Three-Phase Motor Current Calculator
Enter the motor rating, voltage, power factor and efficiency to get formula amps beside the NEC table value.
Step-by-step
- Enter valid values to begin.
NEC Table 430.250 is shown for comparison where the voltage is a standard NEC column (200, 208, 230, 460, 575 V) and the horsepower is a table value. Use the table value for conductors, disconnect and short-circuit protection; use the nameplate for the overload relay.
Three-Phase Motor Current: Formula Amps vs. Table Amps
Core Engineering Principles
A motor’s shaft takes mechanical horsepower and the supply has to deliver more than that, because the motor isn’t 100% efficient and it needs magnetizing current. One horsepower is 746 W of output. Divide by efficiency and you have the real input watts. Divide again by the power factor and the √3 × V and you have the line current. That’s the formula on this page, and it’s what you would measure with a clamp meter on a motor running at full load.
Here is the part that surprises new engineers: the formula amps and the table amps are often different, and the code wants the table. A modern premium-efficiency motor may draw 57 A where the NEC table says 65 A for the same 50 HP at 460 V. Both are right. The nameplate and the formula describe the motor you bought; Table 430.250 is a conservative standard that makes sure conductors and breakers don’t depend on which brand was installed.
Conductor = 125% × table FLC (430.22) • Overload = 115–125% × nameplate (430.32)
NEC & Standard References
NEC 430.6(A)(1) requires that conductor ampacity, switch ratings and branch-circuit short-circuit protection come from Table 430.250 for three-phase motors, not the nameplate. 430.6(A)(2) says the overload relay uses the nameplate full-load current. 430.22 sizes the branch conductors at 125%, and 430.110 sizes the disconnect at 115% of table FLC. NEMA MG-1 publishes the efficiency levels (NEMA Premium) and service factors, and NEMA MG-1 12.44 covers the full-load efficiency test method. Typical power factor runs from 0.80 on small motors to 0.90 on large, and drops sharply when the motor is lightly loaded.1. Output: 50 × 746 = 37,300 W.
2. Denominator: 1.732 × 460 × 0.88 × 0.93 = 652.0.
3. IFLA = 37,300 / 652.0 = 57.2 A (formula).
4. NEC Table 430.250 for 50 HP at 460 V = 65 A. Use 65 A for the conductor (65 × 1.25 = 81.3 A) and the short-circuit device.
5. Overload relay: nameplate amps × 1.15 or 1.25, not the table.
- Don’t size the breaker from the clamp reading. A motor at 70% load might read 42 A. That’s not the number the code uses.
- Low voltage means high current. A 460 V motor on a 430 V bus draws about 7% more current for the same shaft load and runs hotter.
- Check the PF and efficiency on the nameplate. Old rewound motors can be 85% efficient or worse.
- Lightly loaded motors. Power factor falls under 50% load; the current doesn’t fall as much as you expect, because magnetizing current stays.