Building energy analysis
- Energy efficiency projects
Energy Allocation and Billing
Class 0.5S / Class 0.2S
Potentially suitable for higher-accuracy applications, subject to the applicable metering regulations and system requirements.
Applications may include:
- Tenant energy allocation
- Commercial energy billing
- High-accuracy energy management
Important Selection Principle
Do not assume that the highest accuracy class is always necessary.
The correct selection should balance:
Accuracy + Application + Cost + Meter Compatibility
Chapter 12 — CT Installation Best Practices in Smart Buildings
Correct CT installation is essential for reliable energy data.
Step 1 — Confirm the Electrical System
Before installation verify:
- Voltage
- Phase configuration
- Rated current
- Maximum current
- Frequency
Step 2 — Identify the Monitoring Objective
Determine whether the CT is being used for:
- General monitoring
- Energy management
- Tenant billing
- HVAC analysis
- Equipment monitoring
Step 3 — Confirm Cable Size
For split core CTs, check:
- Cable diameter
- Busbar dimensions
- CT window size
- Installation space
The conductor must fit properly inside the CT opening.
Step 4 — Confirm CT Polarity
Typical orientation:
Power Source
↓
P1
CT
P2
↓
LoadIncorrect orientation can cause:
- Reverse power readings
- Incorrect energy calculations
- Abnormal EMS data
Step 5 — Ensure the CT Core Is Fully Closed
For split core CTs:
The magnetic core must close completely.
An improperly closed core can affect measurement accuracy.
Step 6 — Match CT With the Energy Meter
Confirm:
- CT secondary output
- Meter current input
- CT ratio
- Accuracy requirements
Step 7 — Configure Communication
For an RS485 Modbus system, configure:
- Device address
- Baud rate
- Communication parameters
- Register mapping
Step 8 — Verify the Measurement
After installation, compare:
- Current
- Voltage
- Power
- Energy
with expected electrical operating conditions.
Then verify data transmission to:
- EMS
- BMS
- SCADA
- Cloud platform
Chapter 13 — When Should a Building Use a Multi-Circuit Energy Meter?
Multi-circuit energy meters become particularly valuable when a building has many independent feeders.
Typical applications:
- Multiple tenant circuits
- Multiple HVAC feeders
- Multiple floors
- Multiple EV charging circuits
- Multiple equipment groups
Traditional Architecture
Circuit 1 → Meter 1
Circuit 2 → Meter 2
Circuit 3 → Meter 3
Circuit 4 → Meter 4This increases:
- Hardware quantity
- Wiring
- Panel space
- Communication nodes
Multi-Circuit Architecture
Circuit 1 → CT ─┐
Circuit 2 → CT ─┤
Circuit 3 → CT ─┤
Circuit 4 → CT ─┤
│
▼
Multi-Circuit Meter
│
▼
EMSKey Benefits
Lower Hardware Requirements
One device can collect multiple measurement channels.
Reduced Panel Space
Useful in compact distribution cabinets.
Simplified Communication
Fewer devices can simplify network architecture.
Easier Retrofit
Particularly useful when adding energy monitoring to existing buildings.
Chapter 14 — YADA Smart Building Energy Monitoring Solution
From Current Measurement to Intelligent Building Energy Management
A smart building energy management system should not simply collect electricity data.
It should create a complete measurement chain:
Electrical Load
↓
Current Transformer
↓
Energy Meter
↓
RS485 Modbus
↓
Gateway
↓
EMS / BMS
↓
Energy Analysis
↓
OptimizationYADA’s product portfolio can support this architecture through three key layers:
Current Measurement
- SCT Series Split Core CT
- CTF Series Current Transformer
Energy Measurement
- Smart Energy Meters
- Three-Phase Energy Meters
- Multi-Circuit Energy Meters
System Integration
- RS485 Modbus RTU
- EMS Integration
- Building Energy Monitoring
This allows building owners, facility managers and system integrators to create scalable energy monitoring systems from individual circuits to complete buildings.
Chapter 15 — YADA Split Core CT for Smart Building Retrofit Projects
Retrofit Without Rebuilding the Existing Electrical System
A large proportion of smart building energy projects involve existing buildings.
The electrical system may already contain:
- Installed cables
- Existing distribution panels
- Operating HVAC systems
- Tenant feeders
- Existing switchgear
Replacing the electrical infrastructure simply to add energy monitoring is often impractical.
