1 A or 5 A CT Secondaries: Calculate the Long-Cable Burden

1. Calculate the complete secondary circuit

For a remote relay panel, cable burden can dominate the CT's external load. A 1 A secondary reduces resistive cable losses relative to 5 A at the same resistance, but choosing it requires compatible CTs, relay inputs and test equipment. Do the circuit calculation before buying larger CTs or copying an old panel specification. Schneider's application guidance explicitly includes the connected devices and secondary wiring. This guide addresses long-lead arithmetic; the instrument-transformer selection guide covers the broader selection process. Burden is not, by itself, proof of protection accuracy during a fault.

2. Establish the actual current path

Record the CT ratio and tap, nominal secondary current, relay burden at that current, conductor material and cross-section, one-way route length, terminals, test switches and intermediate devices. Obtain cable resistance at the relevant operating temperature rather than treating a room-temperature value as permanent. For a dedicated outgoing and return pair, use the full loop length. Shared returns, residual connections and multiple series devices need their actual circuit model. State whether each device burden is resistive or has a power factor; arithmetic addition of VA magnitudes is a screening approximation when the phase angles are unknown.

3. Worked cable comparison

Assume a dedicated copper pair with 150 m one-way length, 2.5 mm² conductors and illustrative resistivity 0.0175 Ω·mm²/m at 20 °C. The loop is 300 m and its resistance is 2.10 Ω. Assume a resistive 0.20 VA relay burden at either selected nominal input and 0.10 Ω additional connections.

At 1 A, cable burden is 2.10 VA and connection burden is 0.10 VA, giving 2.40 VA total. At 5 A, the same cable is 52.50 VA and connections are 2.50 VA, giving 55.20 VA total. These values assume nominal sinusoidal current and the stated temperature. They exclude CT internal winding resistance and do not quantify transient saturation.

4. Compare alternatives on the same basis

For the last row the cable resistance falls to 0.525 Ω, while connection burden remains 2.50 VA and relay burden 0.20 VA. The resistive cable advantage follows from current squared times resistance. Schneider confirms this relationship. Increasing cable area helps, but it does not reproduce the entire benefit of lower secondary current in this example.

5. Separate burden from fault performance

A protection CT review also needs maximum fault current, system asymmetry, remanence assumptions, CT excitation characteristics, internal resistance and relay-specific performance requirements. At high current, secondary voltage demand rises and saturation can distort the signal. The SEL CT-sizing paper explains the resulting effects on protection elements. A nominal VA total cannot be substituted for that assessment. For metering, verify the certified burden range of the actual accuracy class; arbitrarily reducing load may not match the certificate. IEC 61869-2 supplies the inductive-CT scope, not a public abstract-based permission to assign an accuracy class.

6. Make conversion a controlled procurement change

Specify matching secondary ratings at every input, the correct ratio settings, available CT taps, terminals and safe test facilities. Changing a setting does not convert a physical 5 A input into a 1 A input. Ask for nameplate and connection evidence, measured loop resistance, device burdens, updated schematics and a protection performance review. Demonstrate scaling from injected secondary current to displayed primary current and the intended operating logic. Avoid opening a CT secondary while primary current may flow; Schneider explains the shorting-block requirement. Review the independent neutral input in an REF scheme rather than assuming its leads match the phase circuits.

7. Common calculation questions

Is one-way cable length sufficient?

Only if the model explicitly accounts for the return elsewhere. A dedicated pair uses the outgoing and return paths.

Is 1 A always the better choice?

No. Existing hardware, certified metering burden, fault performance and maintenance compatibility can outweigh the cable advantage.

Can CT internal resistance be ignored?

It is excluded from the external burden table, but must be included where the manufacturer's excitation or protection calculation requires it.

8. Primary references

• Schneider Electric — CT burden
• SEL — CT sizing
• IEC 61869-2:2012
• Schneider Electric — CT