CT Burden Formulas: Lead Resistance and VA

Current transformer burden formulas: lead resistance, burden in VA for 5 A and 1 A secondaries, total burden against rated burden, and primary current.

The formulas

Is is the rated secondary current (5 A or 1 A) and L the one-way length of the cable in metres. The total burden must not exceed the rated burden of the CT, or its accuracy falls and a protection CT may saturate. Use the burden of the relay or meter at the rated current from its data sheet. A 1 A secondary has a burden 25 times lower than a 5 A secondary for the same resistance.

Worked example: 50 m of 2.5 mm² copper to a relay

The relay burden is 2 VA at 5 A, and the CT is rated at 10 VA.

1. RL = 1.72 × 10⁻⁸ × 2 × 50 / (2.5 × 10⁻⁶) = 0.688 Ω
2. Rrelay = 2 / 5² = 0.08 Ω, so Rtotal = 0.768 Ω
3. With a 5 A secondary: S = 5² × 0.768 = 19.2 VA, which is above the 10 VA rating
4. With a 1 A secondary: S = 1² × 0.768 = 0.768 VA

Worked example: primary current

1. A 400/5 A CT reads 3.2 A, so Ip = 3.2 × 400 / 5 = 256 A

Lead burden for 2.5 mm² copper

Frequently asked questions

Why choose a 1 A secondary?

The lead burden is 25 times lower, which allows long cable runs to a remote relay with a small CT. The 5 A secondary is common for short runs in switchgear.

What happens if the burden is too high?

The CT loses accuracy for a meter, and for protection it can saturate at a fault current and give a distorted signal to the relay.

What if I open the secondary circuit?

Never open the secondary of a CT that carries primary current. The voltage can rise to dangerous values. Short the secondary first.

Is the lead resistance the cable resistance or twice it?

Twice the one-way value for a two-wire connection, because the current goes out and returns. Some star-connected schemes differ, so check the connection.

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