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How this is calculated#

The tool gives the full load currents of a transformer and its secondary fault current. It uses the rating and the impedance from the nameplate.

1. Inputs. You enter the rating in kVA and the number of phases. You also enter both voltages and the impedance in percent.
2. Primary full load current. On three phases, I = kVA × 1000 ÷ (√3 × primary voltage). On one phase, it divides by the voltage alone.
3. Secondary full load current. The tool applies the same formula at the secondary voltage.
4. Secondary fault current. It divides the secondary full load current by the impedance as a fraction. For 6 %, that means dividing by 0.06.

The tool reads no table. You enter the voltages and the impedance.

Worked example#

• 500 kVA, 11 kV to 400 V, 6 % impedance, three phase. The primary full load current is 26.24 A. The secondary full load current is 721.69 A. The secondary fault current is 12,028.13 A.

Limits#

The fault current assumes an infinite upstream source. A real source has impedance, so the true fault current is lower. Use the fault level calculator to include the source. The tool does not model the cable after the transformer. It does not size the protection. Impedance is a nameplate value with a tolerance.

Use the primary current to check the supply side device. Use the secondary current to check the main switch and the busbar. Take the impedance from the nameplate, because it has a tolerance.

See also: Transformer full load current, 1000 kVA at 11 kV, Transformer secondary fault current, 500 kVA and Prospective fault current at a board.