The decision is already made — go read your panel
Most "which should I choose?" questions in backup power have genuine trade-offs. This one mostly doesn't: a standby generator must match the voltage and phase of the electrical service it backs up. Your utility made this decision when the building was built. A building served at 208Y/120 V three-phase needs a generator wound and configured for 208Y/120 V three-phase — not 480, not single-phase, not "close."
So the first step costs nothing: look at your electrical service. The panel schedule, the meter, a recent electrical drawing, or ten minutes of your electrician's time will tell you the voltage and phase. Everything else in the project — generator, transfer switch, feeders — flows from that answer.
Single phase and three phase, in plain words
- Single-phase power delivers electricity as one alternating waveform. The standard small-building service is 120/240 V: two hot legs and a neutral, giving 120 V for receptacles and lighting and 240 V for larger equipment. It's the world of houses, small offices, and small storefronts.
- Three-phase power delivers three waveforms, offset by a third of a cycle each — so at any instant, power is flowing on at least one of them. The result is smooth, constant power delivery that motors love and that moves more kilowatts per pound of copper.
The common commercial three-phase services in the U.S.:
- 208Y/120 V — the standard light-commercial service: 120 V for receptacles, 208 V for equipment and motors.
- 480Y/277 V — larger commercial and industrial: 480 V for heavy machinery and big HVAC, 277 V commonly for lighting, with transformers stepping down to 120 V receptacle circuits.
Why amps are the real story
Power (kW) is what you buy; current (amps) is what your conductors, breakers and switchgear must be sized to carry — and current is where three phase earns its dominance. The same 100 kW load at 0.8 power factor draws roughly:
| Service | Voltage | Approximate current for 100 kW |
|---|---|---|
| Single-phase | 240 V | ~520 A |
| Three-phase | 208 V | ~347 A |
| Three-phase | 480 V | ~150 A |
Same energy, dramatically different copper. This is why larger buildings are served at higher voltage and three phase, why a 480 V facility's switchgear looks so compact for its load, and why the kW ↔ amps converter asks for voltage and phase before it answers anything. It also flows straight into transfer-switch selection — the ATS is sized in amps to the service it sits in, which you can check with the ATS sizing tool.
The second big three-phase advantage is motors. Three-phase motors are simpler, smaller for their power, and self-starting without the capacitors and switches single-phase motors need. Commercial HVAC compressors, pumps, elevators and machinery above the small-appliance class are overwhelmingly three-phase — another reason the commercial world runs on it.
What this means for the generator purchase
- Match, don't approximate. Generator alternators are configured for specific voltages — commercial sets are typically available in the standard configurations (120/240 single-phase, 208Y/120, 480Y/277 and others), selected at order time. The Cat Olympian lineup in our models catalog follows exactly this pattern.
- The ATS matches too. Voltage, phase and amperage — the transfer switch is service equipment, and mismatch isn't a discount, it's a redesign. The ATS guide covers sizing to the service.
- Three-phase sets carry single-phase loads. Your 120 V receptacles and office equipment run happily from a three-phase generator — panels distribute single-phase circuits across the three phases. Designers keep the phases roughly balanced; a heavily unbalanced load wastes capacity. The reverse is not true: a single-phase set cannot serve three-phase motors.
- Backing up part of a building follows the same rule. If you're backing only a critical-loads panel, the generator matches that panel's voltage and phase — a downstream-ATS design decision covered in the sizing guide.
Edge cases worth knowing exist
A few situations deserve an electrician's judgment rather than a rule of thumb: buildings on 240 V high-leg delta service (a legacy three-phase flavor with one wild leg), sites considering a service upgrade to three phase as part of the generator project (sometimes worth it for motor-heavy operations — a utility conversation), and phase converters on single-phase sites running a few three-phase machines (the generator then needs to be sized for the converter's behavior, not just the motor). None of these are exotic; all of them are "bring your electrician" items.
Frequently asked questions
How do I know if my building is single-phase or three-phase?
Check the panel: three-phase panels have breakers in groups of three and three hot busbars; the service drawing or meter data confirms it. When in doubt, your electrician answers this in minutes — and the answer determines your generator configuration.
Can I run a three-phase building on a single-phase generator?
No. Three-phase motors and equipment cannot run on single-phase supply. The generator must be three-phase, at the building's service voltage.
Is a three-phase generator more expensive than single-phase?
At commercial sizes the machine is fundamentally the same engine and alternator family — pricing is driven by kW class, fuel and scope far more than by phase. See the cost guide for what actually moves the number.
What voltage generator do I need for a 208 volt building?
A three-phase set configured for 208Y/120 V. Order-time configuration matters: the same model family is wound and connected differently for 208 than for 480. Confirm the exact service voltage from the panel or utility before ordering.
Match the machine to the service — then size it
Phase and voltage are the constraint; kilowatts are the decision. Read your service, convert your loads with the kW ↔ amps tool, size the machine with the selector wizard, and post your project with your service voltage listed so suppliers quote the right configuration the first time. Service classifications and electrical design belong to your licensed electrician and engineer under the NEC — verify your building's specifics with them and your AHJ.