This is where split core CTs become particularly valuable.
Typical YADA Retrofit Architecture
Existing Building Cable
↓
SCT Split Core CT
↓
Smart Energy Meter
↓
RS485 Modbus RTU
↓
Gateway
↓
EMS / BMSWhy Split Core CTs Are Suitable for Building Retrofits
1. Easy Installation
The CT can be opened and installed around an existing conductor.
2. Reduced Electrical Modification
The existing cable does not normally need to be removed simply to install the CT.
3. Faster Deployment
Multiple monitoring points can be added with less installation work.
4. Suitable for Existing Buildings
Applications include:
- Office buildings
- Shopping malls
- Hotels
- Commercial complexes
- Industrial parks
- Smart building retrofit projects
Chapter 16 — YADA Smart Energy Meter for Building Power Monitoring
Converting CT Signals Into Useful Energy Data
The CT provides the current signal.
The energy meter converts that measurement into electrical information that can be used by the building management system.
Basic Measurement Chain
Building Load
↓
Current Transformer
↓
Energy Meter
↓
Electrical Measurement
↓
RS485 Modbus
↓
EMS / BMSDepending on the meter model and configuration, the monitoring system can provide parameters such as:
- Voltage
- Current
- Active Power
- Reactive Power
- Apparent Power
- Power Factor
- Energy Consumption
- Load Information
Why Smart Energy Meters Matter
Without an energy meter:
CT
↓
Current SignalThe building operator cannot directly obtain a complete energy profile.
With an energy meter:
CT
↓
Energy Meter
↓
kW / kWh / PF / Electrical Data
↓
EMSThe measurement becomes useful for:
- Energy analysis
- Cost allocation
- Equipment management
- Building efficiency optimization
Chapter 17 — YADA DTSD3366D-4P Multi-Circuit Energy Monitoring
One Meter for Multiple Three-Phase Circuits
Large buildings may contain many three-phase feeders.
For example:
HVAC Feeder A
HVAC Feeder B
Tenant Feeder A
Tenant Feeder B
EV Charging FeederUsing a separate meter for every circuit can increase:
- Hardware cost
- Panel space
- Wiring complexity
- Communication points
A multi-circuit energy meter can simplify the architecture.
DTSD3366D-4P Application Architecture
Three-Phase Circuit 1
↓
CT
↓
Channel 1
Three-Phase Circuit 2
↓
CT
↓
Channel 2
Three-Phase Circuit 3
↓
CT
↓
Channel 3
Three-Phase Circuit 4
↓
CT
↓
Channel 4
↓
DTSD3366D-4P
↓
RS485 Modbus
↓
EMS / BMSWhy This Architecture Is Valuable for Smart Buildings
1. Multiple Circuits With One Meter
A single multi-circuit device can consolidate multiple three-phase measurement points.
2. External CT Measurement
External CTs provide flexibility for:
- High-current feeders
- Existing cables
- Retrofit installations
3. Reduced Panel Space
Instead of installing multiple independent meters:
Meter 1
Meter 2
Meter 3
Meter 4the system can use:
Multi-Circuit Energy MeterThis is particularly useful where distribution cabinet space is limited.
4. Simplified Communication
Multiple circuits can be consolidated into fewer communication nodes.
This can simplify:
- RS485 wiring
- Device addressing
- Gateway configuration
- EMS integration
Chapter 18 — YADA HVAC Energy Monitoring Solution
Monitoring One of the Building’s Largest Energy Loads
HVAC systems are a priority application for building energy management.
A typical commercial HVAC system may include:
- Chillers
- Pumps
- Cooling towers
- Air handling units
- Fans
HVAC Monitoring Architecture
Utility
↓
Building Distribution
↓
HVAC Distribution Panel
↓
CT
↓
Energy Meter
↓
RS485 Modbus
↓
EMS / BMSMulti-Equipment HVAC Monitoring
For larger facilities:
Chiller 1
↓
CT
↓
Meter Channel 1
Chiller 2
↓
CT
↓
Meter Channel 2
Pump System
↓
CT
↓
Meter Channel 3
AHU System
↓
CT
↓
Meter Channel 4
↓
Multi-Circuit Energy Meter
↓
EMSWhat Can Facility Managers Analyze?
