Motor Branch Circuit Sizing (NEC Article 430)
Why this matters
Motor branch circuits do not follow the simple "ampacity equals breaker" rule. The conductor is sized for 125 percent of motor full-load current; the overcurrent device may be sized at 175 to 250 percent of full-load to allow inrush during starting; the running overload protection is a separate device entirely (the overload relay in the starter) sized at 115 to 125 percent of full-load. An electrician who applies branch-circuit rules to a motor circuit produces an install that either nuisance-trips on every start or carries no real overload protection. NEC Article 430 is its own ecosystem; getting it right is what separates light commercial work from advanced industrial.
The five separate considerations for a motor circuit
NEC 430 separates motor circuit design into five distinct sizing tasks:
- Conductor ampacity (430.22) - sized at 125 percent of motor full-load current
- Branch-circuit short-circuit and ground-fault protection (430.52) - sized at 175 to 250 percent of FLC to allow inrush
- Overload protection (430.32) - sized at 115 to 125 percent of FLC for motor running current
- Disconnect (430.102) - rated for motor + contactor
- Controller / starter (430.81) - rated for the motor and contactor
These are five separate devices in many installations. A combination motor starter integrates the disconnect, the OCPD, the controller, and the overload into one enclosure.
Why inrush matters
A typical induction motor draws 6 to 8 times full-load current during starting (the "locked rotor current" or "starting current"). This lasts 1 to 5 seconds while the rotor accelerates from zero to nominal speed.
A 10A motor draws 60 to 80 A during start. A 30A motor draws 180 to 240 A.
If the branch breaker is sized at 125 percent of FLC (the conductor rule), it trips on every start. The branch OCPD must allow the inrush, which is why 430.52 permits 175 to 250 percent of FLC.
The actual overload protection (the running-current protection) is the overload relay in the starter. It is calibrated to trip on sustained overload (a bearing dragging, a pump shaft seized), not on the brief inrush.
FLC values from NEC Table 430.250 (three-phase) and 430.248 (single-phase)
The full-load current of a motor is taken from NEC tables, NOT from the motor nameplate, for circuit sizing purposes. NEC tables give industry-standard FLC values:
Three-phase 60 Hz 460V (typical for 480V system):
| HP | FLC (A) |
|---|---|
| 1 | 2.1 |
| 1.5 | 3.0 |
| 2 | 3.4 |
| 3 | 4.8 |
| 5 | 7.6 |
| 7.5 | 11 |
| 10 | 14 |
| 15 | 21 |
| 20 | 27 |
| 25 | 34 |
| 30 | 40 |
| 40 | 52 |
| 50 | 65 |
| 60 | 77 |
| 75 | 96 |
| 100 | 124 |
| 125 | 156 |
| 150 | 180 |
| 200 | 240 |
Single-phase 230V:
| HP | FLC (A) |
|---|---|
| 1 | 8 |
| 1.5 | 10 |
| 2 | 12 |
| 3 | 17 |
| 5 | 28 |
| 7.5 | 40 |
| 10 | 50 |
The nameplate FLA (Full-Load Amps) is used only for the overload protection sizing (430.32), not for the branch circuit conductors or OCPD.
Conductor sizing (430.22)
Conductor ampacity must be at least 125 percent of the motor table FLC.
Example: 5 HP 460V three-phase motor, FLC 7.6 A from Table 430.250.
Conductor minimum ampacity = 1.25 × 7.6 = 9.5 A.
NEC 310.16: 14 AWG copper at 75 C = 20 A. 14 AWG meets the ampacity requirement.
If the run is long, voltage drop may force upsizing. Compute per the voltage drop reference.
Branch-circuit OCPD (430.52)
NEC Table 430.52 specifies the maximum OCPD percentage by motor type and protection device type:
| Motor type | Non-time-delay fuse | Time-delay fuse | Inverse-time breaker | Instantaneous-trip breaker |
|---|---|---|---|---|
| Single-phase | 300 percent | 175 percent | 250 percent | 800 percent |
| AC squirrel-cage, design A | 300 percent | 175 percent | 250 percent | 800 percent |
| AC squirrel-cage, design B (most common) | 250 percent | 175 percent | 250 percent | 800 percent |
| DC | 150 percent | 150 percent | 150 percent | n/a |
For a typical 5 HP three-phase motor (design B squirrel-cage), with an inverse-time circuit breaker:
OCPD = 250 percent × 7.6 = 19 A. Standard sizes per 240.6: round UP to the next standard size, 20 A breaker.
Note: 430.52(C) allows rounding up to the next standard size; 430.52(C)(1) Exception 1 allows further increase if the standard size is insufficient to allow the motor to start.
If a 20A breaker still nuisance-trips on start, NEC permits up to 400 percent for design B motors (the upper limit). Branch conductor sizing is unchanged; only the OCPD increases.
Overload protection (430.32)
The overload relay in the motor starter must trip on sustained overload but allow inrush.
For motors with marked service factor 1.15 or greater: overload at 125 percent of nameplate FLA maximum. For motors with marked service factor under 1.15: overload at 115 percent of nameplate FLA maximum.
The overload relay (heaters in older starters, electronic in modern) is sized to the motor nameplate.
Example: 5 HP motor with nameplate FLA of 6.8 A and service factor 1.15.
Overload setting maximum = 1.25 × 6.8 = 8.5 A.
The overload relay is selected with the closest available heater rating not exceeding 8.5 A.
Disconnect (430.102)
Within sight of and not more than 50 feet from the motor controller, except for industrial process motors and large motors with specific exceptions per 430.102(B).
Rating: at least 115 percent of motor FLC, and must be at least the next NEC standard size. For the 5 HP example, FLC × 1.15 = 8.7 A; a 30 A motor-rated disconnect is the smallest commonly available.
The disconnect can be:
- A fused safety switch (NEMA 1 or 3R)
- A unfused safety switch (paired with separate OCPD)
- A circuit breaker (also providing branch OCPD)
- A motor starter with door interlock
For combination starters, the integral disconnect satisfies 430.102.
Combination starter
A combination motor starter integrates:
- Fused or breaker disconnect
- Magnetic contactor (the controller)
- Overload relay
- Enclosure
Brands: Square D 8736, Eaton/Cutler-Hammer A201, Allen-Bradley 509. Modern smart starters add motor protection (phase loss, ground fault, harmonic distortion) and communication interfaces.
For most light commercial motor installs, the combination starter is the right approach: it satisfies all of disconnect, OCPD, controller, and overload in one listed assembly.
Worked example: 5 HP three-phase 480V pump motor
References
- NFPA 70 (NEC) 2023, Article 430 (Motors, Motor Circuits, and Controllers).
- NFPA 70 (NEC) 2023, Article 440 (Air-Conditioning and Refrigerating Equipment) - similar rules for HVAC compressors.
- NFPA 70 (NEC) 2023, Section 240.6 (Standard Ampere Ratings for fuses and breakers).
- NEMA ICS 6 (Industrial Control and Systems: Enclosures).
- NEMA MG-1 (Motors and Generators).
- IEC 60947-4-1 (Low-voltage switchgear and controlgear, motor starter).
- Manufacturer literature: Square D, Eaton, Allen-Bradley, Siemens motor starter catalogs.
- Manuall internal: Electrical Conductor Ampacity Reference, Electrical Voltage Drop Feeder Sizing.