How to Choose a Current Transformer (CT)?

Lucas Moreau Electrical Engineer · Low-Voltage Installation Specialist · 15 Years' Experience

At a Glance

To choose the right current transformer (CT), follow these 5 key criteria: a transformation ratio (Ip/Is) immediately above the maximum current to be measured, a 5 A secondary if the distance to the meter is under 10 m (1 A beyond), a VA rating higher than the total burden of the measurement chain, an accuracy class suited to the application (0.2S–0.5 for tariff metering, 1 for industrial use, 5P/10P for protection), and a physical form factor compatible with your installation (wound, cable window or busbar type).

Why Is Current Transformer Selection Critical?

A current transformer (CT) is a fundamental component in any electrical measurement or protection chain. It converts a high primary current into a proportional, lower secondary current that can be safely used by meters, energy counters and protection relays.

A poorly sized CT leads to direct consequences:

Measurement errors that can distort billing or energy monitoring data.
Nuisance tripping or failure to trip of protection devices.
Core saturation rendering readings unusable above a certain threshold.
Regulatory non-compliance for tariff metering (MID directive, utility requirements).

Key takeaway A CT works by electromagnetic induction: the primary current creates a magnetic field that induces a proportional current in the secondary winding. The secondary circuit of a CT must never be left open while energised — this would cause a dangerous overvoltage that can destroy the device and injure the operator.

Step 1 — Determine the Primary Rated Current (Ip)

1Identify the maximum current you need to measure on your installation. The transformation ratio must be the next standard rating above this current.

Practical Example

Maximum measured current on the feeder: 1,103 A. Available standard ratios are 1000/5 and 1250/5. Choose the 1250/5 CT since 1250 > 1103.

Common Standard Transformation Ratios
Measured Current (A)	Recommended CT Ratio
0 – 40	50/5 (wound CT recommended)
40 – 75	75/5 or 100/5
75 – 100	100/5
100 – 150	150/5 or 200/5
150 – 250	250/5
250 – 400	400/5
400 – 600	600/5
600 – 800	800/5
800 – 1,200	1250/5
1,200 – 1,600	1600/5

Key takeaway Never undersize the ratio: a CT that is too small will saturate and produce inaccurate readings. Always select the next standard rating above the actual maximum current.

Step 2 — Choose the Secondary Current: 1 A or 5 A

2The CT secondary feeds the measuring or protection instruments. Two standards exist:

Comparison: 1 A vs 5 A Secondary
Criterion	5 A Secondary	1 A Secondary
CT-to-instrument distance	≤ 10 metres	> 10 metres
Cable losses	Higher (I² × R)	25 times lower
Cable cross-section	Larger required	Smaller sufficient
Typical use	Sub-distribution boards, close proximity	Main switchboards, long distances, substations
Compatibility	Most instruments	Check instrument compatibility

Key takeaway When in doubt, a 5 A secondary is the default choice for short-distance installations. For main switchboards or distances > 10 m, the 1 A secondary avoids significant losses and reduces the required cable cross-section.

Step 3 — Calculate the Required Burden (VA)

3The CT's VA rating must cover the total burden of everything connected to the secondary:

Instrument burden (meter, relay, ammeter…) — check their datasheets.
Cable burden: P_cable = R × I² (where R depends on cross-section and length).

Calculation Example

Energy meter: 0.5 VA + Protection relay: 1 VA + Cabling (15 m in 2.5 mm², Is = 5 A): ≈ 2.7 VA → Total = 4.2 VA → Choice: 5 VA CT (next standard rating).

Key takeaway Common standard VA ratings are: 1 – 2.5 – 5 – 10 – 15 – 30 VA. Always choose the next value above the calculated total. An oversized VA rating is not a problem; an undersized one degrades accuracy.

Step 4 — Select the Accuracy Class

4The accuracy class defines the maximum allowable error. It must match your application:

Current Transformer Accuracy Classes
Class	Max. Error	Application
0.1	± 0.1%	Calibration, metrology laboratory
0.2 / 0.2S	± 0.2%	High-precision tariff metering
0.5 / 0.5S	± 0.5%	Tariff metering, MID-certified sub-metering
1	± 1%	Industrial sub-metering, energy monitoring
3	± 3%	Indicative display, simple monitoring
5P / 10P	± 5% / ± 10%	Protection (overcurrent relays, earth fault)

⚠️ Accuracy at Low Loads

The smaller the CT ratio relative to the actual current being measured, the less accurate the reading. For reliable measurements with digital instruments, prefer a slightly higher ratio. Example: to measure 40 A, a 100/5 CT will be more accurate than a 50/5 at low load.

Key takeaway Classes with the "S" suffix (0.2S, 0.5S) guarantee extended accuracy even at low loads (down to 1% of rated current). They are recommended for tariff metering where consumption varies significantly.

Step 5 — Choose the CT Type for Your Installation

5The physical form factor depends on the current to be measured, the conductor type and the available space:

Current Transformer Types
CT Type	Current Range	Conductor	Advantages
Wound (closed core)	≤ 40 A	Cable passed through CT	Very accurate, compact, affordable
Cable window	50 – 4,000 A	Cable or busbar passed through	Easy installation, wide range
Busbar type	100 – 6,000 A	Through busbar	Robust, suited for high currents
Split-core (opening)	50 – 4,000 A	Cable or busbar	Retrofit without shutdown, maintenance
Rogowski coil	1 – 10,000 A	Cable or busbar	Flexible, lightweight, no saturation

Key takeaway Between 40 and 150 A, cable window and busbar CTs may have reduced performance. Check the model's datasheet. For retrofits (adding measurement to an existing installation without shutdown), split-core CTs are essential.

