Choose the meter input first. A 5 A input, a 333 mV input and a Rogowski input each take a different sensor, and no CT specification fixes a mismatch. Then record four facts for each circuit: the lowest current you need to measure, the normal operating range, the peak current, and the finished outside diameter of the conductor.
Output type
A current-output CT has a 1 A or 5 A secondary. IEC 61869-2 covers it, and its secondary must always see a low-impedance burden. The 5 A secondary is still the usual choice in switchgear metering. A 1 A secondary suits long runs to the meter, because the lead loss at rated current is 25 times lower on the same cable.
A voltage-output CT has its burden resistor inside the housing and gives a low voltage, typically 333 mV at rated current. IEC 61869-10 covers these low-power passive sensors. They are common in submetering because the output leads are safe to disconnect under load.
A Rogowski coil gives a small voltage proportional to the rate of change of current, stated in mV per kA at 50 Hz. The meter or a separate integrator converts it to current.
A wrong pairing can be dangerous. A 5 A CT on a high-impedance voltage input is close to an open secondary, which produces a dangerous voltage. A 333 mV CT on a 5 A input sees a near short circuit and the meter reads almost zero.
CT types
Solid-core CTs
A solid-core CT is a closed ring, and you pass the conductor through it. With no air gap in the core, it needs less magnetising current than a split-core CT of the same size, so it holds its accuracy better at low current. To fit one, you must disconnect the conductor, and that means a shutdown. On a main incomer, plan the outage or choose a split-core CT or a Rogowski coil.
Split-core CTs
A split-core CT opens and closes around an existing conductor. The joint is an air gap. The gap raises the magnetising current, and ratio and phase error increase, most at low current. Dirt on the pole faces or a latch that is not fully closed makes the gap larger. Clean the faces, close the latch fully, and compare the declared class and the error limits at low current with those of a solid-core CT. A split-core CT removes the need to disconnect the conductor. It does not make work in a live panel safe. Schneider Electric's installation note gives the precautions.
Rogowski coils
A Rogowski coil has no magnetic core. It cannot saturate, its output is linear over a wide range, and a flexible coil fits around large cables and busbars. The output at low current is small, so the noise floor of the integrator and meter sets the low-load limit. The coil is also sensitive to position. One manufacturer's flexible coil is specified at ±1.0% from 3% to 120% of rated current, with a further typical ±0.5% (maximum ±1.0%) error when the conductor is off-centre or tilted (Continental Control Systems). Centre the conductor, keep the coil perpendicular to it, and close the latch fully. Compare the coil, the integrator and the meter as one chain. Our Rogowski coil guide covers this in more detail.
Current rating
The rated primary current sets the ratio. A 200/5 CT gives 5 A at 200 A and 2.75 A at 110 A. Accuracy limits apply up to 120% of rated current. Above that, the error is not specified unless the CT has an extended current rating. IEC 61869-2 (5.203) gives 120%, 150% and 200% as the preferred values of rated continuous thermal current. Check the rated short-time thermal current (Ith, for 1 s) against the fault level at the point of installation.
Size for the load, not the breaker
A breaker rating does not describe the load. Take a feeder on a 400 A breaker that carries 80 to 120 A in the day, 15 A overnight, and 250 A for short periods:
- A 400/5 Class 0.5 CT runs at 20% to 30% of rated current in the day. The limit at 80 A is ±0.75%. The peaks are 62.5% of rated current. Overnight it runs at 3.75%. Class 0.5 has no limit below 5%, so the overnight readings have no specified accuracy.
- A 400/5 Class 0.5S CT holds ±0.5% from 20% of rated current. At 3.75% it is between the ±1.5% limit at 1% and the ±0.75% limit at 5%.
- A 150/5 Class 0.5 CT runs at 53% to 80% in the day and 10% overnight. The 250 A peaks are 167% of rated current, so choose one with a 200% extended current rating.
Both the 400/5 Class 0.5S and the 150/5 with an extended rating are valid choices here. The 400/5 Class 0.5 CT is not, if the overnight base load matters.
