Emergency Generator Sizing

The PG1–PG3 method: running load, voltage dip on motor starting, and instantaneous engine load.

1. What this check does

Takes the loads a generator must carry, including motors and how they start, and computes three required capacities. PG1 covers the running load; PG2 keeps the terminal voltage dip within the allowed value when the motor with the largest starting kVA starts; PG3 keeps the engine within its short-time overload while that start happens with everything else running. The generator must meet the largest of the three.

2. Inputs used

Each load: kW (shaft output for motors, input for other loads), quantity, demand factor, efficiency, power factor.
Motors: starting method (across-the-line, star-delta, autotransformer tap, soft starter or VFD current limit), locked-rotor multiple and starting power factor.
Generator: subtransient reactance Xd'', allowable transient voltage dip, engine short-time overload capability K, rated power factor.

3. Formulas and assumptions

Input kW P = Σ kW · qty · demand / η kVAR Q = Σ P_i · tan(acos PF_i) PG1 = max( √(P² + Q²), P / cosθG ) Starting kVA Q_L = k_start · kW / (η · PF) for one unit of the motor k_start = LR (DOL), LR/3 (star-delta), LR·tap² (autotransformer), current limit (soft starter, VFD) PG2 = Q_L · Xd'' · (1 − ΔV) / ΔV (from ΔV = Xd''·q / (1 + Xd''·q), q = Q_L / PG) PG3 = ( P − P_run,m + Q_L,m · cosθs ) / (K · cosθG) m = the motor whose start adds the most kW Fire pump = Q_L,fp · Xd'' · 0.85 / 0.15 (dip ≤ 15%, when a fire pump is marked) Required kVA = max(PG1, PG2, PG3, fire pump); kW = kVA · cosθG → next listed rating

One motor starts at a time, with all other loads already running (worst case for PG3).
The starting load is treated as a sudden impedance on the subtransient reactance; voltage regulator and governor response are not modelled, which is conservative for the dip.
The listed generator ratings are common standby kW sizes; manufacturers differ, so enter your own list if needed.
Xd″, allowable dip and K are entered by you from the generator and load data. No public table gives Xd″ by rating; typical standard alternators fall around 15–25%, and UFC 3-540-01 D-2.3 limits Xd″ to 0.12 when non-linear loads are 25% or more of the load.
At the selected rating the tool reports the starting dip and the running loading; UFC 3-540-01 H-3.2.3.1 asks for at least 35% of rated kW (without the fire pump) to avoid wet stacking.

4. What is NOT checked

Altitude and ambient temperature derating.
Step-load sequencing and frequency dip.
Harmonic heating from non-linear loads (flagged, not computed).
Fuel, run time and paralleling controls.

5. Reference standards

Pointers only; consult the edition adopted by your authority having jurisdiction.

NFPA 110 — emergency and standby power systems
IEEE Std 446 — emergency and standby power (Orange Book)
Korean building electrical design criteria — PG1–PG3 generator capacity method
NEC 2023 Articles 700, 701, 702 (context); Article 695 (fire pumps)
UFC 3-540-01 Engine-Driven Generator Systems for Prime and Standby Power (Change 3, 2023) — U.S. DoD, approved for public release

6. Validation cases

Hand-calculated cases run on every change. See Validation.

7. Known issues and changes

See the Changelog. Try the tool: Emergency Generator Sizing.