Motor Full Load Current Calculator
Enter a motor's horsepower or kilowatt rating, supply voltage, phase, efficiency and power factor to calculate its full load current. For standard US sizes you also get the NEC table current and the 125% conductor ampacity used for wire sizing.
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Nameplate rating (shaft output)
Line-to-line voltage for three-phase
0.1 to 1 (not used for DC)
How to use the Motor Full Load Current Calculator
- Choose the supply: three-phase AC, single-phase AC or DC.
- Enter the motor rating in hp, kW or W as printed on the nameplate (this is the shaft output).
- Enter the voltage: line-to-line voltage for a three-phase motor.
- Set efficiency and power factor from the nameplate or a datasheet (typical values are pre-filled).
- Read the full load current, the NEC table current and the 125% ampacity, then compare other sizes in the table.
Motor current formulas
| Supply | Full load current | Example |
|---|---|---|
| Three-phase AC | I = Pout ÷ (√3 × V × η × PF) | 10 hp, 460 V, 90%, 0.85 → 12.2 A |
| Single-phase AC | I = Pout ÷ (V × η × PF) | 1 hp, 115 V, 75%, 0.80 → 10.8 A |
| DC | I = Pout ÷ (V × η) | 2 hp, 90 V, 80% → 20.7 A |
Pout is the rated shaft power in watts (hp × 745.7), V is the line voltage, η is the efficiency as a decimal and PF is the power factor. The motor rating is always the mechanical output, so the electrical input is larger by the losses.
NEC full load current, three-phase motors (Table 430.250)
| HP | 200 V | 208 V | 230 V | 460 V | 575 V |
|---|---|---|---|---|---|
| 1 | 4.8 | 4.6 | 4.2 | 2.1 | 1.7 |
| 2 | 7.8 | 7.5 | 6.8 | 3.4 | 2.7 |
| 3 | 11.0 | 10.6 | 9.6 | 4.8 | 3.9 |
| 5 | 17.5 | 16.7 | 15.2 | 7.6 | 6.1 |
| 7-1/2 | 25.3 | 24.2 | 22 | 11 | 9 |
| 10 | 32.2 | 30.8 | 28 | 14 | 11 |
| 15 | 48.3 | 46.2 | 42 | 21 | 17 |
| 20 | 62.1 | 59.4 | 54 | 27 | 22 |
| 25 | 78.2 | 74.8 | 68 | 34 | 27 |
| 30 | 92 | 88 | 80 | 40 | 32 |
| 50 | 150 | 143 | 130 | 65 | 52 |
| 75 | 221 | 211 | 192 | 96 | 77 |
| 100 | 285 | 273 | 248 | 124 | 99 |
Values in amperes for induction-type squirrel-cage and wound-rotor motors running at usual speeds with normal torque. Single-phase values (Table 430.248) are used automatically when you choose single-phase; for example a 1 hp motor is listed at 16 A at 115 V and 8 A at 230 V.
Tips for using full load current
- Wire and breaker sizing: start from the NEC table current, not the nameplate, and multiply by 1.25 for the branch-circuit conductors of a single motor.
- Overload relays: set these from the nameplate full load amps (FLA) and service factor, which is usually lower than the table value.
- Metric motors: for 400 V or 415 V IEC motors the NEC tables do not apply; use the calculated current or the manufacturer's rating plate.
- Check your result: a three-phase 460 V motor draws roughly 1.2 to 1.4 A per horsepower; at 230 V it is about double that.
This calculator is a planning aid. Final installations must follow the edition of the electrical code adopted in your area and the motor manufacturer's data.
Frequently asked questions
How do you calculate motor full load current?
Convert the rated output to watts (1 hp = 745.7 W), divide by the efficiency to get the electrical input, then divide by the voltage and power factor. For three-phase motors also divide by √3 (1.732): I = P ÷ (√3 × V × η × PF). A 10 hp, 460 V motor at 90% efficiency and 0.85 PF draws about 7,457 ÷ (1.732 × 460 × 0.9 × 0.85) ≈ 12.2 A.
Why is the NEC table value different from the calculated current?
The NEC full load current tables (430.248 for single-phase and 430.250 for three-phase) are deliberately conservative values that cover typical motors of each size, including lower-efficiency and lower-speed designs. That is why a 10 hp 460 V motor is listed at 14 A even though a modern motor may draw 12 to 13 A.
Which current should I use to size wire and breakers?
Under NEC 430.6(A)(1), conductors, short-circuit and ground-fault protection and disconnects for general-purpose motors are sized from the table full load current, not the nameplate. Overload protection is set from the nameplate current. Branch-circuit conductors for a single motor must be rated at least 125% of the table current (430.22).
What voltage column should I use for 240 V or 480 V?
The NEC tables list nominal motor voltages. Use the 230 V column for 220 to 240 V systems, the 460 V column for 440 to 480 V systems and the 575 V column for 550 to 600 V systems. This calculator picks the matching column automatically.
What are typical motor efficiency and power factor values?
Small single-phase motors are often 60 to 80% efficient with a power factor of 0.6 to 0.8. Three-phase motors from about 5 to 100 hp are typically 87 to 95% efficient (premium efficiency motors are higher) with a power factor around 0.8 to 0.9 at full load. The nameplate or datasheet gives the exact values.
How much current does a motor draw when starting?
An induction motor started directly across the line typically draws about 5 to 7 times its full load current for a few seconds. The NEMA code letter on the nameplate gives the locked-rotor kVA per horsepower, and soft starters or VFDs reduce the starting current.
Does a motor draw full load current all the time?
No. Full load current is drawn only at rated load. A lightly loaded motor draws less current, but because power factor drops at light load, the current does not fall in direct proportion to the load.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.