If you've ever stared at a current transformer datasheet and wondered what "0.5S", "15VA" or "36 mm bore" actually mean for your project, you're not alone. Current transformers (CTs) look simple — a ring of laminated steel with a coil wound around it — but choosing the wrong one can quietly wreck the accuracy of an energy bill, trip a protective relay for no reason, or simply not fit around the cable you need to measure.

This guide walks through everything you need to check before you buy a measurement CT for a UK installation: accuracy class, current ratio, burden, window/bore size, mounting type, secondary wiring, and which devices you can actually connect to it.

A typical panel/cable-mount instrument current transformer, rated 150/5A, class 0.5s. (Image: pelek.uk)

What a current transformer actually does

A CT is a step-down transformer built for one job: it lets a meter, ammeter or protective relay "see" a large current safely, by producing a small, proportional current in its secondary winding — almost always 5A or 1A at rated primary current. Instead of wiring your meter directly into a 400A distribution busbar, you pass that busbar (or a cable carrying the same current) through the CT's window, and the CT delivers a scaled-down, isolated signal the meter can read.

Because the CT sits in series with the circuit being measured (electrically, via the magnetic core rather than a direct connection), it adds negligible load to the primary system while giving you full electrical isolation between the high-current circuit and your low-voltage metering equipment.

The parameters that matter when selecting a CT

1. Primary and secondary current (the ratio)

This is written as, for example, 150/5A or 400/5A — primary rated current over secondary rated current. Standard secondary currents in the UK/Europe are 5A (most common) or 1A (used where cable runs to the meter are long, since a 1A secondary reduces line losses).

For best accuracy, size the primary rating close to your actual expected load — ideally so normal operating current sits between roughly 25% and 100% of the CT's rated primary current. A CT that's wildly oversized for the load it measures (e.g. a 1000/5A CT reading a 40A circuit) will read that current in the least accurate part of its range.

2. Accuracy class

In the case of metering CTs, accuracy class is typically: 0.1 - 0.2 - 0.2S - 0.5 - 0.5S - 1 - 3 - 5. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The product standard relating to current transformers is IEC 61869-2.

This standard requires an accuracy ratio equivalent only to the accuracy class from:

• 100% to 120% In for classes 0.1 to 1 (e.g. 0.5% for a 0.5 class)
• 50% to 120% In for classes 3 and 5.
• 20% to 120% In for classes 0.2s and 0,5s.

The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor. In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT.

Accuracy class	Ratio error ±% at 1% current	Ratio error ±% at 5% current	Ratio error ±% at 20% current	Ratio error ±% at 100% current	Ratio error ±% at 120% current
0.1		0.4	0.2	0.1	0.1
0.2		0.75	0.35	0.2	0.2
0.5		1.5	0.75	0.5	0.5
1		3	1	1	1
0.2S	0.75	0.35	0.2	0.2	0.2
0.5S	1.5	0.75	0.5	0.5	0.5

 

Accuracy class	Ratio error ±% at 50% current	Ratio error ±% at 120% current
3	3.0	3.0
5	5.0	5.0

3. Burden (VA rating)

Burden is the electrical load the CT's secondary winding is designed to drive — expressed in VA. For the measurement instrument or protection relay operating via a transformer, in order to operate them, the primary current has to induce the power required in the secondary current of the instrument or relay. This induced power must be equal or higher than the losses in the power line + consumption of the measuring instrument or protection relay. The burden imposed on a current transformer consist mainly of:

• The impedance of lead wire between current transformer and instruments/relay
• The impedance of the instrument / relay
• The sum of the above constitute the external burden required

If the real burden in your installation (long cable runs, multiple instruments in series, etc.) exceeds the CT's rated burden, accuracy suffers.

The power consumption of metering device and cable connection should be between the rated power output (rated burden) and ¼ the rated power output of the current transformer. Then the secondary current will be inside the given precision class of the CT.

for SCTr > 2,5VA SCTr >SCable + SMeter>¼ SCTr
for SCTr ≤ 2,5 VA SCTr >SCable + SMeter >½ SCTr

SCTr = Rated power output of the CT (rated burden)
SCable = Power consumption of the cable connection
SMeter = Power consumption of the metering device

Burden Presented By Copper Leads (PW):

PW = (I²*2*L) / (ACU*56)
PW = Burden presented by copper leads in VA
I = Amperage
L = Length of leads in m
ACU= Cross section in mm²