The system can compare:
- Chiller energy consumption
- Pump energy consumption
- Air handling energy
- HVAC load trends
- Operating periods
Example Energy Analysis
Suppose:
| HVAC System | Monthly Energy |
|---|---|
| Chiller 1 | 18,500 kWh |
| Chiller 2 | 21,200 kWh |
| Pumps | 8,400 kWh |
| AHU | 6,800 kWh |
The facility manager can immediately identify which systems deserve further investigation.
Chapter 19 — YADA Tenant Energy Monitoring Solution
Accurate Energy Allocation for Commercial Buildings
Tenant energy monitoring is particularly important for commercial property owners.
Typical buildings include:
- Office towers
- Shopping centers
- Business parks
- Mixed-use developments
Tenant Sub-Metering Architecture
Main Distribution Board
↓
Tenant Feeder
↓
Split Core CT
↓
Energy Meter
↓
RS485 Modbus
↓
EMS
↓
Tenant Energy ReportTenant Energy Data Can Support
Energy Allocation
Determine each tenant’s electricity consumption.
Billing Support
Provide measured energy data for internal billing or cost allocation, subject to local metering requirements.
Consumption Analysis
Compare:
- Tenant A
- Tenant B
- Tenant C
Energy Management
Identify tenants or areas with unusually high energy consumption.
Chapter 20 — YADA EV Charging Monitoring in Smart Buildings
Integrating EV Charging Into Building Energy Management
EV charging is increasingly becoming part of commercial building infrastructure.
However, EV charging creates a new electrical load that should be considered alongside:
- HVAC
- Lighting
- Elevators
- Tenant loads
EV Charging Monitoring Architecture
Building Grid
↓
Main Distribution
↓
EV Charging Distribution
↓
Current Transformer
↓
Energy Meter
↓
RS485 Modbus
↓
EMS / BMSWhat Can Be Monitored?
- Charging energy
- Charging power
- Load profile
- Peak charging periods
- Total EV electricity consumption
Building + EV Charging Energy Architecture
Building Grid
│
┌──────────┼──────────┐
↓ ↓ ↓
HVAC Lighting Tenants
│ │ │
CT CT CT
│ │ │
└──────────┼──────────┘
↓
Energy Meters
│
↓
EMS
↑
│
EV Charging
│
CT
│
Energy MeterThis provides a unified view of building and EV charging electricity consumption.
Chapter 21 — YADA Multi-Circuit Monitoring for Building Distribution Panels
A Practical Solution for High-Density Measurement Points
Consider a commercial building with:
- 4 HVAC circuits
- 4 tenant circuits
- 2 EV charging circuits
- 2 auxiliary circuits
A conventional approach may require numerous independent meters.
Multi-Circuit Approach
Circuit Group
↓
Multiple CTs
↓
Multi-Circuit Energy Meter
↓
RS485
↓
EMSBenefits
Reduce Hardware Quantity
Multiple circuits can be consolidated into fewer meters.
Save Distribution Cabinet Space
Particularly useful in retrofit projects.
Simplify Wiring
Fewer meters can reduce communication and power wiring complexity.
Improve System Scalability
Additional measurement points can be integrated into the broader monitoring architecture.
Chapter 22 — YADA Smart Building EMS/BMS Integration
From Measurement Devices to a Building-Level Energy Platform
The final objective is not simply to collect data.
The objective is to make that data useful.
Typical Communication Architecture
CT
↓
Energy Meter
↓
RS485 Modbus RTU
↓
Communication Gateway
↓
Ethernet / IP Network
↓
EMS / BMS
↓
Cloud / DashboardRole of RS485 Modbus
RS485 Modbus RTU is widely used in industrial and building automation applications because it provides a practical way to connect field measurement devices to higher-level monitoring systems.
Typical configuration parameters include:
- Device address
- Baud rate
- Data format
- Register mapping
EMS Functions
Once measurement data reaches the EMS platform, the system can provide:
Real-Time Monitoring
- Current
- Voltage
- Power
- Energy
Trend Analysis
- Hourly consumption
- Daily consumption
- Monthly consumption
Energy Comparison
- Floor vs floor
- Tenant vs tenant
- HVAC vs lighting
- Building vs EV charging
Alarm Management
Identify abnormal electrical conditions or unexpected energy consumption.
