What Size Generator Do You Need to Run a Motor?
A motor's running power is the easy part. The generator has to survive the start — when the motor briefly draws several times its running current.
Last updated · 7 min read
Sizing a generator for a motor goes wrong in a predictable way: someone adds up the running watts, buys a generator that covers them, and then watches it stall or trip the moment the motor starts. An induction motor started directly on the line briefly draws several times its running current. That starting surge, not the running load, usually decides the generator size.
Step 1: The Running Load
The running load is what the motor draws once it is up to speed. The most reliable figure is the full-load current (FLA) on the nameplate. From it:
Three-phase kVA = 1.732 × volts × amps ÷ 1,000
If you prefer to think in kilowatts, running power is roughly the motor's rated horsepower × 0.746 ÷ efficiency. A 2 hp motor at 80% efficiency draws about 1.9 kW at full load.
Step 2: The Starting Load (Code Letter)
The nameplate of most AC induction motors carries a locked-rotor code letter. It tells you how many kVA per horsepower the motor draws at the instant of starting, with the rotor not yet turning. The ranges are set out in NEC Table 430.7(B):
| Code letter | Starting kVA per hp | Code letter | Starting kVA per hp |
|---|---|---|---|
| A | 0 – 3.14 | G | 5.6 – 6.29 |
| B | 3.15 – 3.54 | H | 6.3 – 7.09 |
| C | 3.55 – 3.99 | J | 7.1 – 7.99 |
| D | 4.0 – 4.49 | K | 8.0 – 8.99 |
| E | 4.5 – 4.99 | L | 9.0 – 9.99 |
| F | 5.0 – 5.59 | Letters continue beyond L for higher inrush designs | |
Worked Examples
These figures use the NEC table currents, which are deliberately conservative; a specific motor's nameplate current is often lower. Use the nameplate figure when you have it.
Matching the Numbers to a Generator
- Continuous rating. The total running load of everything connected — motor plus lights, tools and other equipment — should sit comfortably inside the generator's continuous (running) rating, not at its limit.
- Starting capability. Portable generators usually quote a "surge" or "starting" figure. Larger standby generators are often specified for motor starting in kVA or even in horsepower; if the manufacturer publishes a motor-starting rating, use it, because it accounts for how far the voltage is allowed to dip.
- kW versus kVA. Three-phase generator sets are commonly rated in kW at a 0.8 power factor, so their kVA rating is about 25% higher than the kW figure. A motor starts at a low power factor, so compare starting loads in kVA.
- Voltage dip. Even if the generator does not stall, a heavy start pulls the voltage down for a moment. Sensitive electronics on the same generator may reset, so leave margin.
Ways to Reduce the Starting Surge
- Start the biggest motor first. When several motors share a generator, start the largest one while the generator is otherwise lightly loaded, then bring the others on one at a time.
- Soft starter. Ramps the voltage up over a few seconds, cutting the inrush compared with a direct-on-line start.
- Variable-frequency drive (VFD). Starts the motor at low frequency and voltage, so it can accelerate while drawing little more than its running current. This is the most generator-friendly option, but a VFD has its own compatibility requirements — check with the drive and generator suppliers.
- Hard-start kits on single-phase compressors. Common on air-conditioning compressors; they help a struggling start but do not remove the need for starting capacity.
Quick Checklist
- Read the motor nameplate: horsepower, voltage, phase, full-load amps and code letter. Our guide to reading a motor nameplate explains each field.
- Work out running kVA from the full-load amps.
- Work out starting kVA from the horsepower and the code letter.
- Add the running load of everything else on the generator.
- Choose a generator whose continuous rating covers the total running load with margin, and whose starting capability covers the motor's start while everything else is running.
For permanent installations, standby generators and anything connected to a building's wiring, have a licensed electrician size and connect the system: transfer switches, conductor sizing and protection all have code requirements of their own. To convert motor horsepower to running current first, use the HP to amps calculator.
Frequently Asked Questions
It depends mostly on the starting current. A 1 hp single-phase motor at 115 V draws about 16 A running (about 1.8 kVA), but its start can be several times higher — up to about 9 kVA for a code letter K motor. Choose a generator whose starting capability covers that, not just the running load.
Typically several times its running current for a direct-on-line start. The nameplate code letter gives the starting kVA per horsepower: for example, a code G motor draws 5.6–6.29 kVA per hp at the instant of starting.
It is the locked-rotor indicating code letter from NEC Table 430.7(B). Each letter stands for a range of starting kVA per horsepower, from A (lowest) upward.
Yes. A variable-frequency drive starts the motor gradually, so the starting surge is far smaller than a direct-on-line start. Check compatibility with both the drive and generator suppliers.
Use kVA for motor starting. Motors start at a low power factor, so the kVA demand is much higher than the kW figure suggests.