Transformer calculator
Calculate transformer full load current and impedance.
Try a job 5
Each job fills in example values. Replace them with your values.
- Check a transformer secondaryThe secondary current and the fault level of a 1000 kVA unit.
- Find the currents of a 500 kVA transformerThe primary and secondary currents of a 500 kVA unit.
- Find fault level of a 300 kVA unitThe secondary fault current at 4 % impedance.
- Check an isolating transformerThe currents of a 5 kVA single‑phase unit.
- Check a 22 kV transformerThe primary current of a 1500 kVA, 22 kV unit.
Inputs
A worked example with the default inputs.
- Rating
- 500 kVA
- Phases
- Three phase
- Primary voltage
- 11,000 V
- Secondary voltage
- 400 V
- Impedance
- 6 %
Secondary full load current
721.69A
- Primary full load current
- 26.24A
- Secondary fault current
- 12,028.13A
This result is a design aid. Verify it against the current standard.
Calculation steps
- Primary full load current26.24A
- Secondary full load currentResult721.69A
- Secondary fault current12,028.13A
About this calculator
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.
- Inputs. You enter the rating in kVA and the number of phases. You also enter both voltages and the impedance in percent.
- Primary full load current. On three phases, I = kVA × 1000 ÷ (√3 × primary voltage). On one phase, it divides by the voltage alone.
- Secondary full load current. The tool applies the same formula at the secondary voltage.
- 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.
Questions
The result is a design aid. Verify against the current standard.
What does the transformer calculator do?
The tool reports the primary full load current, the secondary full load current and the secondary fault current. It works from the rating in kilovolt amperes, the primary and secondary voltages, the impedance percent and the phase count.
Which inputs matter most?
The rating and the two voltages set the two full load currents. The impedance percent sets the secondary fault current: a lower impedance gives a higher fault current. The phase count applies the square root of three to both currents.
What are the common gotchas?
The fault current assumes an infinite upstream source, so it is a worst case figure and the tool says so with every result. Cable and network impedance are not included. The impedance percent is limited to 30. The inputs sit in the page address, so a link reproduces the result.
Worked examples
Background
- Short-circuit withstand and the k factorThe adiabatic check in plain terms: prospective fault current, clearing time and k, worked on 25 mm² copper XLPE at 10 kA for 0.1 s.
- Arc flash basicsArcing current, incident energy, boundary and working distance explained, with three engine runs that show clearing time beating fault current every time.
Related tools
- Cable size calculatorFind the smallest cable size that passes each check.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Fault level calculatorProspective fault current at a board from its source.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Transformer inrush and its breakerCheck a transformer’s switch-on inrush against its breaker.AS/NZS 3000
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