Most commercial supplies and large sub-mains are metered through current transformers, because no meter can carry hundreds of amps through its own terminals. The meter then sees a scaled copy of the current, and a number on a label (the CT ratio) connects what it counts to what the circuit used. This guide explains when a supply or a submeter needs CT metering, how the CT ratio turns the meter's count into kWh, and how to check that the result is right.
What is a current transformer explains how the CT itself works, from the ampere-turn balance to the open-secondary hazard.
CT meter vs whole-current meter
| Whole-current meter | CT-operated meter | |
|---|---|---|
| How current reaches the meter | The load current flows through the meter's terminals | CTs on each phase give a scaled secondary current or voltage |
| Current limit | The meter's own rating | The CT rating, up to thousands of amps |
| Voltage connection | Direct | Direct, through fused voltage connections |
| Ratio to set | None | The CT ratio, unless the meter displays secondary energy |
| Fitting | The circuit must be disconnected | CTs fit around conductors; split-core CTs close around a connected cable |
| Where the error comes from | The meter | The meter and the CTs |
Whole-current meters stop at about 100 A per phase. The three direct-connected Modbus meters in the meter comparison are rated 63 A, 65 A and 100 A. In the Great Britain settlement codes, whole-current metering covers supplies up to 100 A per phase, which is about 72 kW on a three-phase supply (Elexon CP1273). Larger supplies are CT metered, and other countries set their own limits. For a submeter, the limit is the rating of the meter you choose, and the rest of this guide applies to any CT-operated submeter.
How a CT meter is connected
A three-phase, four-wire CT meter has three current inputs and four voltage connections:
- A CT on each phase conductor, with its arrow towards the load. Its secondary connects to the meter's current input for that phase.
- A fused voltage connection from each phase, and the neutral. Each current input must pair with the voltage of the same phase.
- For a current-output CT (1 A or 5 A), a test terminal block with shorting links between the CTs and the meter, so the meter can be removed without opening a CT secondary.
The meter calculates power on each phase from that phase's current and voltage, then adds the three. The three-phase power guide explains why each current must pair with its own phase voltage.
The CT ratio and the multiplier
The CT ratio is the rated primary current over the rated secondary current. A 400/5 CT gives 5 A when 400 A flows, so its ratio is 80. The meter only sees the secondary side. How it handles the ratio decides how you read it:
- The meter is set with the ratio. You enter the CT primary and secondary in the meter, and it displays primary values: amps, kW and kWh on the circuit. Many multifunction and Modbus CT meters work this way. The SDM630MCT, for example, holds the CT ratio in holding register 40063 and the 1 A or 5 A secondary rating in another register.
- The meter counts secondary energy. Its display shows energy at the CT secondary, and the nameplate or the meter record gives a multiplier. Multiply each advance of the register by the multiplier to get the energy on the circuit.
Where voltage transformers (VTs) are also fitted, on high-voltage supplies, the multiplier is the CT ratio times the VT ratio.
A wrong ratio is one of the most common commissioning faults. Every reading is then wrong by the same factor, so it is easy to miss. The data still looks plausible, and only a check against another reference finds it.
Accuracy of a CT meter
The meter's class applies to the meter alone. IEC 62053-22 sets classes 0.1S, 0.2S and 0.5S for transformer-operated meters. The CTs have their own class under IEC 61869-2, and their errors add to the meter's. A Class 1 CT is allowed ±1% ratio error at rated current and ±3% at 5% of it. On a circuit that runs far below the CT rating, the CT error can be larger than the meter's.
For billing, the meter must also be of an approved design. In the EU, the Measuring Instruments Directive covers active energy meters in Annex V (MI-003), and the annex covers the meter only. External CTs are outside it. The high-accuracy metering guide explains the difference between a class and an approval, and the CT selection guide shows how to size and class the CTs.
Check a CT meter
- Read the CT label. Record the ratio, the class and the rated output of each CT. All three phases should have the same ratio.
- Read the meter setting. Compare the CT primary and secondary set in the meter, or the multiplier on the meter record, with the label.
- Compare current. Under load, measure each phase current with a clamp meter on the primary conductor. The meter's displayed current should agree within the combined tolerance of the clamp meter, the CT and the meter.
- Check each phase. Each phase should show positive kW for a load that imports, and a power factor that suits the load. Negative kW on one phase points to a reversed CT. A power factor near 0.5 on a resistive load points to a CT paired with another phase's voltage.
- Check the multiplier on the bill or report. Where a supplier or a report applies a multiplier, it must match the CTs that are fitted, including after any CT replacement.
The worked check above shows a 400/5 CT at 120 A: a secondary current of 1.5 A, 30% of rated current, with the meter input and the lead loop inside the CT's rated output.
CT meters as submeters
A CT-operated submeter is the usual choice for a sub-main, an incomer or any circuit above about 100 A. Retrofits normally use split-core CTs, which close around the existing cable. The split-core CT guide covers how to fit and test them, and the submetering guide covers where to put the meters and how to reconcile them with the main meter. The three-phase Modbus meter guide covers reading CT meters over Modbus.
CT metering with EpiSensor
The ZEM-65 electricity monitor is a CT-based three-phase monitor whose current sensors are supplied with it: a 120 A split-core CT, or 300 A, 1 kA or 3 kA Rogowski coils. EpiSensor connects and calibrates the sensors with the meter before it ships, and states Class 0.5S to IEC 62053-22 for the meter and sensors together. There is no CT ratio to set or multiplier to apply, so the ratio fault above cannot occur. The ZEM's accuracy class is not a billing approval. Where an installed CT meter already has a Modbus port, a ZMB Modbus interface or the Gateway can read it instead.
Common questions
What is a CT meter?
An electricity meter that measures current through current transformers fitted on each phase, instead of carrying the load current through its own terminals. It still takes its voltage directly from the conductors. CT meters are used for supplies and circuits above the rating of a whole-current meter.
What is the difference between a CT meter and a whole-current meter?
A whole-current, or direct-connected, meter carries the full load current through its terminals, so it is limited to its own rating, typically up to 100 A per phase. A CT meter measures a scaled-down secondary current or voltage from CTs, so it suits any current that the CTs are rated for.
How do you read a CT meter?
Check whether the meter displays primary energy or secondary energy. If its display or nameplate states a multiplier, or if it counts energy at the CT secondary, multiply the difference between two register readings by the CT ratio (for 400/5 CTs, by 80). If the meter has been set with the CT ratio, the display already shows primary energy.
When is CT metering required?
When the current per phase is above the rating of a whole-current meter. In the Great Britain settlement codes, whole-current metering covers supplies up to 100 A per phase, about 72 kW on a three-phase supply. For a submeter, the same limit comes from the rating of the meter you choose.