Size a 3 phase generator from running kW or line amps with a 0.8 power factor and 25% reserve preset, an optional motor-start check, or find amps from kVA.

Balanced three-phase planning comparison. Defaults are PF 0.8 and running reserve 25%; use applicable values. This is not a final generator selection.

3 Phase Generator Sizing Formula

The following equation estimates a running-load allowance before any motor-start comparison or rounding. It does not establish a final generator rating.

GS = (L / PF) × SF
  • Where GS is the running-load allowance (kVA)
  • L is the total load (kW)
  • PF is the power factor
  • SF is the entered reserve multiplier: 1 + reserve percent / 100 (default 1.25)

Divide running electrical input kW by PF, then multiply by the entered reserve factor. From line amps, running kVA = √3 × line-to-line RMS voltage × line current / 1000. The optional motor comparison assumes one motor starts at the entered current multiple while other loads run: other running kVA + motor running kVA × multiple. The largest motor must already be included in total running kW, and the tool uses the same running PF for it. The greater of running allowance and simplified start is rounded upward using selected comparison sizes; above 2500 kVA, rounding uses extrapolated 250 kVA steps. This does not verify equipment availability or starting capability.

What is a 3-Phase Generator Sizing Calculator?

Definition:

This calculator provides a planning load comparison. Actual selection must separately satisfy engine kW and alternator kVA, starting PF, load sequencing, acceptable voltage/frequency dips, site derating and duty rating. Starting kVA is not itself a required generator nameplate rating. Manufacturer transient-performance data is needed; see Cummins application manual T-030 and Caterpillar’s generator sizing guide.

How to Use the 3-Phase Generator Sizing Calculator?

Example Problem:

The following example shows the running-load comparison without a separate motor-start calculation.

Choose “Compare size from running kW” and “No separate start comparison.” Complete the visible fields; the tool does not use a missing-field selection rule.

Enter total running electrical input 100 kW, PF 0.8, reserve 25% and line-to-line voltage 480 V. Voltage determines the displayed current.

Calculate the running-load allowance using the formula above:

GS = (100 / 0.8) * 1.25

GS = 125 * 1.25

GS = 156.25 kVA. The next selected comparison size is 175 kVA. Neither value alone establishes a suitable generator.

FAQ

What factors affect the sizing of a 3-phase generator?

Running real and apparent load, starting PF and current, load sequence, engine and alternator transient response, allowable voltage/frequency dips, duty and ambient/altitude derating can affect selection.

Why is a safety factor used in the calculation?

Reserve adds a chosen allowance to running kVA. The default is 25%; it does not guarantee that motor starts, load steps or other operating limits will be met.

Can this calculator determine values other than generator size?

The three modes compare size from running kW, compare size from line amps, and calculate line current from nameplate kVA. Select a mode and complete its visible fields. At a specified kVA, PF affects displayed real power but not current. The 80% output is a comparison point, not a universal duty limit.