3 Phase Motor Amperage Calculator — Line Current from Horsepower, and Motor Load from a Measured Current

Background.

A three-phase motor's amperage is not printed on the nameplate in a way you can use directly for every purpose, and the number that is printed there is not the number the electrical code makes you design around. This calculator handles the physics half of that problem in both directions: what current a motor of a known rating draws, and — clamp meter in hand — how heavily a running motor is actually loaded.

The forward calculation starts from a fact people routinely get backwards. A motor's horsepower or kilowatt rating is its SHAFT OUTPUT, the mechanical power it delivers to the pump, fan or conveyor. The electrical power it draws is always larger, by exactly the reciprocal of its efficiency. So the input power is the rated output divided by efficiency, and on a balanced three-phase supply that input power is √3 × V × I × PF. Rearranged for current: I = P_shaft ÷ (√3 × V × η × PF). A 25 hp motor on 460 volts at 93.6 percent efficiency and a power factor of 0.86 draws 29.08 amperes, takes 19.93 kW from the supply to deliver 18.65 kW at the shaft, and throws away the 1.28 kW difference as heat.

The voltage in that expression is the LINE-TO-LINE voltage. This is the single most common error in the calculation, and it is not a small one — using the line-to-neutral voltage moves the answer by a factor of √3, about 73 percent. The US Department of Energy fact sheet this page's √3 relation comes from is explicit about it: V is the "RMS voltage, mean line-to-line of 3 phases".

The reverse calculation is the one that saves money. Measure the running current, multiply by √3, the line-to-line voltage and the power factor, and you have the input power. Multiply by efficiency and you have the shaft power. Divide by the nameplate rating and you have the load factor — the quantity Equations 2 and 3 of that same DOE fact sheet exist to estimate. Motors are habitually oversized, and one running at 40 percent of its rating is doing so at a power factor far below its nameplate figure, which is precisely how a plant ends up paying a demand charge for reactive power it never needed.

One thing this page will not give you, and states beside the result rather than in a collapsed FAQ: a code sizing current. NEC 430.6(A)(1) requires that, other than for low-speed, high-torque and multispeed motors, the full-load current values in NEC Tables 430.247 through 430.250 be used to determine conductor ampacities and the ampere ratings of switches and branch-circuit short-circuit and ground-fault protection — instead of the current marked on the nameplate, and therefore instead of the current computed here. The point of that rule is standardisation: every 25 hp 460-volt motor gets designed around the same number regardless of who made it, so replacing the motor does not invalidate the wiring. This page does not reproduce Table 430.250, because the reproductions available to its author disagree with one another in several cells, and a full-load current taken from a wrong table is worse than no table at all. Look the value up in the code edition adopted where you are. Local amendments govern, adopted editions vary between jurisdictions, and a licensed electrician or electrical engineer must sign off before any work proceeds.

What is 3 phase motor amperage calculator?

Three-phase motor amperage is the line current an induction motor draws from a balanced three-phase supply. It follows from the motor's mechanical output rather than being an independent property: shaft power, divided by efficiency to get electrical input power, divided by √3 times the line-to-line voltage times the power factor to get current.

The vocabulary that matters is on the nameplate. Rated output is mechanical, in horsepower or kilowatts. Full-load efficiency is the fraction of input power that reaches the shaft at rated load. Power factor is the ratio of real power to apparent power — the extent to which the current is doing useful work rather than magnetising iron. Full-load amperes (FLA) is the manufacturer's own current figure at rated load. Locked-rotor amperes or the NEMA code letter describe the far larger inrush at start.

The calculator is valid for balanced, sinusoidal, steady-state operation of a three-phase induction motor. It is not valid across a variable-frequency drive without care — a VFD's input current and its motor-side current are different quantities and its harmonics break the √3 identity — and it says nothing about starting current, torque, temperature rise, service factor or duty cycle. Efficiency and power factor both fall at part load, power factor much faster, so a lightly loaded motor draws more current than a straight proportion of the full-load figure.

How to use this calculator.

1. 01Choose the direction. Mode one goes from a nameplate rating to a current. Mode two goes from a measured current to a load percentage.
2. 02Enter the nameplate rated output and pick its unit. If it says 'hp', electrical horsepower at 746 W is the conventional reading; the mechanical option is there because the DOE fact sheet and some manufacturers use 745.6999 W instead.
3. 03Enter the LINE-TO-LINE supply voltage. On a 460 V three-phase system that is 460, not 266.
4. 04Enter the nameplate full-load efficiency as a percentage and the nameplate power factor as a decimal. Both are printed on modern motors; if yours is old enough that they are not, the answer will only ever be an estimate and should be labelled as one.
5. 05In mode two, enter the measured line current — the mean of the three phases if they differ.
6. 06Read the line current, then the input power beneath it. The gap between input power and shaft output is the heat the motor makes, and it is what the ventilation has to remove.
7. 07Do not take the current to the conductor or breaker calculation. NEC 430.6(A)(1) requires the value from Table 430.250 for that, and this page deliberately does not reproduce it.

