Choose the generator configuration that matches the electrical system and loads being backed up. Three-phase is not an upgrade you select simply because a building is commercial. A small office and a machine shop may need different supplies, even when their total kilowatt demand is similar.
What the phase labels mean
A typical 120/240-volt single-phase service provides 120 volts from either line to neutral and 240 volts between lines. It is often called split-phase. Having two line conductors does not make it a three-phase supply.
A three-phase supply has three alternating voltages displaced in time. Its connection arrangement determines the available line-to-line and line-to-neutral voltages. That is why the complete voltage designation matters when matching a generator, switch, and equipment.
| Supply arrangement | Line to neutral | Line to line |
|---|---|---|
| 120/240V single-phase, split-phase | 120V | 240V |
| 120/208V three-phase, wye | 120V | 208V |
| 277/480V three-phase, wye | 277V | 480V |
These are not all possible services. Delta systems require their own identification; do not apply the wye table to them. Equipment marked 208V, 230V, or 240V needs a review of its permitted input range rather than an assumption that those numbers mean the same thing.
A real product family comes in several configurations
Generac’s Protector RG04845 specification lists 120/240V single-phase and several three-phase configurations, including 208/120V and 480/277V. See specification sheet Rev. G, October 14, 2024, page 2. Those are model configurations to specify correctly; the table does not authorize an owner to reconnect an installed unit.
Why the same kilowatts can mean different amps
For a balanced three-phase load, current is approximately:
Amps = watts ÷ (1.732 × line-to-line volts × power factor).
For an illustrative 60kW electrical load at 0.80 power factor, that is about 208.2 amps at 208V or 90.2 amps at 480V. The power has not changed; the voltage and current relationship has. Cummins includes the underlying relationships in its application manual, useful formulas appendix.
This calculation does not size conductors, breakers, or a transfer switch. It assumes balanced loading and excludes motor-starting demand. It also does not mean a 480V generator can directly supply a 120V appliance. Any needed transformation and distribution must be designed.
What if the building has both single- and three-phase loads?
That is a reason to document the distribution system, not to guess from the largest motor. Ask the designer to identify which loads connect to each phase, how single-phase loads will be balanced, and what limits the proposed alternator places on unbalanced loading.
A single-phase generator cannot directly reproduce a three-phase supply. A phase converter or drive, where suitable, is a separate engineered application with its own input requirements. Do not assume its presence makes any generator compatible.
What to send with a quote request
Provide existing drawings and safely accessible equipment nameplate photos. Have the electrician verify service voltage, phase, neutral arrangement, motor phase rotation, and the transfer-equipment configuration. Do not remove energized covers to collect the information.
Next, use the business sizing guide to document demand and the three-phase transfer-switch checklist to specify the switching arrangement.
Connected guidance
Commercial Planning Related to Single-Phase vs. Three-Phase Generators
Continue exploring this Topic with Clean Power and Total Harmonic Distortion in Generators. Then connect this decision to What Changes the Cost of Generator Installation? and Running Watts vs. Starting Watts for Generator Sizing; those adjacent choices can change equipment, installation scope, operating readiness, and total ownership cost.