The Conversion Formula
Horsepower to amps conversion requires knowing voltage, phase, efficiency, and power factor. One HP = 746 watts (mechanical). The electrical input power is higher than mechanical output because of motor losses.
Single-Phase: I = (HP × 746) / (V × η × PF). Three-Phase: I = (HP × 746) / (V × √3 × η × PF). Where η = motor efficiency (typically 0.85-0.95) and PF = power factor (typically 0.80-0.90).
For NEC circuit sizing: ALWAYS use the FLC values from NEC Tables 430.248/250 rather than calculating from the formula. The NEC table values are standardized and account for typical motor characteristics. The formula is useful for estimation and verification only.
NEC Table Quick Reference
Three-Phase Motor FLC (NEC 430.250): 1 HP/460V = 2.1A, 3 HP/460V = 4.8A, 5 HP/460V = 7.6A, 10 HP/460V = 14A, 15 HP/460V = 21A, 20 HP/460V = 27A, 25 HP/460V = 34A, 30 HP/460V = 40A, 40 HP/460V = 52A, 50 HP/460V = 65A, 75 HP/460V = 96A, 100 HP/460V = 124A.
For 230V three-phase: double the 460V values. For 208V three-phase: multiply 460V values by 2.21.
Single-Phase Motor FLC (NEC 430.248): 1 HP/230V = 8A, 2 HP/230V = 12A, 3 HP/230V = 17A, 5 HP/230V = 28A, 7.5 HP/230V = 40A, 10 HP/230V = 50A. Single-phase motors draw approximately 2× the current of three-phase motors at the same HP and voltage.
Step-by-Step Examples
Example 1: 25 HP, 460V, 3-phase motor. From NEC Table 430.250: FLC = 34A. Conductor sizing: 34 × 1.25 = 42.5A minimum ampacity → 8 AWG copper (50A at 75°C). Breaker: 34 × 2.5 = 85A → 90A inverse-time breaker.
Example 2: 5 HP, 230V, single-phase motor. From NEC Table 430.248: FLC = 28A. Conductor: 28 × 1.25 = 35A → 8 AWG copper (50A). Breaker: 28 × 2.5 = 70A → 70A inverse-time breaker.
Example 3: Using formula for estimation — 50 HP, 460V, 3Φ, 92% efficiency, 0.87 PF: I = (50 × 746) / (460 × 1.732 × 0.92 × 0.87) = 37,300 / 637.9 = 58.5A. NEC table value: 65A. The formula gives a lower value because the NEC tables use conservative efficiency/PF assumptions.
Why NEC Tables vs Nameplate vs Formula
NEC Table FLC: Used for conductor sizing (430.22), SCGF protection (430.52), and disconnect sizing (430.110). These values represent typical motors and provide standardized sizing for all circuit components.
Motor Nameplate FLA: Used ONLY for overload protection (430.32). The nameplate shows actual operating current for that specific motor, which may differ from the NEC table value due to actual efficiency and service factor.
Formula Calculation: Used for estimation, verification, and applications where NEC tables don't cover (unusual voltages, special motors). The formula requires knowing actual motor efficiency and power factor, which vary by manufacturer and load.
Common Mistakes to Avoid
Using nameplate amps for conductor sizing — NEC requires using Table FLC values. Using nameplate values may undersize conductors since some motors have higher FLC than nameplate FLA.
Forgetting the voltage scaling — When a motor operates at a non-standard voltage (e.g., 208V instead of 230V), current increases proportionally. A motor rated 28A at 230V draws approximately 31A at 208V (28 × 230/208).
Assuming single-phase and three-phase draw the same current — At the same HP and voltage, single-phase motors draw about 1.73× more current than three-phase motors. A 5 HP motor draws 28A at 230V single-phase but only 15.2A at 230V three-phase.
Using HP formula output for NEC calculations — Always use NEC table values for code compliance. The formula gives approximate values that may differ from NEC standards.