Reading amps off a motor nameplate or clamp meter and wondering what horsepower that represents? This calculator converts current and voltage into mechanical horsepower, accounting for efficiency and power factor — the reverse of our HP to amps calculator.
Amps to HP Formulas
3-φ: HP = (A × V × 1.732 × η × PF) ÷ 746
DC: HP = (A × V × η) ÷ 746
The result is the mechanical power at the shaft, which is why efficiency appears: a motor drawing 1,440 W of electricity at 85% efficiency delivers only ~1,224 W (1.64 HP) of shaft power. For code-compliant wire and breaker sizing, use the NEC tables on our HP to amps page rather than a calculation.
Common Amps → HP (Single-Phase, 85% η, 0.9 PF)
| Amps | @ 120 V | @ 230 V |
|---|---|---|
| 5 | 0.6 HP | 1.2 HP |
| 8 | 1.0 HP | 1.9 HP |
| 12 | 1.5 HP | 2.8 HP |
| 16 | 2.0 HP | 3.8 HP |
| 20 | 2.5 HP | 4.7 HP |
| 30 | 3.7 HP | 7.1 HP |
Worked Example
Three-Phase Amps to HP
Most industrial motors are three-phase, where the √3 factor applies. This table assumes 90% efficiency and 0.87 power factor — reasonable for a modern motor at full load.
| Amps | @ 208 V | @ 230 V | @ 460 V | @ 575 V |
|---|---|---|---|---|
| 5 | 1.9 HP | 2.1 HP | 4.2 HP | 5.2 HP |
| 10 | 3.8 HP | 4.2 HP | 8.4 HP | 10.5 HP |
| 15 | 5.7 HP | 6.3 HP | 12.5 HP | 15.7 HP |
| 20 | 7.6 HP | 8.4 HP | 16.7 HP | 20.9 HP |
| 30 | 11.3 HP | 12.5 HP | 25.1 HP | 31.4 HP |
| 40 | 15.1 HP | 16.7 HP | 33.4 HP | 41.8 HP |
| 60 | 22.7 HP | 25.1 HP | 50.2 HP | 62.7 HP |
| 100 | 37.8 HP | 41.8 HP | 83.6 HP | 104.5 HP |
What Efficiency and Power Factor to Assume
If the nameplate does not state them, these are reasonable starting values. Both rise with motor size — a large motor is inherently more efficient than a small one.
| Motor size | Typical efficiency | Typical power factor |
|---|---|---|
| Under 1 HP | 70–80% | 0.70–0.80 |
| 1–5 HP | 80–87% | 0.78–0.84 |
| 5–20 HP | 87–91% | 0.83–0.87 |
| 20–100 HP | 91–95% | 0.86–0.90 |
| Over 100 HP | 94–96% | 0.88–0.92 |
Why This Disagrees With the NEC Table
Run a code-table current through this calculator and the answer will not match the motor's nameplate horsepower. That is expected, and it is not an error in either place.
Take a 10 HP motor at 460 V three-phase. The NEC table gives 14 A. Feed 14 A into this calculator at 90% efficiency and 0.87 PF and you get about 11.7 HP — noticeably more than 10.
The reason is that code table values are deliberately conservative. They are standardised worst-case figures chosen so that conductors and protective devices remain adequate whatever motor is installed, including older, less efficient designs. A modern premium-efficiency motor genuinely draws less than the table says.
- To estimate shaft power from a measurement — use this calculator with the actual measured current.
- To size conductors, breakers and disconnects — use the code table, never a calculation. See the motor full-load amps chart.
- To set overload protection — use the nameplate FLA and the service factor printed on that specific motor.
Don't Use a Starting-Current Reading
A direct-on-line motor start draws several times its running current — commonly six to eight times full-load amps for a fraction of a second. Feeding that inrush figure into this calculator returns a horsepower number that is nonsense.
Let the motor settle into steady running under its normal load before taking a clamp-meter reading. If you are sizing a generator or transformer rather than estimating shaft power, starting kVA is the number that governs, not running amps.
Getting a Trustworthy Reading
- Clamp one conductor only. Clamping two or three at once on a three-phase supply gives a near-zero reading, because the currents cancel.
- Measure under the real working load, not free-running. An unloaded motor tells you almost nothing about the power it delivers in service.
- Check all three phases on a three-phase motor. More than a few percent of imbalance between them points at a supply or winding problem and makes any single reading unreliable.
- Use a true-RMS meter where the motor is fed by a variable frequency drive — an averaging meter misreads the drive's non-sinusoidal waveform badly.
To check a measured current against the published figure for that motor size, see the motor full-load amps chart.
Frequently Asked Questions
At 120 V single-phase with typical efficiency, about 1.5 HP. At 230 V the same current supports roughly 2.8 HP. Voltage matters as much as amps.
Nameplate amps are at full rated load; a lightly loaded motor draws fewer amps. Also, your efficiency and PF guesses may differ from the motor's real values — check the nameplate for both.
No — electrical code requires the standardized NEC full-load-current tables, not calculations. This tool is for understanding and load estimation; wiring is a licensed electrician's job.
About 16.7 HP at 460 V, or 8.4 HP at 230 V, assuming 90% efficiency and 0.87 power factor. Voltage matters as much as current.
Both rise with motor size. Roughly 75% and 0.75 under 1 HP, 85% and 0.82 at 1–5 HP, 90% and 0.87 at 5–20 HP, and 93% and 0.88 above 20 HP. Use the nameplate values whenever they are printed.
Because power factor collapses at part load — from 0.87 down to 0.50 or below — while efficiency also drops. The motor still draws magnetising current that does no useful work. Always measure under the real working load.
No. Inrush is commonly six to eight times full-load amps for a fraction of a second, and feeding it in returns a meaningless figure. Let the motor settle into steady running first.
Code table values are deliberately conservative — standardised worst-case figures so the wiring is adequate for any motor. A 10 HP motor listed at 14 A on the table calculates to about 11.7 HP here, because a modern efficient motor genuinely draws less than the table allows for.