Generator Sizing Chart — Full-Load Amps by Size & Voltage

How to use the amp chart

Add up the running amps of everything you need to power, then account for the higher startup (surge) amps of the largest motor. Match that total to a generator size at your service voltage — making sure the generator's full-load amps exceed your peak demand. When a load is close to a generator's rated amps, size up to the next unit to keep headroom for surges.

Example: a job needs 400 running amps at 208V three-phase. Reading the chart, a 125 kW generator delivers ~434A at 208V 3-phase — enough, but with little headroom. If large motors will surge on startup, stepping up to a 150 kW (~520A) gives safer margin.

What size generator do I need for my amp service?

A common way to size a generator is to match it to your electrical service rating (the amps your main panel is rated for). To back up a full service you generally want a generator whose full-load amps at your voltage meet or exceed the service size, though most jobs only need to power essential loads rather than the entire panel. Use this as a starting guide, then call us to size for your actual load:

Approximate generator sizes to fully back a service of each rating at a 0.8 power factor. Most applications power selected circuits, not the whole panel — so you may need less. We'll help you right-size for the actual load.

Frequently asked questions

How do I read the generator amp chart?

Find your generator's kW size in the left column, then read across to the voltage and phase you're running. The number is the approximate full-load amps (FLA) that generator can deliver at that voltage — in other words, the maximum continuous amperage available from that size.

What is FLA (full-load amps)?

Full-load amps is the current a generator delivers at its rated output. Match it to your equipment's amperage draw and panel rating to size correctly. Keep your total load below the FLA figure for your generator and voltage.

Why is single-phase blank for larger sizes?

Generators above about 100 kW are almost always run three-phase — single-phase at those sizes draws impractically high amps. That's why the 1-phase column only lists smaller units. Ask us if you have a large single-phase need.

Single-phase vs three-phase amps?

At the same kW, three-phase delivers lower amps per leg than single-phase, which is why large loads use three-phase. Higher voltage also lowers amps — 480V draws far fewer amps than 208V for the same power.

Are these amp figures exact?

They're planning estimates based on a 0.8 power factor. Actual amperage varies with power factor, voltage and load. Call us to confirm sizing for your job.

What size generator do I need for a 400 amp service?

To fully back a 400 amp service, plan on roughly a 150 kW generator at 208V three-phase, about 320 kW at 480V three-phase, or around 100 kW at 240V single-phase. Most jobs only power essential circuits, so you may need less — we'll size it to your actual load.

What size generator do I need for a 200 amp service?

A 200 amp service typically pairs with about an 80 kW generator at 208V three-phase or roughly 50 kW at 240V single-phase to cover the full panel. If you're only backing up key circuits, a smaller unit is usually enough.

What size generator for a 100 or 125 amp service?

For a 100 amp service, plan on about 40 kW at 208V three-phase (~25 kW single-phase); for 125 amp service, about 50 kW three-phase (~30 kW single-phase). These back the full service — powering only essentials needs less.