Special Case: Sizing a CT for a Motor Circuit

Electric motors draw a starting current (Is) far higher than their rated current (In), typically 5 to 8 times In. To prevent CT saturation during transient phases:

Motor sizing rule:

CT primary Ip = I_starting / 2

Example

Motor with In = 85 A and I_starting = 510 A (factor of 6). Choose Ip = 510 / 2 = 255 A → 300/5 CT (next standard rating).

Key takeaway This "I_starting/2" rule is a trade-off between steady-state accuracy and the ability to withstand inrush currents during start-up without saturating the CT's magnetic core.

Standards and Safety for Current Transformers

Current transformers must comply with applicable international standards:

IEC 61869-1: general requirements for instrument transformers.
IEC 61869-2: specific requirements for current transformers.
EN 50463: for railway applications.
MID Directive (2014/32/EU): for regulated tariff metering.

⚠️ Critical Safety Warning

The secondary of an energised CT must never be left open-circuited. Before any work, short-circuit the secondary terminals or use a dedicated shorting terminal block. An open secondary generates dangerous voltages that can reach several kilovolts.

Choose recognised and certified manufacturers: Schneider Electric, Socomec, CIRCUTOR, Chauvin Arnoux, IME, Eastron or WAGO for DIN-rail solutions.

Summary Table: Choose Your CT in 6 Criteria

CT Selection Criteria Overview
Criterion	Recommendation	Common Mistake
Primary current (Ip)	Next standard ratio above maximum current	Choosing a ratio too close → saturation
Secondary current (Is)	5 A if distance < 10 m, 1 A if > 10 m	5 A over long distance → excessive losses
Burden (VA)	≥ total burden (instruments + cabling)	Forgetting cable losses → undersizing
Accuracy class	0.5S for metering, 1 for industrial, 5P for protection	Class too low for tariff metering
Physical type	Wound < 40 A, cable/busbar > 150 A, split-core for retrofit	Closed core for retrofit → shutdown required
Motor circuit	Ip = I_starting / 2	Sizing on In → saturation at start-up

Frequently Asked Questions — Current Transformers

What is a current transformer and what is it used for?

A current transformer (CT) is a device that converts a high electrical current into a proportional lower current (typically 1 A or 5 A). It allows measurement, metering or protection of an electrical circuit without interrupting the main conductor or exposing measuring instruments to high currents.

Why must the secondary of a CT never be left open while energised?

When the secondary is open while the primary carries current, all the magnetic energy concentrates and generates a very high voltage (several kilovolts) across the secondary terminals. This can destroy the CT's insulation, cause an arc flash, and pose a lethal danger to the operator. Always short-circuit the secondary before disconnecting any instrument.

What is the difference between a measuring CT and a protection CT?

A measuring CT (classes 0.1 to 3) is optimised to provide accurate readings within the normal operating range (5% to 120% of rated current). A protection CT (classes 5P, 10P) is designed to faithfully transmit high fault currents (up to 20 times In depending on the accuracy limit factor) so that protection relays can trip correctly.

When should I use a split-core CT instead of a solid-core CT?

A split-core CT is essential for retrofits, i.e. when adding measurement to an existing installation without being able to shut down the supply or disconnect cables. It clips around the conductor without interrupting service. However, a solid-core CT offers better accuracy and lower cost: prefer it for new installations.

How do I choose between a 1 A and 5 A secondary?

The choice depends on the distance between the CT and the measuring instrument. Below 10 metres, a 5 A secondary works perfectly and is compatible with most instruments. Beyond 10 metres, a 1 A secondary reduces cable losses by a factor of 25 (I² × R law) and allows the use of smaller cable cross-sections.

What is an "S" accuracy class (0.2S, 0.5S)?

The "S" suffix (for "Special") indicates that the CT maintains its accuracy over an extended range, including at low loads (from 1% to 120% of rated current, compared to 5% to 120% for standard classes). S-classes are recommended for tariff metering where consumption varies between low-load and high-load periods.

What is a Rogowski coil and when should I use one?

A Rogowski coil is a flexible current sensor without a magnetic core that clips around a conductor. Unlike conventional CTs, it never saturates and can measure very high currents (up to tens of kA). It is particularly suited for tight spaces, large busbars and transient measurements. Its output signal requires an electronic integrator.

Key Takeaways

Choose a transformation ratio (Ip/Is) where the primary is the next standard rating above the maximum measured current.
Use a 5 A secondary for short distances (< 10 m) and 1 A for long distances (> 10 m).
Calculate the total burden (instruments + cabling) and select the next standard VA rating above it.
Match the accuracy class to the application: 0.5S for tariff metering, 1 for industrial monitoring, 5P/10P for protection.
Select the physical type based on your installation: wound (< 40 A), cable window/busbar (> 150 A), split-core for retrofits.
For motor circuits, size on Ip = I_starting / 2 to avoid saturation.
Never leave the secondary open while energised — always short-circuit before working on the circuit.