Future load growth
If the circuit will grow from 100 A to 180 A, a 200/5 CT avoids a later replacement. At today's 100 A it runs at 50% of rated current, which is inside the tightest part of every class. The risk is a low base load. At 10 A it runs at 5%, where Class 0.5 allows ±1.5% and Class 0.5S allows ±0.75%.
Accuracy class
For measuring CTs, IEC 61869-2 (5.6.201.3, Tables 201 and 202) sets these limits at rated frequency, with the burden anywhere from 25% to 100% of rated output:
| Class | Error, at % of rated current | 1% | 5% | 20% | 100% | 120% |
|---|---|---|---|---|---|---|
| 0.5 | Ratio error | not specified | ±1.5% | ±0.75% | ±0.5% | ±0.5% |
| 0.5 | Phase displacement | not specified | ±90′ | ±45′ | ±30′ | ±30′ |
| 0.5S | Ratio error | ±1.5% | ±0.75% | ±0.5% | ±0.5% | ±0.5% |
| 0.5S | Phase displacement | ±90′ | ±45′ | ±30′ | ±30′ | ±30′ |
| 0.2S | Ratio error | ±0.75% | ±0.35% | ±0.2% | ±0.2% | ±0.2% |
| 0.2S | Phase displacement | ±30′ | ±15′ | ±10′ | ±10′ | ±10′ |
The S classes reach the same limits at lower current. The Class 0.5S limit at 1% equals the Class 0.5 limit at 5%, and its limit at 5% equals the Class 0.5 limit at 20%. That matters for circuits that spend most of their time at low load, such as lighting, HVAC at night and spare feeders.
Phase displacement matters more than the ratio error at low power factor. The active-power error is approximately the phase error in radians multiplied by tan φ. A Class 0.5 CT at rated current may have 30′ (0.5°) of phase error. At a power factor of 0.5 that is about 1.5% error in kW, three times its ratio limit. At 5% of rated current and the same power factor, the allowed 90′ gives about 4.5%.
A CT class covers the CT alone. The meter has its own class, for example IEC 62053-22 for transformer-operated active-energy meters. The errors of the two add, and neither class alone establishes the accuracy of the installation or its suitability for billing. Beckhoff's terminology guide explains the terms that datasheets use.
The class limits apply at rated frequency only. On the supply side of a variable-speed drive or an inverter, the current contains harmonics, for example the 5th and 7th at 250 Hz and 350 Hz on a 50 Hz system. Ask for the frequency response if harmonic power matters. Measure a drive on its supply side. The drive output runs at variable frequency, and an iron core runs closer to saturation at low frequency.
A metering CT may also carry an instrument security factor, FS 5 or FS 10 (5.6.201.6). At FS times rated primary current, with rated burden, its composite error reaches 10%. Above that the secondary current stops rising in proportion, which protects the meter during a fault. With a burden well below rated, the CT follows more of the fault current before it saturates.
Burden
Burden is the load on the CT secondary: the meter input, the leads, and every terminal and test block in the loop. Datasheets state it in VA at rated secondary current (VA = I² × Z). IEC 61869-2 gives standard rated outputs of 2.5, 5, 10, 15 and 30 VA.
The class limits apply between 25% and 100% of rated output. Above rated output, the core runs at higher flux and the ratio and phase errors grow, most at high current. Below 25%, the CT is outside its tested range. A 15 VA CT that feeds a 0.5 VA electronic meter over short leads is well below its 3.75 VA minimum. Choose a lower rated output, or a CT with the extended burden range in 5.6.201.4, which holds the class from 1 VA to rated output (rated output 15 VA or less).
