A motor has two very different currents, and confusing them is how installations go wrong. Full-load amps (FLA) is the running current — it sizes the cable, the breaker, and the continuous-load rating. Starting current is the locked‑rotor surge — it sizes the contactor, the starter, and the generator.
Full-load amps is the steady current the motor draws at rated output. It is modest, continuous, and it sizes the cable, the breaker, and the continuous-load rating.
The inrush surge — six to eight times the full-load amps for the first second or two of acceleration. It is violent, brief, and it sizes the contactor, the starter, and the generator.
FLA = running — "How many amps does my motor draw while running?"
Starting = starting — "How many amps does my motor draw while starting?"
You need both.
Get the FLA wrong and the cable runs hot — the breaker trips
on overload or the insulation fails over time. Get the starting current
wrong and the generator trips the moment the motor comes up to speed.
The full calculation for the starting side — locked‑rotor amps, starting methods,
and starting kVA — lives on our companion motor starting current calculator,
which takes the full‑load amps from here as its starting point.
Full-load amps determines the conductor size, the circuit breaker, and the overload protection settings. Use the 3 Phase Motor Amps Calculator to get it right.
Locked‑rotor amps determines the generator rating, the contactor, and the starter. This is the number that ensures the motor actually starts.
Designers who size the generator on FLA consistently under‑buy. The starting current is 6–8× the FLA — the generator must handle that surge, not just the running load.
1. Use this page for FLA (cable & breaker).
2. Use the motor starting current calculator for the surge (generator).
Together they cover the complete installation.
The formula takes the motor's mechanical output and walks it back to the electrical current the supply must deliver, accounting for the two losses along the way: efficiency and power factor.
From horsepower:
I = hp × 746 ÷ (√3 × V × n × PF)
From kilowatts:
I = kW × 1000 ÷ (√3 × V × n × PF)
If you do not have the nameplate data, use 0.90 for efficiency and 0.85 for power factor — the typical values for a standard industrial motor.
Two rules of thumb:
The tool runs the exact figure for your numbers. The kVA the motor draws is then:
and it is the kVA, not the amps alone, that the generator must carry.
Enter the rated horsepower of your motor, along with the supply and motor parameters.
Enter the rated kilowatt output of your motor, along with the supply and motor parameters.
Here is the detail that separates a code-correct installation
from a hopeful one: the code does not trust the nameplate.
For sizing conductors, switches, and branch-circuit protection,
the NEC requires you to use the full-load current values in
Table 430.250 (for three-phase motors), not the
nameplate FLA. The table values are deliberately conservative,
and they are almost always higher than the calculated figure —
because they are built to cover the worst motor in each horsepower
class, not your specific one.
| Horsepower (hp) | Amps at 230 V | Amps at 460 V |
|---|---|---|
| 1/2 hp | 2.2 A | 1.1 A |
| 1 hp | 4.2 A | 2.1 A |
| 3 hp | 9.6 A | 4.8 A |
| 5 hp | 15.2 A | 7.6 A |
| 7.5 hp | 22 A | 11 A |
| 10 hp | 28 A | 14 A |
| 15 hp | 42 A | 21 A |
| 20 hp | 54 A | 27 A |
| 30 hp | 80 A | 40 A |
| 50 hp | 130 A | 65 A |
| 75 hp | 192 A | 96 A |
| 100 hp | 248 A | 124 A |
| 150 hp | 360 A | 180 A |
| 200 hp | 480 A | 240 A |
Three examples, calculated with the formulas above, to show the shape of the answer.
Classic small pump motor
I = 5 × 746 ÷ (1.732 × 460 × 0.90 × 0.85) =
6.1 A
NEC table value: 7.6 A
125% for conductors: 9.5 A
Common industrial motor
I = 11,000 ÷ (1.732 × 415 × 0.85 × 0.90) =
20.0 A
NEC table (approx. 15 hp @ 460 V is 21 A, similar)
Smaller workshop motor
I = 7,500 ÷ (1.732 × 400 × 0.86 × 0.90) =
14.0 A
NEC table (approx. 10 hp @ 460 V is 14 A)
For a quick reference across the range, the rule of thumb holds up: 1.25 amps per horsepower at 460 V — so a 100 hp motor is about 124 A, which is exactly what the NEC table says. Run the calculator for your exact voltage, efficiency, and power factor, and feed the resulting kVA into the generator path.
Use the 3 phase motor amps calculator above, then check the starting surge with our motor starting current calculator, convert the kVA with our generator kVA to amps calculator, and find the machine with our generator sizing calculator.
A: Divide the motor power by the product of voltage, efficiency, power factor, and the square root of three. From horsepower: I = hp × 746 ÷ (√3 × V × η × PF). From kilowatts: I = kW × 1000 ÷ (√3 × V × η × PF).
A: Full-load amps is the current a motor draws at its rated output power, running at full load. It is the steady-state current, distinct from the higher starting current, and it is the number used to size cables, breakers, and continuous-load ratings.
A: It depends on the horsepower, voltage, efficiency, and power factor. A 5 hp, 460 V motor draws about 6 A, a 50 hp motor about 65 A, and a 100 hp motor about 124 A (NEC Table 430.250 values at 460 V).
A: Use I = hp × 746 ÷ (√3 × V × η × PF). Multiply the horsepower by 746 to get watts, then divide by voltage, efficiency, power factor, and 1.732. For example, a 5 hp, 460 V motor at 90% efficiency and 0.85 PF draws about 6.1 A.
A: Use I = kW × 1000 ÷ (√3 × V × η × PF). An 11 kW, 415 V motor at 85% efficiency and 0.90 PF draws about 20 A. For code sizing, use the NEC Table 430.250 value rather than the calculated figure.
A: Full-load amps (FLA) is the running current at rated output; starting current (locked-rotor amps) is the surge drawn during acceleration, typically six to eight times higher. FLA sizes the cable and breaker; starting current sizes the contactor and generator.
A: NEC Table 430.250 lists the full-load current values for three-phase motors by horsepower and voltage, used for sizing conductors and protection. The values are conservative — a 10 hp, 460 V motor is listed at 14 A — and per NEC 430.6(A)(1) they override the nameplate FLA.
A: A 10 hp, 460 V three-phase motor draws 14 A per NEC Table 430.250, and about 9 to 10 A by the formula at 90% efficiency and 0.85 PF. At 230 V the same motor draws 28 A, because current rises as voltage falls.