Work out the primary and secondary current of a transformer from its kVA rating and voltages, and the fault current at its terminals from its impedance.
Secondary current, each line
1,443A
Full load, three-phase.
Check the values to see a result.
Single-phase I = S × 1000 ÷ VThree-phase I = S × 1000 ÷ (√3 × VLL)Short-circuit Isc = IFL × 100 ÷ Z%
A 1,000 kVA 11 kV to 400 V three-phase transformer at full load carries 52.49 A on the primary and 1,443 A on the secondary, in each line. At 6% impedance it can deliver up to 24.06 kA into a fault at its terminals.
Tip: Metering CTs are sized on the load current per phase, not the transformer rating, unless the transformer runs near full load.
A transformer is rated in kVA, so its full-load current follows from the rating and the voltage alone: divide the kVA by √3 times the line-to-line voltage for a three-phase unit. A 1,000 kVA transformer carries 1,443 A in each line at 400 V and 52.49 A at 11 kV. The impedance on the nameplate then sets the largest current it can deliver into a fault.
I = S × 1000 ÷ (√3 × VLL)
S is the rating in kVA and VLL the line-to-line voltage of the winding. Work it out once for the primary and once for the secondary.
I = S × 1000 ÷ V
No √3: the kVA is carried by one winding at its own voltage.
I = IFL × loading % ÷ 100
A transformer at 60% of its kVA carries 60% of its full-load current. Power factor does not appear, because kVA already includes it.
Isc = IFL × 100 ÷ Z%
Z% is the impedance on the nameplate. The result assumes a source of unlimited strength upstream, so it is the most the transformer can deliver; cables and the network reduce the real figure.
At full load the secondary carries 1000 × 1000 ÷ (1.732 × 400) = 1,443 A in each line, and the 11 kV primary 52.49 A. With 6% impedance the fault current at the secondary terminals can reach 1,443 × 100 ÷ 6 = 24.06 kA, which is the level the main switchboard has to withstand.
Secondary current, each line 1,443 A Open in the calculator
A typical North American unit substation: 601.4 A on the 480 V side and 20.92 A on the 13.8 kV side. At 5% impedance the secondary fault current is up to 12.03 kA.
Secondary current, each line 601.4 A Open in the calculator
Loaded to 75% of 630 kVA, a 20 kV to 400 V transformer carries 682 A in each secondary line. For metering, size the CTs on this measured load rather than on the 909 A full-load rating if the transformer never runs near its nameplate.
Secondary current, each line 682 A Open in the calculator
Full-load line current for standard three-phase ratings at common voltages.
| Rating (kVA) | 400 V (A) | 480 V (A) | 11 kV (A) | 20 kV (A) |
|---|---|---|---|---|
| 50 | 72.2 | 60.1 | 2.6 | 1.4 |
| 100 | 144.3 | 120.3 | 5.2 | 2.9 |
| 160 | 230.9 | 192.5 | 8.4 | 4.6 |
| 200 | 288.7 | 240.6 | 10.5 | 5.8 |
| 250 | 360.8 | 300.7 | 13.1 | 7.2 |
| 315 | 454.7 | 378.9 | 16.5 | 9.1 |
| 400 | 577.4 | 481.1 | 21.0 | 11.5 |
| 500 | 721.7 | 601.4 | 26.2 | 14.4 |
| 630 | 909.3 | 757.8 | 33.1 | 18.2 |
| 800 | 1,154.7 | 962.3 | 42.0 | 23.1 |
| 1,000 | 1,443.4 | 1,202.8 | 52.5 | 28.9 |
| 1,250 | 1,804.2 | 1,503.5 | 65.6 | 36.1 |
| 1,600 | 2,309.4 | 1,924.5 | 84.0 | 46.2 |
| 2,000 | 2,886.8 | 2,405.6 | 105.0 | 57.7 |
| 2,500 | 3,608.4 | 3,007.0 | 131.2 | 72.2 |
For a three-phase transformer, divide the kVA by √3 times the voltage in kV: 1,000 kVA ÷ (1.732 × 0.4 kV) = 1,443 A. For single-phase, divide by the voltage in kV.
The rating is apparent power, which already accounts for the power factor of whatever load is connected. What heats the windings is current, and kVA sets it directly.
The figure on the nameplate or test certificate. Distribution transformers are commonly between 4% and 6%, with larger units at the upper end. A lower impedance means a higher fault current.
No, it is the upper limit. The upstream network and the cables between the transformer and the fault add impedance and reduce the current. A protection study uses the full network.
A ZEM on the transformer’s low-voltage side measures the real loading, with split-core CTs or flexible Rogowski coils for large conductors and busbars.