Chapter 23 — Smart Building Application Example
Commercial Office Building
Consider a multi-floor office building containing:
- 8 floors
- Central HVAC
- Multiple tenants
- EV charging
- Parking
- Common-area lighting
Monitoring Architecture
Main Grid
│
▼
Main Distribution
│
┌────────────┼─────────────┐
↓ ↓ ↓
HVAC Tenants EV Charging
│ │ │
CT CT CT
│ │ │
└────────────┼─────────────┘
↓
Energy Meters
↓
RS485 Modbus
↓
EMS/BMSEnergy Management Results
The building operator can understand:
Total Building Consumption
How much electricity the building uses.
HVAC Consumption
How much energy is used for cooling and ventilation.
Tenant Consumption
How much electricity each tenant consumes.
EV Charging Consumption
How much energy is used by EV charging.
Common-Area Consumption
How much electricity is used by:
- Lighting
- Elevators
- Pumps
- Parking systems
Chapter 24 — Smart Building Energy Monitoring Upgrade Path
One of the advantages of CT-based monitoring is that a building does not need to monitor every circuit on day one.
A phased approach can be used.
Phase 1 — Main Energy Monitoring
Main Feeder
↓
CT
↓
Energy Meter
↓
EMSObjective:
Establish the building energy baseline.
Phase 2 — Major Energy Consumers
Add monitoring for:
- HVAC
- Lighting
- EV charging
Phase 3 — Tenant and Floor Sub-Metering
Add:
- Tenant feeders
- Floor feeders
- Commercial areas
Phase 4 — Equipment-Level Monitoring
Add:
- Pumps
- Elevators
- Equipment
- Critical electrical loads
Final Architecture
Building
↓
Multiple CT Measurement Points
↓
Smart / Multi-Circuit Energy Meters
↓
RS485 Modbus
↓
EMS / BMS
↓
Building Energy IntelligenceChapter 25 — Why Choose YADA for Smart Building Energy Monitoring?
1. Complete CT + Meter Product Portfolio
YADA can support multiple measurement layers:
Current Transformer
↓
Energy Meter
↓
Multi-Circuit Energy Meter
↓
EMS Integration
2. Strong Retrofit Compatibility
Split core CT solutions are particularly suitable for existing commercial buildings where minimizing electrical modification is important.
3. Multi-Circuit Monitoring Capability
The DTSD3366D-4P provides a practical option for applications requiring multiple three-phase feeder measurements.
4. Industrial Communication Integration
RS485 Modbus communication enables connection with higher-level:
- EMS
- BMS
- SCADA
- Energy monitoring platforms
5. Suitable for Multiple Building Applications
YADA measurement solutions can be applied to:
- Office buildings
- Shopping malls
- Hotels
- Commercial complexes
- Business parks
- Smart buildings
- EV charging facilities
Chapter 26 — Common Current Transformer Selection Mistakes in Smart Buildings
Choosing the wrong CT can affect the accuracy and reliability of the entire building energy monitoring system.
Common problems include:
- Incorrect current measurement
- Wrong energy calculation
- Reverse power readings
- Poor low-load measurement
- Communication or integration problems
- Difficult installation
- Unnecessary project costs
Mistake 1 — Selecting CT Only According to Maximum Current
A common mistake is selecting the CT based only on the maximum theoretical current.
For example:
A tenant feeder may have a maximum current of:
500A
But its normal operating current may only be:
150–250A
Selecting an excessively large CT may not provide the best measurement performance across the actual operating range.
Better Approach
Consider:
- Minimum operating current
- Normal operating current
- Maximum operating current
- Future load expansion
The CT should be selected based on the actual electrical design and required measurement accuracy.
Mistake 2 — Using the Same CT Ratio for Every Building Circuit
Different circuits have different electrical loads.
For example:
Main Feeder → High Current CT
HVAC Feeder → Medium Current CT
Lighting Feeder → Lower Current CT
Tenant Feeder → Circuit-Specific CT
EV Charger → Charger-Specific CTUsing one oversized CT ratio for all circuits is usually not the best engineering approach.
Mistake 3 — Ignoring Accuracy Requirements
Not every building measurement point requires the same accuracy.
For example:
General Equipment Monitoring
Class 1.0 may be sufficient.
Building Energy Management
Class 0.5 may be more appropriate.
Energy Allocation / Higher-Accuracy Applications
Class 0.5S or 0.2S may be considered depending on the application and applicable metering requirements.
