Understand the relationship between generator kVA, voltage, phase and current before detailed electrical design. This guide supports early planning; the final generator and electrical installation should be checked against the actual equipment, site conditions and duty.
Three-phase current
For a balanced three-phase load, amps are calculated as kVA × 1,000 ÷ (√3 × line voltage). For example, 100 kVA is about 144 A at 400 V.
Single-phase current
For single phase, amps are calculated as kVA × 1,000 ÷ voltage. The result is therefore very different from a three-phase calculation at the same kVA.
Why the figure is only a guide
Cable capacity depends on installation method, ambient temperature, grouping and length. Breaker selection also depends on fault current, protection settings and discrimination.
Unbalanced loads
A three-phase set should not be selected from total current alone when one phase will carry materially more load. Provide a phase-by-phase schedule.
Non-linear equipment
UPS systems, variable-speed drives and electronic power supplies can introduce harmonics and affect alternator selection. Ask the equipment supplier for input-current and harmonic data.
From kVA to current
The familiar three-phase formula assumes a balanced load and uses line-to-line voltage. At 400 V, 20 kVA is about 29 A, 100 kVA about 144 A and 500 kVA about 722 A. If the system voltage changes, the current changes too. Do not copy a 415 V figure into a 400 V design or apply the three-phase formula to a single-phase circuit.
Why kW and amps answer different questions
kW describes real power consumed in doing work, while kVA includes the effect of power factor. Current is needed for conductors and protective devices, but generator engine power and alternator capacity must both be considered. A low power-factor or harmonic-rich load can use alternator capacity without using the equivalent share of engine kW.
Use final equipment data for design
A guide conversion helps establish scale. It does not replace the generator data sheet, cable calculations, voltage-drop assessment, fault study or protection settings. Long cable runs, ambient temperature, grouping and installation method can all change the conductor requirement. A competent electrical designer should coordinate the breaker, cable, earthing and downstream protection.
Information to send with an enquiry
Share the site location, required voltage and phase, intended prime or standby duty, expected running hours and a schedule of the equipment to be powered. Include motor starting methods, UPS or variable-speed-drive data, the largest load step, fuel-runtime target, noise constraints and the proposed installation position where known. Photographs, electrical drawings and measured load information help the team distinguish firm requirements from early assumptions. FW Power can then discuss an appropriate generator, control arrangement, delivery, installation, commissioning and ongoing service scope without treating a guide calculation as a finished design.