The lead loop burden at rated secondary current is I² × R. At 5 A, every 0.1 Ω of loop resistance adds 2.5 VA. For example, a meter 15 m from a 200/5 CT, wired in 2.5 mm² copper, has a 30 m loop of about 0.207 Ω at 20 °C. That adds 5.2 VA. With a 1 VA meter input, the total is 6.2 VA, which is over a 5 VA rating:
There are three fixes. In 4 mm² secondary leads, the loop is 0.129 Ω and the total is 4.2 VA. A 10 VA CT accepts 6.2 VA and still has more than 2.5 VA connected. A 1 A secondary on the same 2.5 mm² leads adds only 0.21 VA, if the meter has a 1 A input. If you extend an existing secondary, recalculate the loop in the same way. Get the loop resistance from the conductor resistance calculator and check the total with the CT ratio and burden calculator.
Physical fit and polarity
Measure the finished outside diameter of the cable, including insulation, before you specify the aperture. The nominal conductor area does not give it. The conductor size guide shows why. For a busbar, check the width and the thickness against the CT's rectangular window. Check the outside dimensions of the CT against the space between phases and to the enclosure.
Where a phase has parallel cables, pass all of them through one CT, or fit one CT on each cable and add the readings in the meter. Never pass two phases, or a phase and the neutral, through the same CT. It then measures their sum, which is a residual current, not a load.
Fit each CT with its P1 side (or the arrow) towards the supply, and connect S1 and S2 to the matching meter terminals. Wiring mistakes here are a common commissioning fault, and each has a signature:
- A reversed CT reads negative power on that phase. On a balanced three-phase load, the total reads about one third of the true value.
- A CT connected against the wrong voltage phase gives a power factor that does not suit the load. On a resistive load the power factor reads about 0.5, and the power reads about half the true value with a negative sign.
Check the sign and the power factor of each phase against the load, and compare each current with a clamp meter reading.
Safety
A voltage-output CT has its burden inside the housing. An open output lead therefore does not produce the high voltage of an open current secondary, provided that the internal burden is intact. Follow the wiring instructions for the exact sensor.
Voltage rating
The CT insulation must suit the system voltage and the position in the installation. IEC 61010-1 places measurements at the service entrance, ahead of the main overcurrent device, in CAT IV, and measurements in distribution boards and fixed installations in CAT III (Continental Control Systems). A CT around a bare busbar depends on its own insulation. Check its rated insulation voltage against the busbar voltage.
CT selection checklist
- Confirm which sensor inputs the meter accepts: 1 A, 5 A, 333 mV or Rogowski.
- Record the lowest, normal and peak current of each circuit.
- Choose the ratio and class so that the lowest current you need to measure is inside a specified limit, and the peak is inside 120% or the extended current rating.
- Check the short-time thermal current against the fault level.
- Measure the finished conductor diameter or busbar size, and check the aperture and the space in the panel.
- For a current-output CT, add the meter burden and the lead loop in VA, and keep the total between 25% and 100% of the rated output.
- Check the insulation rating and the measurement category for the position of the CT.
- At commissioning, check the polarity and the phase of each CT against the readings.
CTs for EpiSensor monitors
A ZEM electricity monitor ships with its current sensors connected and calibrated against the meter. Its Class 0.5S to IEC 62053-22 therefore covers the meter and the sensors together, and there is no CT ratio, burden or class to match on site. You choose the sensor option: split-core CTs with a 14 mm aperture for circuits up to 120 A, or Rogowski coils for 300 A, 1000 A and 3000 A. The ZEM integrates the Rogowski signal itself, so there is no separate integrator.
Current transformer selection builds a generic selection brief from the current range, insulated conductor dimensions, installation allowance and meter input. You can check a candidate sensor against that brief. For this 14 mm cable, a 2 mm allowance on each side requires an 18 mm round opening:
The optional EpiSensor matching section compares the brief with published ZEM or ZDR options. It does not approve installation access, accuracy at low current or a third-party sensor connection. The polarity and phase checks above still apply at commissioning. If you are not sure which option fits, talk to an engineer and give the circuit's current profile and conductor dimensions.
Third-party wireless current sensors and clamp meters are in the current transformers section of the Device Directory, each with its documented readings and how it connects to Edge.