Plan three-phase generator capacity for commercial and industrial loads, including imbalance and motor starting. This guide supports early planning; the final generator and electrical installation should be checked against the actual equipment, site conditions and duty.
Confirm the distribution voltage
Use the actual line-to-line voltage and frequency. UK low-voltage projects commonly use 400 V and 50 Hz, but the selected set and site design govern.
Balance the phases
Record current on L1, L2 and L3. Significant imbalance can limit usable capacity even when the arithmetic total appears acceptable.
Sequence large loads
Staging chillers, pumps and production equipment can reduce the largest load step and improve voltage and frequency recovery.
Coordinate protection
The generator fault contribution differs from the utility supply. Protection settings, breaker operation and discrimination need to be checked for generator mode.
Check neutral and harmonics
Triplen harmonics from electronic loads can add in the neutral. The alternator, neutral conductor and protective arrangements must suit the load profile.
Create a phase-by-phase schedule
Record each single-phase circuit on its intended phase and each three-phase load once. The highest phase current can become the limiting value even when total kW looks acceptable. Where circuits can be redistributed, improve the balance before increasing generator size. Allow for the actual neutral current, particularly with electronic loads.
Account for transient behaviour
A three-phase motor can demand high current while developing starting torque. Transformers may draw magnetising inrush, and UPS rectifiers can impose a rapid step after transfer. Establish which event produces the largest voltage or frequency disturbance and whether controls can reconnect loads in stages. Manufacturer transient-performance tools are valuable for critical projects.
Design for generator-mode protection
Utility fault current is often much higher than generator fault current. A downstream device that trips promptly on mains may respond differently on the generator. Review protection discrimination, earth-fault paths, neutral switching and earthing arrangements in both operating modes. Commissioning should prove phase rotation, voltage, frequency, transfer logic and protective operation.
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.