Generator Sizing Chart: kW, kVA and Amps at Common Voltages

Three-Phase kW to Amps Chart

Values assume a balanced three-phase load at a power factor of 0.8, the standard rating basis for most diesel generating sets. Amps are full-load current per phase, rounded.

Read across a row to see what a generator of a given kW delivers in kVA and current. Read down a voltage column to see why large sets are often connected at medium voltage: at 208 V the current of even a mid-size set becomes very large, demanding heavy cables and big breakers.

The Formulas Behind the Chart

• kVA = kW / power factor
• Three-phase amps = kW x 1000 / (1.732 x volts x power factor)
• Single-phase amps = kW x 1000 / (volts x power factor)
• kW = kVA x power factor

The figure 1.732 is the square root of three, which appears in every three-phase calculation. For example, a 250 kW set at 480 V and 0.8 power factor gives 250,000 divided by 665, or about 376 amps per phase.

From a Load List to a Generator Size

1. List every load that must run on generator power, with its running kW or amps and its voltage.
2. Separate motors and other inductive loads, and note their starting current, which can be several times running current.
3. Decide the starting sequence. Starting large motors one at a time, in steps, keeps the peak demand down.
4. Add up running load, then add the largest step’s starting demand to find the peak the generator must carry without excessive voltage dip.
5. Identify non-linear loads such as UPS systems, variable speed drives and LED lighting, which may need a larger alternator than the kW suggests.
6. Apply site derating for high ambient temperature and altitude, using the manufacturer’s published curves.
7. Add margin for future loads, and choose the rating type, standby or prime, that matches the duty.

Our generator size calculator runs these steps for you and gives a recommended kW and kVA.

Why kW and kVA Both Matter

kW is real power: the work that turns motors and lights lamps, and it determines how hard the engine works. kVA is apparent power, which includes the reactive current motors and transformers draw; it determines how hard the alternator and its cables work. A generator can be within its kW limit and still overloaded in kVA if the load’s power factor is poor. That is why proposals should state both, and why the alternator on a set feeding many motors or UPS systems is sometimes oversized compared with the engine.

Single-Phase kW to Amps

Small standby sets for shops, offices and homes are often single-phase. Single-phase loads have no square-root-of-three term, so current is higher for the same power. At a power factor of 1.0, typical for resistive loads such as heaters and lighting, the current is simply kW multiplied by 1000 and divided by the voltage.

Motor Starting and Voltage Dip

Motors draw far more current when they start than when they run, and that surge shows up as a voltage dip on the generator. A dip that is too deep can drop out contactors, trip drives and stop the transfer. Manufacturers publish motor starting performance for each alternator, usually as the starting kVA it can accept for a given dip.

Ways to reduce the size needed include starting motors in sequence, using soft starters or variable speed drives, and fitting a larger alternator on the same engine. Each option changes the chart reading, so treat the chart as the first step and the motor starting check as the second.

A Worked Example

Take a small commercial building at 480 V three-phase. Its essential loads are lighting and sockets at 40 kW, IT and controls at 25 kW behind a UPS, and two 15 kW pumps that start one after the other. Running load is 95 kW. The first pump to start adds a short burst of demand well above its running kW, so the generator must ride that step on top of the loads already running without a deep voltage dip.

Allowing for the motor starting step, the UPS input and a sensible margin for growth, a set in the region of 150 kW would be checked against the manufacturer’s motor starting data. At 480 V and 0.8 power factor, the chart shows about 225 amps for that size, which sets the breaker and cable sizes.

Common Sizing Mistakes

• Using nameplate amps of every device added together, which overstates load because not everything runs at once.
• Ignoring motor starting, which causes voltage dips, contactor dropout and failed transfers.
• Forgetting that a generator rated for standby duty is not meant to run as the everyday supply.
• Mixing single-phase and three-phase figures in one total without converting them.
• Leaving no margin, so the first new tenant or production line overloads the set.

For large single-set installations, our page on big emergency sets for campuses and plants covers layout, fuel and paralleling choices. USP&E sizes, supplies and commissions generating sets and power plants, with 150+ power projects in 45+ countries over 24+ years and ISO 9001 and ISO 45001 systems. Before you size for another country, check supply voltage and frequency by country.