Mistake 4 — Choosing a CT Without Checking the Energy Meter
The CT and energy meter must be compatible.
Before ordering, confirm:
- CT ratio
- Secondary output
- Input type of the meter
- Accuracy requirements
- Wiring configuration
For example:
CT
500A / 5A
↓
Energy Meter
5A Current InputThe CT output must match the meter’s input specification.
Mistake 5 — Ignoring the Physical Installation Space
Electrical specifications are not the only consideration.
The CT must physically fit the installation location.
Check:
- Cable diameter
- Busbar dimensions
- CT window size
- Panel clearance
- Cable routing
- Installation direction
This is particularly important for split core CT projects.
Mistake 6 — Using Solid Core CTs in Difficult Retrofit Projects
For an existing commercial building, removing conductors can create:
- Additional labor
- Electrical risks
- Building downtime
- Higher project costs
For many retrofit applications, a split core CT provides a more practical installation approach.
Mistake 7 — Ignoring CT Polarity
Incorrect CT orientation can produce abnormal measurement results.
Typical orientation:
Power Source
↓
P1
CT
P2
↓
LoadIncorrect polarity may cause:
- Negative active power
- Reverse energy
- Incorrect load analysis
Mistake 8 — Installing CTs Without Considering Future Expansion
A building’s electrical demand can change.
Future projects may include:
- EV chargers
- Additional tenants
- New HVAC equipment
- Solar PV
- Battery storage
- Additional office areas
The monitoring architecture should therefore consider future expansion wherever practical.
Chapter 27 — Smart Building CT Installation Checklist
Before installation, engineers should verify the following.
Electrical Parameters
☐ Rated voltage
☐ Rated current
☐ Maximum current
☐ Frequency
☐ Single-phase / three-phase configuration
CT Parameters
☐ CT ratio
☐ Accuracy class
☐ Secondary output
☐ Core type
☐ Window size
☐ Installation method
Energy Meter Compatibility
☐ Current input
☐ Voltage input
☐ CT ratio configuration
☐ Communication protocol
☐ Wiring configuration
Physical Installation
☐ Cable diameter
☐ Busbar dimensions
☐ Panel space
☐ CT clearance
☐ Cable direction
☐ CT core fully closed
Communication
☐ RS485 wiring
☐ Device address
☐ Baud rate
☐ Modbus register mapping
☐ Gateway compatibility
System Verification
☐ Current reading
☐ Voltage reading
☐ Power reading
☐ Energy reading
☐ Power factor
☐ Communication status
☐ EMS/BMS data
Chapter 28 — How to Match CTs With Energy Meters
A reliable smart building monitoring system requires compatibility between the CT and energy meter.
The basic relationship is:
Electrical Load
↓
CT
↓
Current Signal
↓
Energy Meter
↓
Electrical Parameters
↓
EMS / BMSStep 1 — Confirm CT Output
Typical CT outputs may include:
- 5A
- 1A
- mA-level outputs
The actual output must match the energy meter specification.
Step 2 — Configure the CT Ratio
Example:
300A / 5A
The corresponding CT ratio must be configured correctly in the meter.
If the physical CT ratio and meter configuration do not match, the displayed current and energy data can be incorrect.
Step 3 — Confirm Phase Configuration
The monitoring system should match the electrical system:
- Single-phase
- Three-phase three-wire
- Three-phase four-wire
Step 4 — Verify Measurement Direction
Check CT polarity and phase sequence.
Incorrect phase mapping can result in:
- Incorrect active power
- Abnormal power factor
- Incorrect energy calculation
Chapter 29 — Smart Building Energy Monitoring FAQ
Q1. Why are current transformers used in smart buildings?
Current transformers allow electrical systems with high currents to be measured safely and connected to energy meters.
They are commonly used for:
- Main feeders
- HVAC
- Lighting
- Tenant circuits
- EV charging
- Equipment monitoring
Q2. Where should CTs be installed in a commercial building?
Typical locations include:
- Main distribution boards
- Sub-distribution panels
- HVAC feeders
- Tenant feeders
- EV charging panels
- Equipment distribution circuits
The exact measurement points should be determined according to the building’s energy management objectives.
Q3. Are split core CTs suitable for existing buildings?
Yes.
Split core CTs are particularly useful for retrofit projects because they can be installed around existing conductors without the same level of conductor modification normally associated with solid core CT installation.
