Motor Circuit Components
A complete motor branch circuit consists of four elements, each governed by different NEC sections: (1) Branch circuit conductors — sized per NEC 430.22 using FLC from Tables 430.248/250. (2) Overload protection — sized per NEC 430.32 using motor nameplate FLA. (3) Short-circuit and ground-fault protection (SCGF) — sized per NEC 430.52 using FLC. (4) Disconnect — sized per NEC 430.110 using FLC.
The critical distinction: conductors and SCGF protection use NEC table FLC values (430.248 for single-phase, 430.250 for three-phase). Overload protection uses the actual motor nameplate full-load amps (FLA). These values are different — the NEC table values are typically higher to accommodate motor starting current and service factor.
Conductor Sizing (NEC 430.22)
Motor branch circuit conductors must have ampacity of at least 125% of motor FLC from Table 430.248 (single-phase) or Table 430.250 (three-phase). This 125% factor accounts for the motor's continuous duty cycle.
Example: 10 HP, 460V, 3-phase motor. FLC from Table 430.250 = 14A. Minimum conductor ampacity = 14 × 1.25 = 17.5A. From Table 310.16 at 75°C: 14 AWG = 20A (✓). However, most electricians use 12 AWG minimum for motor circuits.
For multiple motors on one feeder: feeder ampacity = 125% of largest motor FLC + sum of all other motor FLCs (NEC 430.24).
Overload Protection (NEC 430.32)
Overloads are sized using the motor nameplate FLA (NOT the NEC table FLC). For motors with service factor ≥ 1.15 or temperature rise ≤ 40°C: maximum overload = 125% of nameplate FLA. For all other motors: maximum overload = 115% of nameplate FLA.
If the motor won't start with the calculated overload size, NEC 430.34 allows increasing: to 130% for SF ≥ 1.15, or 120% for all others. If it still won't start, verify the motor and load — do not continue increasing overload size.
Overload devices include: thermal overload relays (heaters) in motor starters, electronic overload relays, or integral thermal protectors built into the motor windings.
Short-Circuit Protection (NEC 430.52)
SCGF protection for motors is intentionally oversized compared to conductor ampacity to allow motor starting current (typically 6-8× FLC) to flow without tripping. NEC Table 430.52 provides maximum fuse/breaker sizes:
Inverse-time circuit breaker: maximum 250% of FLC. Example: 14A FLC × 2.5 = 35A → use 35A breaker. If unable to carry starting current, may increase to maximum 400%.
Dual-element time-delay fuse: maximum 175% of FLC. Example: 14A × 1.75 = 24.5A → next standard: 25A fuse. May increase to maximum 225%.
Instantaneous-trip breaker (MCP): maximum 800-1300% of FLC depending on motor type. Used with motor starters that provide overload protection.
Common Mistakes to Avoid
Using nameplate FLA for conductor sizing — Conductors are sized using NEC table FLC values (430.248/250), NOT the nameplate. Using nameplate values often undersizes conductors.
Using NEC table FLC for overload sizing — Overloads are the one component sized from the nameplate FLA. Using the table FLC defeats the purpose of overload protection.
Applying standard 125% breaker rule to motors — Motor SCGF devices use Table 430.52 multipliers (250% for inverse-time, 175% for dual-element fuses), not the standard 125% from NEC 210.20.
Omitting the disconnect — Every motor must have a disconnect within sight and within 50 feet of the motor per NEC 430.102. The disconnect rating must be ≥ 115% of motor FLC.