Q: How do you calculate transformer fault current?

The prospective fault current at the secondary is If = full-load current ÷ (Z% ÷ 100) , where Z% is the transformer impedance (typically 4–6%). For example, a transformer with 909 A full-load current and 5% impedance gives about 18 kA. Protective devices must have a breaking capacity above this.

Question 1

How do you size a distribution transformer?

Accepted Answer

Take the maximum demand (connected load × diversity), then apply the 125% rule — the transformer should be rated at least 1.25× the demand — and round up to the nearest standard kVA size (50, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000 kVA…). This leaves headroom for growth and inrush.

Question 2

How do you calculate transformer fault current?

Accepted Answer

The prospective fault current at the secondary is If = full-load current ÷ (Z% ÷ 100), where Z% is the transformer impedance (typically 4–6%). For example, a transformer with 909 A full-load current and 5% impedance gives about 18 kA. Protective devices must have a breaking capacity above this.

Question 3

What is the 125% rule for transformers?

Accepted Answer

Continuous loads should not exceed 80% of a transformer's rating, which is the same as sizing the transformer to at least 125% of the maximum demand. It prevents continuous full-load operation and allows for load growth and harmonics.

Question 4

How do you size a busbar?

Accepted Answer

Size the busbar so its rated current (after ambient temperature derating) is at least the transformer secondary full-load current. Copper busbar ratings depend on the cross-section and cooling — e.g. a 50×10 mm bar carries around 900 A at 35 °C. Then check the volt drop over the busbar run.

Transformer & Busbar Sizing Calculator