How this instrument works
The formula converts a motor's mechanical output rating into the current its supply conductors actually carry. Horsepower times 746 turns the nameplate rating into watts of mechanical output — 746 W being the engineering rounding of the exact 745.699872 W in one mechanical horsepower, the value NEMA and IEEE motor standards have used for a century. That output figure, though, is never what the motor pulls from the wall.
Two real losses sit in the denominator, and both push the current up. Efficiency accounts for friction, winding resistance, and core losses inside the motor: a machine that is only 88% efficient must draw more input power than it delivers as shaft work, so dividing by η (as a decimal, 0.88) restores that larger input figure. Power factor accounts for the reactive current the motor's magnetic field pulls from the line without doing any work; dividing by PF converts real power back into the total current a clamp meter actually registers.
This particular formula is written for a single-phase supply — there is no √3 term, which a three-phase version needs. It is also an idealization: a perfect motor, 100% efficient at unity power factor, would draw only 16.22 A for the same 5 hp, a floor no real machine reaches, since efficiency and power factor are always somewhat below 1. At the other extreme, zero horsepower output draws zero current no matter what voltage or power factor is entered — there is nothing there to move.
- Enter the nameplate rating into Motor power, hp — this is the motor's rated mechanical output, not its electrical input.
- Set Voltage to the motor's rated single-phase supply voltage, typically 115 or 230 V for shop-sized motors.
- Enter Efficiency, % from the nameplate or datasheet; if it is missing, 82–90% is typical for motors this size.
- Enter Power factor from the nameplate, usually somewhere between 0.75 and 0.95 at full load.
- Read Current draw in amps — the current the supply conductors must carry, before any code safety margin is added.
Worked example — sizing a 5 hp shop compressor motor
A 5 hp single-phase air compressor motor is nameplated at 230 V, 88% efficient, with a 0.9 power factor. The formula gives I = (5 × 746) ⁄ (230 × 0.88 × 0.9) = 3730 ⁄ 182.16 = 20.4765 A, which reads out as 20.48 A — the actual current the branch circuit has to carry, not the horsepower-times-746-over-volts shortcut some installers still reach for.
Skip efficiency and power factor entirely — divide 3730 W by 230 V alone — and the estimate drops to 16.22 A, roughly 4.26 A short of reality. That gap is exactly why the National Electrical Code sizes motor branch circuits from tabulated full-load current values rather than a bare watts-over-volts guess: a wire or breaker picked from the naive number would be undersized for the current the motor genuinely pulls.
Questions
Why does the current increase when efficiency and power factor drop?
Because both sit in the denominator, so a smaller η or PF makes the fraction bigger. The same 5 hp motor at 100% efficiency and unity power factor draws only 16.22 A; drop it to a realistic 88% efficient, 0.9 power-factor machine and the draw rises to 20.48 A — a jump from losses alone that never shows up on the horsepower nameplate.
Does this formula work for a three-phase motor?
Not as written — it has no √3 term, so it is single-phase only. A three-phase motor needs I = (hp × 746) ⁄ (√3 × V × η × PF); applying the single-phase version to a three-phase circuit overstates the current by a factor of √3, about 73% too high.
Where does the 746 constant come from?
From the definition of mechanical horsepower: 550 foot-pounds of work per second, which converts exactly to 745.699872 W. NEMA and IEEE motor standards round that to 746 W for nameplate and sizing calculations, and that is the value this instrument uses.
Should I use this number to size a breaker or wire?
Not directly. This is a calculated estimate of typical operating current; NEC branch-circuit and overload sizing (Article 430) is based on the motor's nameplate full-load amps or, lacking that, the Code's own tabulated full-load current tables. Use this figure to sanity-check a nameplate value or plan ahead of choosing a motor, not to replace either one.
What if I don't know the motor's efficiency or power factor?
Check the nameplate or manufacturer datasheet first — both are normally printed there. Lacking that, general-purpose motors in the 1–10 hp range typically run 82–90% efficient with a 0.75–0.9 power factor at full load, but treat those as planning estimates only; an exact figure needs the specific motor's rated values.