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Horsepower to amps calculator
Amps = hp x 746 / (volts x 1.732 x efficiency x power factor) for a three-phase motor; drop the 1.732 for single-phase. A 10 hp, 460 V motor draws about 11.7 A, as TECO’s data sheet lists, and a 1 hp, 115 V single-phase motor about 14 A. Enter yours below, or work from amps back to horsepower.
Updated October 12, 2026
The formulas
| Supply | Amps from horsepower |
|---|---|
| Three-phase | hp x 746 / (V x 1.732 x efficiency x PF) |
| Single-phase | hp x 746 / (V x efficiency x PF) |
| DC | hp x 746 / (V x efficiency) |
746 watts per horsepower (10 CFR 431.25(e)). Efficiency and power factor as decimals. Same formula as Twin City Fan’s motor guide and DOE’s motor fact sheet.
Estimated running amps for common motors
| Three-phase motor | Efficiency used | 208 V | 230 V | 460 V | 575 V |
|---|---|---|---|---|---|
| 1 hp | 85.5% | 2.85 | 2.58 | 1.29 | 1.03 |
| 2 hp | 86.5% | 5.63 | 5.09 | 2.55 | 2.04 |
| 3 hp | 89.5% | 8.17 | 7.38 | 3.69 | 2.95 |
| 5 hp | 89.5% | 13.6 | 12.3 | 6.15 | 4.92 |
| 7.5 hp | 91.7% | 19.9 | 18 | 9.01 | 7.21 |
| 10 hp | 91.7% | 26.6 | 24 | 12 | 9.61 |
| 15 hp | 92.4% | 39.5 | 35.8 | 17.9 | 14.3 |
| 20 hp | 93% | 52.4 | 47.4 | 23.7 | 19 |
| 25 hp | 93.6% | 65.1 | 58.8 | 29.4 | 23.5 |
| 30 hp | 93.6% | 78.1 | 70.6 | 35.3 | 28.2 |
| 40 hp | 94.1% | 104 | 93.6 | 46.8 | 37.5 |
| 50 hp | 94.5% | 129 | 117 | 58.3 | 46.6 |
| 75 hp | 95.4% | 192 | 173 | 86.6 | 69.3 |
| 100 hp | 95.4% | 255 | 231 | 115 | 92.4 |
| 150 hp | 95.8% | 381 | 345 | 172 | 138 |
| 200 hp | 96.2% | 506 | 458 | 229 | 183 |
Computed with the formula: federal minimum efficiency for a 4-pole enclosed motor (10 CFR 431.25, Table 5) and a power factor of 0.85. Real motors vary; the nameplate wins. These are not NEC Table 430.250 values.
| Single-phase motor | Efficiency used | Power factor used | 115 V | 230 V |
|---|---|---|---|---|
| 1/4 hp | 57% | 0.43 | 6.62 | 3.31 |
| 1/3 hp | 60% | 0.55 | 6.55 | 3.28 |
| 1/2 hp | 64% | 0.56 | 9.05 | 4.52 |
| 3/4 hp | 66% | 0.65 | 11.3 | 5.67 |
| 1 hp | 67% | 0.69 | 14 | 7.02 |
| 1.5 hp | 77% | 0.84 | 15 | 7.52 |
| 2 hp | 71% | 0.76 | 24 | 12 |
| 3 hp | 78% | 0.78 | 32 | 16 |
| 5 hp | 75% | 0.94 | 46 | 23 |
Efficiency and power factor from Baldor, WEG and Nidec US Motors data sheets for general-purpose capacitor-start motors, set at the highest nameplate current we found for each size. These are not NEC Table 430.248 values. Published copies of that table list higher currents at every size here except 1/4 hp, where this estimate is the higher one.
Why the NEC table reads higher
NEC® 430.6(A)(1) says that when you size conductors, switches and branch-circuit protection from a motor’s current, you use the full-load current tables in the code (Table 430.248 for single-phase, Table 430.250 for three-phase), not the nameplate. The code tables are conservative and usually sit above both the nameplate and this formula: for the 10 hp, 460 V motor above, the formula and TECO’s data sheet agree at about 11.7 A, and published copies of the code table give a higher figure. Single-phase is the same story at most sizes: the motors we checked draw less than the code table lists. The exception is 1/4 hp, where the Nidec motor that sets our default draws more than published copies of the code table give.
We do not reproduce the NEC tables: they are NFPA’s copyrighted text. NFPA lets you read NFPA 70 free online with a free account. Use this calculator to estimate running current, check a reading against expectations, and size the supply transformer, not to pick wire or breakers.
NFPA 70®, National Electrical Code® and NEC® are registered trademarks of the National Fire Protection Association, Quincy, MA. Morsa is not affiliated with NFPA.
Where the efficiency comes from
General-purpose three-phase motors from 1 to 500 hp made since June 1, 2016 must meet minimum full-load efficiencies set by the Department of Energy in 10 CFR 431.25 (Table 5). Small motors up to 3 hp, including capacitor-start single-phase motors, have their own minimums in 10 CFR 431.446. Leave the efficiency blank on a three-phase motor and the calculator uses the minimum for the size, poles and enclosure you pick, which is a floor: many motors beat it. Power factor moves the answer more: TECO’s 1 hp motor at exactly the federal 85.5% efficiency runs at a 0.78 power factor and draws 1.40 A at 460 V, while the default 0.85 gives about 1.29 A. Use the nameplate when you have it. Sizes between two listed ratings round to the nearer one, with a tie going up, as the regulation does.
Power factor has no federal table. Three-phase motor data sheets give full-load values like 0.78 for a TECO 1 hp motor, 0.87 for a TECO 10 hp and 0.862 for an ABB 100 hp, so for three-phase the calculator assumes 0.85 and shows the 0.80 to 0.90 spread with every answer.
Single-phase motors
Small single-phase motors run at a much lower power factor, so the federal minimum efficiency with a 0.85 power factor would read far too low. For single-phase the calculator takes both efficiency and power factor from manufacturer data sheets: Baldor’s product pages list both, WEG’s catalog lists efficiency and current, and Nidec US Motors lists current. For each size we use the motor with the highest nameplate current, so the estimate leans high. Baldor’s 1 hp L3510M, for example, lists 67% efficiency, a 0.73 power factor and 13.0 A at 115 V; Nidec’s 1 hp T1C2JH lists 14.0 A. The result line also shows the range across the data sheets we read for that size. From about 1.5 hp up, many single-phase motors add a run capacitor, which lifts the power factor above 0.9.
Reading a motor nameplate
HP or kW: rated output at the shaft, not what it draws from the line.
FLA (full-load amps): the current at rated load and voltage. Use it when you have it.
NEMA nominal efficiency and PF: put them in the calculator for a better answer than the defaults.
SF (service factor): a 1.15 SF motor can run at 115% of rated load, drawing more current than FLA.
Volts: a dual-voltage motor (230/460 V) draws half the current at the higher voltage.
Related tools
Sources
10 CFR 431.25, energy conservation standards for electric motors (Table 5)
10 CFR 431.446, small electric motors energy conservation standards
Twin City Fan, Integral AC Motor Selection and Application Guide (FE-800)
TECO-Westinghouse, 10 hp 4-pole ODP motor performance data (DHP0104G)
ABB, 100 hp 4-pole 460 V premium efficiency motor data sheet
Baldor-Reliance, single-phase motor product pages (for example L3510M, L1405T, L1408T, L1409T)
WEG, US100 standard catalog, single-phase general purpose motors
Nidec US Motors, FL600 catalog, single-phase capacitor-start TEFC motors
NFPA 70, National Electrical Code, free access (free NFPA account)
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