Article 430 is one of the most complex sections of the NEC, spanning over 50 pages. But the core logic is straightforward: use table FLC (not nameplate), size conductors at 125%, and select overcurrent protection by motor type. Here's how to get it right every time.
Rule 1: Always Use Table FLC, Not Nameplate
NEC 430.6(A)(1) is clear: use the full-load current values from Tables 430.247 through 430.250 for conductor sizing, overcurrent protection, and switch sizing. Never use the motor nameplate current.
Why? Table values represent the worst-case FLC for a given HP and voltage. Using nameplate current could result in undersized conductors if the motor is replaced with a different manufacturer's unit.
Rule 2: Conductors at 125% FLC
NEC 430.22: Branch circuit conductors for a single motor must have an ampacity ≥ 125% of the motor FLC.
Example: 10 HP, 460V, 3Φ motor → Table 430.250 FLC = 14A. Conductors: 14 × 1.25 = 17.5A → 12 AWG (25A at 75°C).
Rule 3: SCGF by Motor Type
NEC 430.52: Short-circuit and ground-fault protection varies by motor starting type:
Inverse-time breaker for Design B motors: ≤ 250% FLC. Time-delay fuse: ≤ 175% FLC. Instantaneous-trip breaker: ≤ 800% FLC (for Design B).
Example: 14A FLC → Inverse-time breaker: 14 × 2.5 = 35A → Next standard size = 35A. Time-delay fuse: 14 × 1.75 = 24.5A → 25A fuse.
Rule 4: Overload Protection at 115-125% FLC
NEC 430.32: Motor overload protection (in the starter or VFD) is sized at the nameplate FLA (not table FLC).
For motors with service factor ≥ 1.15: overload = 125% of nameplate FLA. For motors with SF = 1.0 or temperature rise ≤ 40°C: overload = 115% of nameplate FLA.
Multi-Motor Feeder Sizing
NEC 430.24: Feeder conductors must be sized at 125% of the largest motor FLC + sum of all other motor FLCs.
Example: Three motors at 14A, 27A, 65A → Feeder = (65 × 1.25) + 27 + 14 = 81.25 + 41 = 122.3A.
Key Takeaways
- •Always use NEC table FLC values — never motor nameplate current — for conductor and SCGF sizing
- •Conductors: 125% FLC. SCGF: up to 250% FLC (inverse-time breaker). Overloads: 115-125% nameplate FLA
- •Multi-motor feeders: 125% of largest motor + sum of all others
- •Keep the NEC 430 flow chart (430.1 Figure) handy — it maps the entire process visually