Q4. What CT accuracy class should I choose?
It depends on the application.
A general guideline is:
| Application | Typical Accuracy Consideration |
|---|---|
| Basic equipment monitoring | Class 1.0 |
| Building energy management | Class 0.5 |
| Higher-accuracy energy allocation | Class 0.5S / 0.2S |
Final selection should follow project specifications and applicable standards.
Q5. Can CTs measure energy consumption directly?
No.
A CT primarily provides a current measurement signal.
An energy meter combines current measurement with voltage and other electrical parameters to calculate power and energy.
Q6. Can one energy meter monitor multiple building circuits?
Yes, when using a suitable multi-circuit energy meter.
For example, a multi-circuit meter can be used to monitor multiple three-phase feeders through external CTs.
This can reduce:
- Meter quantity
- Panel space
- Wiring complexity
- Communication nodes
Q7. Can building energy meters communicate with BMS?
Yes, if the meter and BMS architecture support compatible communication protocols.
RS485 Modbus RTU is widely used for connecting field meters with:
- EMS
- BMS
- SCADA
- Gateways
Q8. Why is HVAC energy monitoring important?
HVAC can represent a significant portion of commercial building electricity consumption.
Monitoring HVAC feeders allows facility managers to analyze:
- Chiller consumption
- Pump consumption
- Fan consumption
- Operating patterns
- Energy efficiency
Q9. Can CTs be used for EV charging monitoring?
Yes.
CTs can be installed on EV charging distribution feeders and connected to suitable energy meters.
This enables building operators to distinguish EV charging consumption from other building loads.
Q10. What is the difference between a CT and a smart energy meter?
A CT primarily measures current.
A smart energy meter processes electrical measurements and provides parameters such as:
- Voltage
- Current
- Power
- Energy
- Power factor
The meter can then transmit data to an EMS or BMS.
Q11. What is the advantage of using a multi-circuit energy meter?
A multi-circuit meter can consolidate several measurement circuits into one device.
This can help:
- Save panel space
- Reduce hardware quantity
- Simplify wiring
- Reduce communication nodes
Q12. How can CT-based energy monitoring help reduce building energy costs?
CT-based monitoring provides detailed energy data.
This allows facility managers to identify:
- High-consumption systems
- Abnormal loads
- After-hours operation
- HVAC inefficiencies
- Excessive peak demand
The data can then support targeted energy-saving actions.
Chapter 30 — Smart Building Energy Management Technical Glossary
Current Transformer — CT
A device used to transform high AC current into a lower measurable signal for electrical measurement systems.
Split Core Current Transformer
A CT with an openable magnetic core designed for easier installation around existing conductors.
Solid Core Current Transformer
A CT with a fixed closed magnetic core, commonly used in new electrical installations.
CT Ratio
The relationship between primary current and secondary current.
Example:
500A / 5A
Accuracy Class
A classification describing the measurement accuracy of a CT under specified conditions.
Energy Meter
A device that measures electrical parameters and calculates electrical energy consumption.
Multi-Circuit Energy Meter
An energy meter designed to measure multiple electrical circuits through multiple measurement inputs.
EMS
Energy Management System
A software and hardware platform used to monitor, analyze and optimize energy consumption.
BMS
Building Management System
A system used to monitor and control building systems such as HVAC, lighting and other building services.
Sub-Metering
Measuring energy consumption at individual:
- Floors
- Tenants
- Equipment
- Systems
- Distribution circuits
rather than only measuring total building consumption.
Modbus RTU
A communication protocol commonly used over RS485 networks for industrial and building automation devices.
HVAC
Heating, Ventilation and Air Conditioning
A major electrical load category in commercial buildings.
