Generator current calculator
Full load and fault current of a generator from its kVA.
Try a job 5
Each job fills in example values. Replace them with your values.
- Check a 60 kVA standby generatorThe full load current and the fault current of a 60 kVA set at 400 V.
- Check a 200 kVA generatorThe full load current of a 200 kVA set at 400 V.
- Check a portable generatorThe current of a 10 kVA single‑phase set.
- Find a generator's fault currentThe fault current of a 500 kVA set at 15 % reactance.
- Check a holiday park backup setThe current and fault level of a 150 kVA set.
Inputs
A worked example with the default inputs.
- Rating
- 100 kVA
- Phases
- Three phase
- Voltage
- 400 V
- Power factor
- 0.8
- Subtransient reactance
- 15 %
Full load current
144.34A
- Real power
- 80kW
- Fault current
- 962.25A
This result is a design aid. Verify it against the current standard.
Calculation steps
- Full load currentResult144.34A
- Real power80kW
- Fault current962.25A
About this calculator
How this is calculated
The tool works from the kVA rating on the generator nameplate. It gives the full load current, the real power and the fault current at the terminals. The steps are plain circuit arithmetic. The tool reads no Standard table.
- Apparent power. You enter the rating in kVA. The tool converts it to VA.
- Full load current. On three phases the tool divides the VA by √3 times the line voltage. On one phase it divides by the voltage.
- Real power. It multiplies the kVA by the power factor you enter. The result is in kW.
- Fault current. It divides the full load current by the subtransient reactance, as a fraction. This is the first-cycle symmetrical value.
| Step | Basis | What it decides |
|---|---|---|
| Full load current | I = S / (√3 × V) | The current at rated output |
| Real power | P = S × power factor | The load the engine must drive |
| Fault current | I / X″d | The first-cycle fault level at the terminals |
Worked example
A 200 kVA set at 400 V on three phases has a power factor of 0.8. Its subtransient reactance is 15 %. The full load current is 288.68 A. The real power is 160 kW. The fault current is 1,924.5 A.
Limits
- The fault current is the initial symmetrical value only. It ignores the decay to the transient and synchronous values.
- It ignores any excitation limit. A few cycles in, the real current is much lower.
- The tool does not size the generator for motor starting or step loads.
- It does not size the cable or the protective device. Use the cable size calculator for the cable.
- The subtransient reactance comes from the maker data sheet. No default fits every set.
See also: Generator fault current from the subtransient reactance, Generator full load current from kVA and the fault level calculator.
Questions
The result is a design aid. Verify against the current standard.
What does the generator calculator do?
The tool reports the full load current, the real power output and the fault current of an alternator. It works from the rating in kilovolt amperes, the voltage, the phase count, the power factor and the subtransient reactance.
Which inputs matter most?
The rating and the voltage set the full load current. The power factor sets the real power in kilowatts. The subtransient reactance sets the fault current, which defaults to 15 % in the tool. A lower reactance gives a higher fault current.
What are the common gotchas?
The fault current is the initial subtransient value, taken as the full load current divided by the per unit reactance. Use the machine test sheet value when you have one. Cable impedance is not included. The inputs sit in the page address, so a link reproduces the result.
Worked examples
- Generator fault current calculation at 400 VA 500 kVA 400 V standby set at 12 % subtransient reactance gives 721.69 A full load and 6014.07 A fault current, with the decay the figure leaves out.
- What size generator for a dairy shedSize a standby set for a dairy shed from its load list. The vacuum pump's start sets the size, then staggered steps, a drive and a soft starter shrink it.
- Generator full load current: 200 kVA to ampsA 200 kVA 400 V standby set converted to 288.68 A of full load current, 160 kW of real power and 2,405.63 A of initial fault current from 12 % reactance.
- Generator sizing for a 150 kVA holiday parkA 150 kVA three‑phase standby set at 400 V gives 216.51 A full load current, 120 kW real power and 1443.38 A fault current from one nameplate.
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