Intermediate
Motor Circuit Formulas
Motor branch circuit sizing formulas per NEC 430 — conductor sizing, overload, SCGF, and disconnect calculations.
Formulas
Conductor ≥ 1.25 × FLC
Motor branch circuit conductor minimum ampacity (NEC 430.22)
Overload ≤ 1.25 × FLA (SF≥1.15)
Overload protection max — service factor ≥ 1.15 (NEC 430.32)
Overload ≤ 1.15 × FLA (SF<1.15)
Overload protection max — service factor < 1.15
Breaker ≤ 2.50 × FLC
Inverse-time breaker maximum (NEC 430.52)
Fuse ≤ 1.75 × FLC
Dual-element fuse maximum (NEC 430.52)
Disconnect ≥ 1.15 × FLC
Motor disconnect minimum rating (NEC 430.110)
Variable Definitions
FLC=Full Load Current (from NEC Tables 430.248/250)(amperes (A))
FLA=Full Load Amps (from motor nameplate)(amperes (A))
SF=Service Factor (from motor nameplate)(dimensionless)
Worked Example
Size branch circuit for 10 HP, 460V, 3Φ motor (NEC table FLC = 14A, nameplate FLA = 13.2A, SF = 1.15).
- Conductor: 14 × 1.25 = 17.5A → 14 AWG (20A) minimum
- Overload: 13.2 × 1.25 = 16.5A (using FLA, SF ≥ 1.15)
- Breaker: 14 × 2.50 = 35A inverse-time
- Disconnect: 14 × 1.15 = 16.1A → 30A disconnect minimum
Answer: 14 AWG conductor, 16.5A OL, 35A breaker, 30A disconnect
When to Use
- •Sizing motor branch circuit components per NEC 430
- •Selecting overload relays for motor starters
- •Determining maximum SCGF (breaker/fuse) size for motors
- •Sizing motor disconnects
Standards & References
- NEC 430.22 — Motor Conductor Sizing
- NEC 430.32 — Overload Protection
- NEC 430.52 — Short-Circuit/Ground-Fault Protection
- NEC 430.110 — Disconnect Rating