Power metering

CT Ratio & Burden Calculator (Metering & Protection)

Select a reference current-transformer ratio from the primary current, then add up the secondary burden from the connected meters and relays and the pilot leads, so the CT’s rated burden (VA) and class can be chosen. The exact accuracy class and saturation still come from the CT datasheet.

Primary current (A)

Secondary

Connected device burden (VA)

Lead length one-way (m)

Lead cross-section (mm²)

ⓘLead burden rises with the square of the secondary current, so a 1 A CT loses about 25× less in the leads than a 5 A CT — the usual choice for long runs.

Reference CT ratio

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Lead burden

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Total burden

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Select CT rated burden

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Ratio and burden together

A CT is specified by two things: its ratio (primary current to a 5 A or 1 A standard secondary) and its rated burden (the VA it can drive at rated secondary while holding its accuracy class). Load the CT beyond its rated burden and the accuracy collapses — and a protection CT may saturate and fail to operate the relay.

Lead burden VA = I_sec² × ρ × (2 × L) / S Total burden = device VA + lead burden VA Select CT: rated burden ≥ total burden

The leads count twice (go and return), and copper resistivity ρ ≈ 0.0175 Ω·mm²/m. The device VA is the sum of the connected meters, transducers, and relays at rated secondary. The tool rounds up to the next standard CT burden (2.5, 5, 10, 15, 30 VA).

Why 1 A for long runs

Because lead burden scales with the square of the secondary current, dropping from 5 A to 1 A cuts the lead loss to one twenty-fifth. For switchrooms with long pilot cables, a 1 A CT keeps the burden within a modest rating; for short local metering, 5 A is common and convenient.

⚠Metering and protection CTs are not interchangeable. A metering CT (class 0.2 / 0.5) is accurate around rated current and saturates early to protect instruments. A protection CT (class 5P / 10P with an ALF) must stay linear up to the fault current. Match the class to the job.

Frequently asked questions

What if my burden is well below the CT rating?

A lightly loaded metering CT is fine and stays accurate. A lightly loaded protection CT is also fine — but never exceed the rated burden, or the accuracy and saturation behaviour are no longer guaranteed.

Does the accuracy class come from this calculation?

No. The class (0.2, 0.5, 5P10, 10P20, and the accuracy limit factor) is a CT property from the datasheet. This tool sizes the burden the CT must be rated to drive; the class is chosen to suit metering or protection.

How do I pick the primary rating?

Choose a standard primary at or just above the maximum load current so the CT runs near its accurate range without saturating in normal service. This tool rounds up to the nearest standard CT primary.

Complete the metering scheme

With the ratio, burden, and class set, match the CT and the associated protection relay to the switchboard.

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Standards referencedIEC 61869-2 (current transformers) · standard secondary currents 5 A / 1 A · standard burdens 2.5–30 VA.

Estimate for guidance only. The final CT ratio, rated burden, accuracy class, and saturation performance must be confirmed by a qualified person against the CT datasheet and the metering or protection scheme.