Generator Current Calculator
Calculate generator output current from a kVA or kW rating, or estimate the generator size needed for a known load current. This calculator covers single-phase and three-phase generator amps, power factor, voltage, and reserve capacity.
Generator current formulas
A generator current calculator helps estimate the amps available from a generator rating, or the generator size needed for a load. The important inputs are rating, voltage, phase, and power factor.
Worked example: 50 kVA generator at 400 V three-phase
The generator may be rated for about 72 amps per phase, but many real installations avoid running continuously at the absolute maximum rating.
Common generator amperage table
| Generator | 230 V 1φ | 400 V 3φ | 480 V 3φ |
|---|---|---|---|
| 10 kVA | 43.48 A | 14.43 A | 12.03 A |
| 25 kVA | 108.70 A | 36.08 A | 30.07 A |
| 50 kVA | 217.39 A | 72.17 A | 60.14 A |
| 100 kVA | 434.78 A | 144.34 A | 120.28 A |
Do not forget starting current
Motors, pumps, compressors, refrigerators, air conditioners, and some welders can draw much higher current when starting than when running. A generator that can handle the running amps may still struggle if multiple motor loads start at the same time. For practical generator sizing, consider starting current, load sequence, altitude, temperature, and manufacturer derating.
Common questions
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For single-phase generators, amps = kVA × 1000 ÷ volts. For three-phase generators, amps = kVA × 1000 ÷ (√3 × volts).
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At 400 V three-phase, a 50 kVA generator produces about 72.17 amps per phase. At 230 V single-phase, it produces about 217.39 amps.
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A 25 kVA generator at 400 V three-phase produces about 36.08 amps per phase.
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First convert kW to kVA using kVA = kW ÷ power factor, then convert kVA to amps using the generator voltage and phase.
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Generators are rated in kVA because the alternator capacity depends on voltage and current. Real kW output depends on load power factor.
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Many installations avoid continuous operation at the maximum rating. A common planning approach is to keep continuous load below the full rating, such as 70% to 80%, but always follow the generator manufacturer data.
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Yes. Motors and compressors can draw several times their running current during startup. This can require a larger generator even when the running load seems acceptable.
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It can estimate current and apparent power, but complete generator sizing also needs starting loads, load diversity, future capacity, altitude, temperature, fuel type, and local standards.
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A = kVA × 1000 ÷ (√3 × line-to-line volts).
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If the generator rating is given in kW, use the rated power factor or the expected load power factor. A common rough value is 0.8, but the nameplate value is better.