	Cross section 1.5 mm²…..10 mm2
Return leads (m)	At 1A secondary current			At 5A secondary current
	2.5 mm²	4 mm²	6 mm²	1.5 mm²	2.5 mm²	4 mm²	6 mm²	10 mm²
1	0.01			0.6	0.36	0.22	0.15	0.09
2	0.03			1.19	0.71	0.45	0.30	0.18
3	0.04				1.07	0.67	0.45
4	0.06			2.38	1.43	0.89	0.60	0.54
5	0.07				1.78	1.12	0.74
6	0.09			3.57	2.14	1.34	0.89	0.54
7	0.10				2.50	1.56	1.04
8	0.11			4.76	2.86	1.79	1.19	0.71
9	0.13				3.21	2.01	1.34
10	0.14	0.09	0.09	5.95	3.57	2.24	1.49	0.89
20	0.29	0.18	0.12		7.10	4.50	3.00
30	0.43	0.27	0.18		10.7	6.70	4.50
40	0.57	0.36	0.24		14.3	8.90	6.00
50	0.72	0.45	0.30		17.8	11.2	7.40
60	0.86	0.54	0.36			13.4	8.90
70	1.00	0.63	0.42			15.6	10.4
80	1.14	0.71	0.48			17.9	11.9
90	1.29	0.80	0.54			20.1	13.4
100	1.43	0.89	0.60			22.4	14.9

 

Burden Presented By Instruments And Relay:

Device Category	Device	Current Path VA
Ammeters	Moving iron	0.7........1.2
Ammeters	Moving coil with rectifier	0.001.....0.25
Ammeters	Moving coil with bimetal	3.5
Ammeters	Recorder	0.4.....9
Wattmeters	Indicating	0.2.....5
Wattmeters	Recording	0.01.....12
Wattmeters	Power factor meters	2....5
Wattmeters	kWh meters	0.4....1
Wattmeters	Reactive power Regulators	2.....3.5
Relays	Overcurrent, electronic	0.25
Relays	Overcurrent, others	2.....25
Relays	Time-lag overcurrent, thermal	6.....11
Relays	Distance, static	<1
Relays	Distance, other	2.....30
Relays	Directional, others	<1
Relays	Directional, static	1......5
Relays	Earth fault, static	<0.06
Relays	Earth fault, others	0.001.....3.5

 

4. Window / bore size and mounting type

This determines what the CT physically fits over or how it's fixed in a panel:

• Window-type / bar-type CT — a fixed, closed core with a bore (e.g. Ø 30 mm, Ø 36 mm) that a cable or busbar is threaded through. Common for new panel builds.
• Split-core CT — the core opens (usually hinged) so it can be clipped around a cable without disconnecting it. Ideal for retrofits and sub-metering projects where the circuit can't be interrupted.
• DIN rail mount — CT clips onto standard 35 mm DIN rail inside a distribution board, often with a bore adapter.
• Panel / busbar mount — bolts to a mounting plate or fits directly over a busbar of a given cross-section (check both round-cable bore and rectangular busbar dimensions on the datasheet).

A split-core CT, which opens to clip around an existing cable without disconnecting the circuit. (Image: pelek.uk)

Always check the bore diameter against the outer diameter of the insulated cable  (not the cross section) or busbar dimensions it needs to pass.

 

Accuracy classes explained — and what to use them for

UK and European CTs are tested to BS EN 61869-2 (which replaced the older BS 3938 / BS 7626). The accuracy class number is the maximum current error (%) permitted at rated current. Classes with an "S" suffix are extended-range classes, tested down to 1% of rated current, and are what utilities and MID-compliant billing meters require.

 

Accuracy class	Typical error at rated current	Best suited for
0.1 / 0.2	±0.1% / ±0.2%	Reference-grade and laboratory measurement, calibration standards, high-value utility revenue metering
0.2S / 0.5S	±0.2% / ±0.5% (accurate across an extended 1–120% current range)	Billing-grade sub-metering, tenant/landlord recharging, embedded generation and export metering, MID-certified commercial meters
0.5	±0.5% at rated current	Energy monitoring, power quality analysers, general commercial sub-metering, multifunction panel meters
1	±1%	General-purpose panel ammeters/voltmeters, industrial control panels, non-billing monitoring and indication
3	±3% (index/indication class — no phase-angle guarantee)	Basic current indication only — simple panel ammeters, retrofit split-core CTs for rough load monitoring, alarms
5P / 10P	Defined by accuracy limit factor (ALF), not a simple % — designed to stay linear up to many times rated current	Protection CTs feeding overcurrent, earth-fault or differential relays and circuit breaker trip coils — not for accurate measurement

 

Rule of thumb: if money changes hands based on the reading (tenant billing, export/import metering, DNO revenue metering), specify 0.2s or 0.5s. If it's for internal energy dashboards or general monitoring, 0.5s, 0.5 or 1 is normally sufficient and considerably cheaper. If the CT is feeding a protective relay rather than a meter, you need a protection class, not a measurement class — the two are not interchangeable.

