Formulas
A motor’s nameplate power is its mechanical output. The electrical power it draws is higher because of losses (efficiency) and reactive power (power factor). The full-load current is:
Single-phase: I = P ÷ (V × efficiency × PF)
Three-phase: I = P ÷ (√3 × V × efficiency × PF)
with P in watts (1 hp = 745.7 W) and V the line-to-line voltage for three-phase.
Example
A 5 hp three-phase motor on 460 V with an efficiency of 89% and a power factor of 0.85 draws about 3,729 ÷ (1.732 × 460 × 0.89 × 0.85) = 6.2 A. Its electrical input is 4.19 kW and its apparent power 4.93 kVA.
Typical values
| Motor size | Efficiency | Power factor at full load |
|---|---|---|
| under 1 hp | 65% to 80% | 0.65 to 0.80 |
| 1 to 10 hp | 82% to 91% | 0.78 to 0.87 |
| 10 to 100 hp | 91% to 95% | 0.85 to 0.90 |
Premium efficiency motors (NEMA Premium, IE3) are at the top of these ranges. Efficiency and power factor drop at light load.
What the NEC requires
For sizing conductors, short-circuit protection and disconnects, the National Electrical Code (430.6) requires the full-load current from its tables: Table 430.248 for single-phase and 430.250 for three-phase motors. These table values are deliberately conservative: a 5 hp, 460 V three-phase motor is listed at 7.6 A. Overload protection, on the other hand, is sized from the nameplate current. Use this calculator to estimate or check, not to replace the code tables.
Starting current
When started across the line (direct-on-line), an induction motor draws a locked-rotor current of about 6 to 8 times its full-load current for a few seconds. Star-delta starters reduce it to about a third, while soft starters and variable frequency drives limit it further. Size breakers and generators with this inrush in mind.
Tips
- Measure the actual current with a clamp meter on each phase; unbalanced currents point to a supply or winding problem.
- A motor drawing much more than its nameplate current is overloaded or failing.
- Power factor correction capacitors reduce the current drawn from the supply, not the current in the motor itself.