Chapter 31 — Smart Building Energy Monitoring: Complete System Architecture
The complete solution can be summarized as:
UTILITY GRID
│
▼
MAIN DISTRIBUTION BOARD
│
┌────────────┼────────────┐
│ │ │
▼ ▼ ▼
CT CT CT
│ │ │
HVAC Lighting Tenant
│ │ │
└────────────┼────────────┘
│
▼
ENERGY METERS
│
▼
RS485 MODBUS RTU
│
▼
EMS / BMS
│
┌──────────────┼──────────────┐
▼ ▼ ▼
Dashboard Reports Analysis
│ │ │
└──────────────┼──────────────┘
▼
ENERGY OPTIMIZATIONChapter 32 — YADA Smart Building Energy Monitoring Solution
From CT to Complete Building Energy Visibility
YADA can support a complete measurement architecture:
Current Measurement
SCT / CTF Current Transformers
↓
Energy Measurement
Smart Energy Meters
↓
Multi-Circuit Measurement
DTSD3366D-4P Multi-Circuit Energy Meter
↓
Communication
RS485 Modbus RTU
↓
Management
EMS / BMS
Typical YADA Application Scenarios
Commercial Office Buildings
Monitor:
- Main power
- HVAC
- Lighting
- Tenants
- Elevators
Shopping Malls
Monitor:
- HVAC
- Retail areas
- Restaurants
- Common areas
- EV charging
Hotels
Monitor:
- Guest areas
- HVAC
- Kitchen
- Laundry
- Lighting
Business Parks
Monitor:
- Individual buildings
- Tenant feeders
- Shared facilities
- EV charging
- Common areas
Chapter 33 — Why Choose YADA for Smart Building Energy Monitoring?
Complete Measurement Ecosystem
YADA can provide solutions covering:
Current Transformer
→
Energy Meter
→
Multi-Circuit Energy Meter
→
Communication
→
Energy Management
Retrofit-Friendly CT Solutions
Split core CTs can simplify the addition of energy monitoring to existing electrical systems.
Multi-Circuit Measurement
DTSD3366D-4P can be used where multiple three-phase circuits need to be monitored with external CTs.
Industrial Communication
RS485 Modbus communication provides a practical connection between field meters and higher-level management platforms.
Multiple Application Scenarios
The solution can be applied to:
- Smart buildings
- Office buildings
- Shopping malls
- Hotels
- Business parks
- Commercial complexes
- EV charging facilities
Chapter 34 — Key Takeaways
Current Transformers Are the Foundation of Smart Building Energy Monitoring
A modern building energy management system needs more than a single utility meter.
The complete measurement architecture should provide visibility from:
Building Level
to
Distribution Level
to
System Level
to
Tenant / Equipment Level
The fundamental architecture is:
Current Transformer
↓
Energy Meter
↓
RS485 Modbus
↓
EMS / BMS
↓
Energy Analysis
↓
Energy OptimizationThe Five Most Important Points
1. CTs Enable Safe Current Measurement
They provide the current measurement interface between electrical circuits and energy meters.
2. Split Core CTs Are Valuable for Retrofit Projects
They can simplify installation around existing conductors.
3. CT Ratio Must Match the Actual Application
Selection should consider:
- Normal load
- Maximum load
- Future expansion
4. Multi-Circuit Meters Simplify High-Density Monitoring
They can reduce hardware and panel-space requirements when multiple feeders need to be monitored.
5. Energy Data Must Reach an EMS or BMS
Measurement becomes valuable when it supports:
- Analysis
- Reporting
- Optimization
- Energy-saving decisions
Final Conclusion
Building a Smarter Energy Management System Starts With Accurate Measurement
Smart buildings require detailed visibility into how electricity is consumed.
A complete solution combines:
Current Transformers
Smart Energy Meters
Multi-Circuit Energy Monitoring
RS485 Modbus Communication
EMS / BMS Integration
YADA’s product architecture can support this measurement chain from electrical current acquisition to building-level energy monitoring.
For new construction, solid core CT solutions can provide permanent measurement installations.
For existing buildings, split core CTs provide a practical option for energy monitoring retrofits.
For buildings with multiple feeders, multi-circuit energy meters can simplify the measurement architecture.
The result is a scalable energy monitoring system that helps building owners and facility managers move from:
“How much electricity did the building use?”
to:
“Where is the electricity being used, when is it being used, and how can we use it more efficiently?”
Related Bolgs
Ultimate Guide to Current Transformers
What Is a Current Transformer?
How Does a Current Transformer Work?
Current Transformer Accuracy Class Explained
Split Core Current Transformer Guide
Split Core CT vs Solid Core CT
Current Transformer Ratio Selection Guide
Current Transformer for Energy Meter
Current Transformer for Solar PV Systems
Current Transformer for EV Charging Infrastructure
Current Transformer for Data Center Power Monitoring
Current Transformer for Industrial Energy Management
Current Transformer Manufacturer Guide