Generator Sizing Calculator

Generator Sizing Guide for AS/NZS 3010:2014

Generator sizing is the process of selecting a genset with enough capacity to supply the connected electrical load while maintaining voltage and frequency within acceptable limits. In Australia, generator installations must comply with AS/NZS 3010:2014, which covers wiring requirements, protection, earthing, and fuel systems for generating sets. Whether you are specifying a standby generator for a commercial building, a prime power unit for a remote site, or a portable set for temporary supply on a construction project, the sizing process follows the same fundamental steps: calculate the total running load, account for the largest motor starting transient, apply derating factors for site conditions, and select the next standard genset size.

Key concepts

• Standby vs prime vs continuous rating. Standby rating is the maximum output available during a mains failure, typically limited to 200 to 500 hours per year. Prime rating is the maximum output for unlimited running hours at variable load. Continuous rating is for a constant load running 24/7 (base-load generation). Standby rating is typically 10 to 15 percent higher than prime for the same engine.
• Motor starting inrush. When the largest motor on the generator supply starts, the inrush current (typically 6 to 8 times full load current) causes a voltage dip proportional to the generator impedance. The generator must be large enough to hold the voltage dip below 15 to 20 percent, or sensitive equipment may malfunction or trip.
• Derating factors. Generators are derated for altitude (above 1000 m, typically 3.5 percent per 300 m), ambient temperature (above 40 degrees Celsius, typically 2 percent per degree), and fuel type (natural gas engines produce less power than diesel for the same displacement). All applicable factors must be applied to the nameplate rating before comparing to the site load.
• Fuel consumption estimation. Diesel generators consume approximately 0.27 litres per kWh at full load. At 75 percent load, which is the most efficient operating point, consumption drops to approximately 0.22 litres per kWh. These figures feed directly into fuel storage sizing and operational cost estimates.

Common scenarios

1. Commercial building standby power. A multi-storey office building with a 350 kW maximum demand and a 75 kW fire pump as the largest motor. The generator must supply 350 kW steady state while handling the fire pump inrush (approximately 225 kVA transient demand). After applying a 25 percent growth allowance, the calculated minimum is around 550 kVA standby. The next standard size up, typically 600 or 650 kVA, would be selected.
2. Remote mine site prime power. A processing plant at 1200 m altitude with 45 degree Celsius ambient temperature and a 1.2 MW base load. The altitude derating (approximately 7 percent) and temperature derating (approximately 10 percent) reduce the effective output of a 1500 kVA genset to around 1245 kVA. Two gensets running in parallel with load sharing may be required to provide both redundancy and the necessary capacity.
3. Temporary construction supply. A construction site requiring 100 kW for tower cranes, welders, and site sheds. The crane motor starting transient is the critical sizing factor. A 200 kVA portable genset is typically specified to handle both the running load and the crane starting inrush, with margin for additional temporary loads as the project progresses.