The formula.

Start from the energy balance rather than from a formula to memorise. Whatever electrical power goes into a motor either comes out of the shaft or comes out as heat, so input power equals shaft power divided by efficiency. For the 25 hp worked example that is 25 × 746 = 18,650 W of shaft power, and 18,650 ÷ 0.936 = 19,925.21 W of electrical input.

Now convert input power into current. On a balanced three-phase supply the real power is √3 × V × I × PF, where V is the line-to-line voltage — Equation 1 of the DOE fact sheet, quoted there as "Pi = V x I x PF x √3 / 1000" with V defined as the mean line-to-line voltage of the three phases. Rearranging: I = Pi ÷ (√3 × V × PF) = 19,925.21 ÷ (1.7320508076 × 460 × 0.86) = 29.0794 A. Substituting the first expression into the second gives the one-line form I = P_shaft ÷ (√3 × V × η × PF).

The √3 is not an approximation and it is not 1.732. It is carried here at forty significant digits. Where a source has rounded it, the difference shows up in the third significant figure, which is exactly what happens in the DOE fact sheet's own example — see the FAQ on the 22.9 kW figure.

Apparent power is the same product without the power factor: S = √3 × V × I = 23.1689 kVA for this motor, against 19.9252 kW of real power. That ratio is the power factor, and it is why a generator or transformer feeding motors is sized in kVA rather than kW.

Reading the identity backwards gives the load estimate. With a measured current, Pi = √3 × V × I × PF; multiply by efficiency for shaft power; divide by the rated shaft power for the load factor. That is the DOE sheet's Equation 2, printed there as "Pir = hp x 0.7457 / Efl", combined with its Equation 3, "Load = Pi / Pir x 100%" — together algebraically identical to the form used here, with 0.7457 kW being the mechanical horsepower this page exposes as a unit option.

Nothing is rounded at an intermediate step. Efficiency and power factor are applied as exact decimals, √3 at full precision, and the results are rounded once at the boundary.

A worked example.

Frequently asked questions.

References& sources.

1. [1]US Department of Energy, Office of Energy Efficiency and Renewable Energy, 'Determining Electric Motor Load and Efficiency', fact sheet DOE/GO-10097-517 (January 1997). Source of Equation 1, 'Pi = V x I x PF x √3 / 1000', with V defined as 'RMS voltage, mean line-to-line of 3 phases' and I as 'RMS current, mean of 3 phases'; and of the load estimate, its Equation 2, 'Pir = hp x 0.7457 / Efl' (input power at full-rated load), combined with its Equation 3, 'Load = Pi / Pir x 100%' — together Load = Pi ÷ (hp × 0.7457 ÷ Efl) × 100%. Also the source of the worked example (469.7 V, 37 A, PF 0.763) used as this page's independent check. Authority: US DOE / EERE. Document revision: January 1997. Locator: Equations 1, 2 and 3, and the worked example. Access: open. Independence: primary. Retrieved 2026-07-29.
2. [2]NIST Special Publication 811, 'Guide for the Use of the International System of Units (SI)', Appendix B.9 'Factors for units listed alphabetically'. Source of both horsepower definitions: horsepower (electric) = 7.46 E+02 W exactly, and horsepower (550 ft·lbf/s) = 7.456 999 E+02 W. The exact mechanical value used here, 745.6998715822702 W, follows from the international foot (0.3048 m) and pound-force (4.4482216152605 N) and is asserted in the test suite. Authority: National Institute of Standards and Technology. Document revision: SP 811, 2008 edition. Locator: Appendix B.9. Access: open. Independence: primary. Retrieved 2026-07-29.
3. [3]NFPA 70, National Electrical Code, 430.6(A)(1) 'Ampacity and Motor Rating Determination — General Motor Applications', reproduced by ElectricalLicenseRenewal: other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Tables 430.247 through 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches and branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. This is the rule that makes the current computed on this page unusable for code sizing, and it is stated beside the result. Authority: NFPA, via an open code-reproduction host. Locator: 430.6(A)(1). Access: gated at NFPA, open at the reproduction. Independence: primary text, secondary host. Retrieved 2026-07-29.
4. [4]EC&M (Electrical Construction & Maintenance), 'Motors and the NEC' — independent confirmation that NEC Article 430 sizes motor circuits from the Table 430.247–430.250 full-load current rather than the nameplate value, and that motor branch-circuit protection and overload protection are separate devices under 430.52 and 430.32 respectively. Used to check the scope statement, not to source any numeric value. Authority: EC&M / Endeavor Business Media. Access: open. Independence: secondary-check. Result: agree. Retrieved 2026-07-29.