Adding a meter to an existing circuit usually means measuring the current in a cable that cannot be disconnected. A split-core current transformer (CT) is made for that job, because its core comes apart and clamps on. That is why split-core CTs are the usual choice for retrofit energy monitoring and submetering.
The convenience has a cost in accuracy, and the CT only works with a meter input made for its output. This guide covers how a split-core CT works, what to check on a datasheet, how to fit it and how to test it. The CT selection guide covers accuracy classes and burden in more depth.
How a current transformer works explains the ratio, magnetising current and burden that apply to every CT, split-core or not.
How a split-core CT works
A CT is a transformer. The conductor through its window is the primary, a single turn. The secondary is a winding of many turns on a magnetic core. The alternating current in the conductor magnetises the core, and the core induces a secondary current in proportion:
secondary current = primary current ÷ ratio
A 200:5 CT gives 5 A when 200 A flows in the conductor, and 3 A at 120 A. The CT needs alternating current to magnetise its core, so an ordinary CT does not measure DC.
In a split-core CT, the core is made in two parts. A hinge, a clip or a separate bar lets you open it, put it around the conductor and close it again. The joint leaves a small air gap in the magnetic path. The core then needs more magnetising current, and that current is the part of the primary current that does not reach the secondary. The ratio error and the phase error both grow, most at low current, where the magnetising current is a larger share of the total.
Output types
Split-core CTs are made with three kinds of output. The meter's input decides which one you need:
| Output | What it gives | Typical meter input | Disconnecting under load |
|---|---|---|---|
| Current, 1 A or 5 A | A secondary current in proportion to the primary current | 1 A or 5 A CT input; switchgear meters | Dangerous: short the secondary first |
| Current, milliamps | A small secondary current, for example 100 mA at rated current | A meter made for that sensor | Follow the manufacturer's instructions |
| Voltage, usually 333 mV | A voltage across a burden resistor inside the CT, 0.333 V at rated current | 333 mV input; submeters and data loggers | Safe while the internal burden is intact |
IEC 61869-2 covers current-output CTs. IEC 61869-10 covers low-power passive sensors, including voltage-output CTs. A wrong pairing does not just read wrongly. A 5 A CT on a high-impedance voltage input is close to an open secondary and can develop a dangerous voltage. A 333 mV CT on a 5 A input sees a near short circuit, and the meter reads almost zero.
Split-core vs solid-core CTs
| Split-core CT | Solid-core CT | |
|---|---|---|
| Fitting | Closes around a connected cable | Conductor must be disconnected and passed through |
| Air gap in the core | Yes, at the joint | No |
| Accuracy at low current | Lower for the same size and class | Better |
| Sensitivity to installation | A dirty or partly open joint adds error | Little |
| Typical use | Retrofit monitoring, submetering, temporary measurement | New panels, switchgear metering, billing installations |
Where the conductor is a busbar, a group of parallel cables or a cable too large for any practical window, a flexible Rogowski coil is the third option. It has no iron core, so it does not saturate, but its small output limits accuracy at low current.
What to check on a datasheet
Output and ratio. The output must match the meter input, and the ratio must cover the circuit. Check the rated primary current and whether the CT has an extended rating above 100%.
Accuracy class and the currents it applies to. IEC 61869-2 sets the limits for each class at 5%, 20%, 100% and 120% of rated current, and the S classes add a limit at 1%. A Class 1 CT is allowed ±1% ratio error at rated current and ±3% at 5% of it. A Class 0.5S CT is allowed ±0.5% from 20% upwards and ±0.75% at 5%. A split-core CT that is accurate at rated current can still be several percent out on a circuit that runs at 5% of its rating.
Phase error. On loads with a low power factor, phase displacement causes more energy error than the ratio error. The CT selection guide shows the limits and the effect on kW.
Window size. Measure the finished outside diameter of the cable, including its insulation. The nominal conductor size does not give it. The conductor size and CT aperture guide explains why.
Insulation and listing. Check the rated insulation voltage against the voltage of the conductor the CT goes around, especially on a bare busbar, and the measurement category for its position in the installation. In North America, current sensors for energy monitoring inside panelboards and switchboards are covered by UL 2808, energy monitoring equipment. Check the listing that your local code requires.
Size the CT to the load
Choose the rating from the current the circuit actually carries, not from the breaker. A feeder on a 400 A breaker that carries 60 A puts a 400 A CT at 15% of its rating, where every class allows more error than at rated current. Overnight, at 20 A, it runs at 5%.
