Motor full-load current (FLA)
Full-load amps from horsepower, voltage, power factor and efficiency — three-phase or single-phase. The number to check a clamp reading against, estimate FLA without a nameplate, or size a starter in the field.
Motor full-load current · three-phase & single-phase
I = HP × 746 ÷ (√3 × V × PF × eff)
What this gives you
This is the theoretical full-load current a motor draws from its rated horsepower, voltage, power factor, and efficiency. It's the number you use to check a clamp-meter reading against the nameplate, to size conductors and starters in a pinch, and to sanity-check a motor that seems to be pulling too much. For three-phase motors the line current is I = HP × 746 ÷ (√3 × V × PF × efficiency); drop the √3 for single-phase.
Working in metric? Flip the power input to kilowatts and the calculator swaps the 746 W-per-HP factor for a straight 1,000 W per kW — the rest of the formula is unchanged. In horsepower three-phase mode it also surfaces the NEC Table 430.250 full-load current for the nearest standard motor at your voltage, so the sizing figure sits right next to the calculated one.
Nameplate FLA vs. calculated vs. the NEC table
There are three "full-load amps" and they are not the same, so don't be surprised when they differ by a few amps. The nameplate FLA is what the motor actually draws at rated load and is the number to program into an overload relay. This calculated value is derived from the power equation and depends heavily on the power factor and efficiency you enter — good for estimating when you don't have the nameplate. The NEC Table 430.250 value is a deliberately conservative figure used only for sizing wire, breakers, and disconnects, and it usually runs higher than both. Use the nameplate for protection, the table for sizing, and this calculator to estimate or to check.
Field note — power factor and efficiency move the answer a lot
The two easiest inputs to get wrong are the ones that swing the result most. A motor at light load has a much lower power factor than at full load, so its running current does not fall as fast as you'd expect when the load drops. If your calculated current is well below a measured value, check the power factor first — a 0.85 assumption on a motor actually running at 0.70 will under-predict the current by roughly 20 percent. When in doubt, use the nameplate PF and efficiency, not a round number.
Worked example
A 10 HP, 460 V three-phase motor at 0.85 power factor and 90 percent efficiency: input power is 7,460 ÷ 0.90 = 8.29 kW, and the line current is 8,289 ÷ (1.732 × 460 × 0.85) = about 12.2 A. The NEC table lists 14 A for the same motor — higher, because it's built for sizing, not for measuring. Both are "right" for their purpose.