Motor Full-Load Current
Full-load current from a motor nameplate for DC, single- and three-phase supplies, with the NEC table value shown alongside it — plus kW ⇄ kVA ⇄ hp, starting current from the NEMA code letter, star-delta and autotransformer starting, and NEC overload, device and conductor sizing from the correct basis for each. Fully offline.
Open Motor Full-Load Current →What is the motor full-load current calculator?
A free motor calculator that runs entirely in your browser. Enter a nameplate — rated power in kW, hp or metric hp, supply voltage, efficiency and power factor — and it gives the full-load current for a DC, single-phase or three-phase supply, together with the electrical input power, apparent and reactive power and the losses in the motor. Beside that answer it shows the NEC table full-load current for the same motor, because those two numbers are different and the code cares which one you use. A second tab handles the kW, kVA and horsepower chain including brake horsepower at part load. A third works out starting current from the NEMA code letter, the reduction star-delta and autotransformer starting give, and the NEC overload, device and conductor sizing — each from the basis the code actually requires. Nothing is uploaded.
How to use Motor Full-Load Current
- Pick the supply — Choose DC, single-phase or three-phase. Power factor disappears on DC because it does not apply, and three-phase voltage is line-to-line.
- Enter the nameplate — Type the rated power and choose its unit, then the voltage, efficiency and power factor. Say whether the rating is shaft power (the usual case) or already electrical input — the tool divides by efficiency only for shaft power.
- Compare the two currents — The left figure comes from your nameplate; the right is the NEC table value for that horsepower and voltage. They will usually differ. If your voltage is not a tabulated column — 400 V and 415 V are not — the tool says so rather than interpolating.
- Size the protection — On the Starting & protection tab, enter both currents in their own fields, pick the motor type and, if you have it, the code letter. Overload, device maxima and conductor sizing all appear, each computed from the right basis.
Frequently asked questions
Why does the protection tab ask for two different currents?
Because the code requires two. NEC 430.6(A)(1) says branch-circuit short-circuit protection and conductor sizing use the table value from 430.248 or 430.250 — explicitly instead of the current marked on the nameplate. NEC 430.6(A)(2) says overload protection uses the nameplate figure. A 25 hp 460 V motor might be 29.3 A on its nameplate and 34 A in the table, so one shared field would make one of the two answers wrong.
My supply is 400 V and the table gives nothing. Why?
NEC tables 430.248 and 430.250 have columns for 115, 200, 208, 230, 460, 575 and 2300 V. There is no 400 V or 415 V column, because those are IEC system voltages. Rather than interpolate a value the code does not publish, the tool tells you the voltage is not tabulated — your nameplate current and the manufacturer's data are the right basis in an IEC installation.
What is the starting current really going to be?
If you have the code letter, use it — the letter states a locked-rotor kVA per horsepower band, so the tool shows a range rather than a single figure. The familiar six to eight times full-load current is an engineering rule of thumb for Design B and IEC Design N cage motors, not a code value; the tool falls back to it only when no code letter is given and labels it as such.
Why can't it size protection for a DC or synchronous motor?
Those have their own full-load-current tables — 430.247 for DC and the synchronous sub-table of 430.250 — which are not reproduced here. Rather than apply the percentages to a nameplate value the code does not permit for that purpose, the tool declines and says why. The nameplate current and power conversion still work normally.
Which NEC edition are the figures from?
The percentages, code-letter bands and standard device ratings used here were checked as stable across the 2005, 2011, 2014, 2017 and 2020 editions. The 2023 edition re-cut Table 430.52(C)(1) to separate Design B premium efficiency motors, so check against the edition in force where you work.
Tips
- Efficiency and power factor come off the nameplate — a generic guess can shift the current by ten percent or more.
- Star-delta starting gives you a third of the current, but also a third of the torque, so it only suits a load that can start unloaded.
- The conductor figure is 125% of the table current for a single continuous-duty motor; a group of motors on one feeder follows a different rule.