Current transformer selection builds a selection brief from the current range, the conductor size and the meter input. For a 25 mm cable that carries 20 A to 120 A, with a 2 mm allowance on each side, it asks for a 29 mm round window:
For a current-output CT, also check the burden. The meter input, the leads and every terminal in the loop must stay between 25% and 100% of the CT's rated output for its class to apply. The CT ratio and burden calculator checks the operating point and the burden together.
Fit a split-core CT
- Check the CT output, ratio and window against the meter input and the conductor.
- Identify the conductor and its phase. Label the CT and the meter channel with the same phase.
- Open the core. Check that both faces of the joint are clean and undamaged.
- Put the CT around one conductor only, or around all the parallel cables of one phase. Never around two phases, or a phase and its neutral: the CT then measures their sum, which is close to zero.
- Fit the P1 side towards the supply, so that current flows from P1 to P2 towards the load. Where the CT carries an arrow, it shows the direction of current and points towards the load. Follow the marking on the CT.
- Close the core until the latch engages fully. Secure the CT so that it cannot hang open on the cable.
- For a current-output CT, keep the secondary shorted until the meter is connected. Connect S1 and S2, or the output leads, to the meter channel of the same phase.
- Configure the meter with the CT ratio or rating, unless the CT and meter were supplied and calibrated as a pair.
Test the installation
Test with the circuit under load. Measure the primary current on the same conductor with a clamp meter, then check the CT output:
- Current output: the secondary current equals the primary current divided by the ratio. A 200:5 CT on a 120 A load gives 3.0 A.
- 333 mV output: the output voltage equals 0.333 V multiplied by the primary current divided by the rated current. A 100 A sensor on a 42 A load gives about 140 mV AC. Measure it with a multimeter on its AC millivolt range.
Then check the meter. Each phase must show positive kW for a load that imports, a power factor that suits the load, and a current within the combined tolerance of the clamp meter and the CT.
| Symptom | Likely cause |
|---|---|
| Negative kW on one phase | CT reversed, or its output leads swapped |
| Reading about half the expected value with a power factor near 0.5 on a resistive load | CT paired with another phase's voltage channel |
| Reading near zero on a loaded circuit | Two conductors in one CT, or the wrong cable |
| Reading low by a few percent, or changing when the CT is moved | Core not fully closed, or dirt on the joint faces |
| Every reading wrong by the same factor | Wrong ratio set in the meter, or wrong output type |
The submetering guide shows how each fault appears in the data after commissioning.
Split-core CTs on EpiSensor monitors
The ZEM-65 electricity monitor is ordered with its current sensors. The 120 A option is a split-core CT with a 14 mm window and 2 m cables. Larger circuits use 300 A, 1 kA or 3 kA Rogowski coils. EpiSensor connects the sensors and calibrates them with the meter before it ships, and states Class 0.5S to IEC 62053-22 for the meter and sensors together. There is no ratio, burden or output type to match on site. The fitting steps above, the arrow direction and the phase labels still apply.
A ZEM reports over the Zigbee mesh to a Gateway running Edge. For a meter that already has CTs and a Modbus port, the three-phase Modbus meter guide covers reading it instead.
Common questions
What is a split-core current transformer?
A current transformer with a core in two parts that hinge or separate, so that it can close around an existing cable or busbar. It measures alternating current through the conductor it encloses, and it is used to retrofit energy monitoring without disconnecting the circuit.
What is the difference between a split-core and a solid-core CT?
A solid-core CT is a closed ring, so the conductor must be disconnected and passed through it. Without a joint, it holds its accuracy better at low current. A split-core CT opens and closes around a conductor that stays connected, at the cost of an air gap in the core that increases error, most at low current.
Are split-core CTs accurate?
They can meet the same accuracy classes as solid-core CTs, but only near their rated current and with the core closed on clean faces. Compare the stated class and the error limits at 5% and 20% of rated current, and size the CT so that the load that carries most of the energy runs above 20% of its rating.
Is it safe to disconnect a split-core CT from the meter?
A voltage-output CT, such as a 333 mV type, has its burden resistor inside, so an open output lead does not produce a dangerous voltage while that resistor is intact. A current-output CT (1 A or 5 A) must never be open-circuited while primary current flows: short its secondary first.
How do I test a split-core CT?
With the circuit under load, measure the primary current with a clamp meter. For a current-output CT, the secondary current should equal the primary current divided by the CT ratio. For a 333 mV CT, the output voltage should equal 0.333 V multiplied by the primary current divided by the rated current.