 

Applications and which class/setup typically suits each

• Tenant/sub-metering and service charge recharging — Class 0.5s or 0.2s, split-core for retrofit, paired with an MID-approved or billing-grade multifunction meter.
• Distribution board / switchboard monitoring — Class 0.5–1-0,5s , DIN rail or panel-mount CTs feeding a multifunction panel meter or power analyser.
• Solar PV / battery storage / generator export metering — Class 0.5s, matched to the inverter's or grid connection's rated current, for accurate import/export reconciliation.
• Motor control centres and industrial machinery — Class 1-0,5s CTs feeding panel ammeters for load indication; separate 5P/10P protection CTs for overload/short-circuit protection relays.
• Data centres and critical power (UPS, standby generators) — Class 0,5- 0.5s CTs feeding power quality analysers/BMS for load balancing and capacity monitoring.
• EV charge point installations — Class 0.5–1-0,5s CTs for dynamic load management (DLM) systems that throttle charging based on building demand.
• General indication and alarms — Class 1, Class 3  split-core CTs are the cheapest option where only a rough current reading or threshold alarm is needed.
  

Connecting the CT: wiring, burden and safety

The secondary circuit wiring between the CT and the connected instrument matters more than people expect, because the resistance of that wire is part of the total burden:

• Use adequately sized conductors. For short runs (a few metres inside the same panel) 1.5–2.5 mm² is usually fine; for longer runs to a remote meter, increase the cross-section (or use a 1A secondary CT) to keep the added resistance — and therefore the burden — within the CT's rated VA.
• Keep secondary leads twisted and short where possible to reduce induced noise, particularly in panels with variable-frequency drives or other switching equipment nearby.
• Never open-circuit a live CT secondary. Unlike a voltage transformer, an energised CT with an open secondary can develop a dangerous high voltage across the open terminals and overheat the core. Always fit and use shorting terminals or a test block if a meter needs to be disconnected while the primary is still live.
• Observe polarity. CTs have a marked P1/P2 (primary) and S1/S2 (secondary) polarity; getting this backwards will reverse the sign of power/energy readings on multifunction meters.

Secondary terminal block on an instrument CT — this is where the meter's current-input wiring connects. (Image: pelek.uk)

 

What can you connect a CT to?

• Panel ammeters — simple analogue or digital current indicators mounted on switchgear doors.
• Multifunction panel meters — display current, voltage, frequency, power factor, kWh and more from a single unit.
• Energy/kWh meters (sub-meters) — used for billing, service charge recharging or carbon reporting; pair with a CT rated for billing accuracy.
• Power quality analysers — log harmonics, sags, swells and demand profiles over time.
• Protective relays and circuit breaker trip units — require a protection-class CT (5P/10P), not a measurement class.
• PLC / BMS / SCADA systems — usually via a CT-to-4–20 mA or CT-to-0–10 V transducer, or a meter with a Modbus/RS-485 or pulse output feeding the building management system.
• Current transformer relays / current-sensing switches — trigger an alarm or output contact when current crosses a threshold (e.g. dry-run protection on a pump).

 

Quick selection checklist

1. What's the maximum expected current on the circuit? → sets the primary rating.
2. Does the reading feed billing, a dashboard, or a protection relay? → sets the accuracy class (0.2S/0.5S, 0.5–1-3, or 5P/10P respectively).
3. Can the circuit be interrupted for installation? → window/bar-type if yes, split-core if no.
4. What's the cable or busbar size that needs to pass through? → sets the bore/window dimensions.
5. How far is the meter from the CT, and what's its input burden? → sets the required VA rating and secondary cable size.
6. What device is it feeding? → confirms whether you need a measurement CT or a protection CT, and whether 5A or 1A secondary suits the run length.

Browse instrument current transformers

Principal Elektrik (pelek.uk) stocks a range of ONKA measurement current transformers covering ratios from 100/5A up to 4000/5A, in accuracy classes 0.5s, 1 and 3, with panel, DIN-rail, cable and split-core mounting options — each supplied with its own test report and BS EN 61869-2 compliance.

FOR SAFETY REASONS current transformers can pose a dangerous high voltage on secondary terminals, which can destroy the current transformer itself, if power is applied to the primary whilst secondary ( S1 and S2 ) is open circuited. Thus either the system shall not be powered (Active) while installing process, or the secondary terminal shall be short circuited.

NOTE: Different types of CT markings: P1 = K, P2 = L, S1 = k, S2 = l

INSTALLATION NOTES.
To ensure that the meter reads correctly install as follow:

1. While installing the current transformer; ensure that the polarity of that is in high importance. P1 should face MAINS and P2 should face LOADS.
2. Secondary connections (S1 and S2) should be made in correct way to avoid metering problems.
3. Ensure that the current transformer is installed around a single conductor. If it is installed with both live and neutral conductors passing through the aperture, then no current will be measured under normal working conditions.
4. Current Transformer cables should be kept as short as possible in order to minimise interference from electrical noise.

This article is a general guide and does not replace the specific installation instructions, wiring diagrams and safety guidance provided with your CT and metering equipment. If you're not confident working with live electrical panels, use a qualified electrician.