Single-Phase vs Three-Phase Motor Circuits — FLA, Wire, and Breaker
A 5 HP single-phase motor draws roughly 28 A; a 5 HP three-phase motor draws about 7.6 A. The circuit that feeds each is sized quite differently.
Where the current difference comes from
Three-phase: I = P / (√3 × VLL × PF × η)
The √3 in the three-phase denominator, plus the higher line-to-line voltage, is why three-phase current is roughly one-third of the single-phase value for the same power.
Table full-load currents (typical, 230 V)
| Motor | 1Φ FLC (A) | 3Φ FLC (A) |
|---|---|---|
| 1 HP | 8 | 3.6 |
| 2 HP | 12 | 6.8 |
| 3 HP | 17 | 9.6 |
| 5 HP | 28 | 15.2 |
| 7.5 HP | 40 | 22 |
Use the PEC motor FLC tables for design; nameplate current is for overload sizing only.
Sizing follows the same rules
- Conductor: 125% × table FLC, then ampacity table and corrections.
- Branch short-circuit protection: up to 250% of table FLC for an inverse-time breaker.
- Overload: 115–125% of nameplate FLA.
Worked: 5 HP, 230 V
3Φ: conductor at 1.25 × 15.2 = 19 A → 3.5 mm²; breaker ≤ 2.5 × 15.2 = 38 A → 30–40 A
The three-phase circuit uses smaller wire and a smaller breaker — one of the practical reasons three-phase is preferred for larger motors.
EngEst Pro’s EE Tools motor-circuit calculator handles both single-phase and three-phase — enter horsepower, voltage, and phase and it returns table FLC, conductor size, breaker rating, and overload setting per the PEC motor rules.
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