NEC Article 430 — Motor Branch Circuit Protection for Industrial

Why Article 430 Matters for Every Motor Installation

NEC Article 430 is the most frequently referenced code article in industrial electrical work because nearly every industrial facility has motor loads — pumps, compressors, fans, conveyors, mixers, and machine tools. Article 430 defines the rules for sizing conductors, overcurrent protection, overload protection, and disconnecting means for motor branch circuits. The NEC Industrial Electrical guide maps how Article 430 fits within the broader NEC framework, and the VFD wiring guide applies these same rules to variable frequency drive installations.

The critical distinction in Article 430 that causes the most sizing errors: conductor sizing and overcurrent protection use Full-Load Current (FLC) from NEC Table 430.250, while overload protection uses the motor nameplate Full-Load Amperage (FLA). These two values are often different. Using nameplate FLA where the NEC requires table FLC (or vice versa) is the #1 Article 430 error found during electrical inspections.

Step 1 — Determine Motor Full-Load Current (430.6)

NEC 430.6(A)(1) requires using NEC Table 430.248 (single-phase), Table 430.249 (two-phase), or Table 430.250 (three-phase) to determine the full-load current for motor branch circuit conductor sizing and overcurrent protection. Do not use the motor nameplate FLA for this step.

Worked example: 50 HP, 460V, three-phase, squirrel-cage induction motor.

• NEC Table 430.250: 50 HP at 460V = 65A FLC
• This value is used for all conductor and overcurrent device sizing in the following steps.
• The motor nameplate might read 62A or 68A — the NEC table value (65A) is what you use regardless.

Step 2 — Size the Branch Circuit Conductors (430.22)

NEC 430.22(A) requires branch circuit conductors with ampacity not less than 125% of the motor FLC from the NEC table.

• 50 HP example: 65A × 1.25 = 81.25A minimum ampacity
• NEC Table 310.16, 75°C column (per 110.14(C) for standard terminations): 4 AWG copper THHN = 85A. Use 4 AWG.
• If ambient temperature exceeds 30°C or conduit fill exceeds 3 current-carrying conductors, apply derating factors per 310.15(B) and (C). At 40°C ambient: 4 AWG derated to 85A × 0.88 = 74.8A — insufficient. Upsize to 3 AWG (100A × 0.88 = 88A).

For feeders supplying multiple motors (430.24), the conductor ampacity must be at least 125% of the largest motor FLC plus the sum of all other motor FLCs. This applies to MCC feeder conductors and bus sizing. The MCC automation article covers multi-motor feeder design.

Step 3 — Size Short-Circuit and Ground-Fault Protection (430.52)

NEC Table 430.52 defines the maximum rating of the branch circuit short-circuit and ground-fault protective device as a percentage of motor FLC:

• Inverse-time circuit breaker: 250% of motor FLC. 50 HP example: 65A × 2.5 = 162.5A. Next standard size per NEC 240.6(A) = 175A breaker.
• Dual-element time-delay fuse: 175% of motor FLC. 65A × 1.75 = 113.75A. Next standard size = 125A fuse.
• Instantaneous-trip breaker (MCP): 800% (adjustable) to 1300% of motor FLC. MCPs are used in motor control center (MCC) combination starters where the overload relay provides the time-delay protection element. 65A × 8.0 = 520A. An MCP set at 520A instantaneous provides short-circuit protection while allowing the motor's starting current (typically 600–700% of FLC for a NEMA Design B motor) to pass without nuisance tripping.

The NEC intentionally sizes motor branch circuit overcurrent protection much higher than the conductor ampacity (175A breaker on 85A conductor) because motor starting current is 5–7 times the running current. The overload relay (Step 4) provides the running overcurrent protection. This two-layer protection scheme is unique to motor circuits and is a frequent source of confusion.

VFD Input Protection

For motors fed by VFDs, the overcurrent protection sizing follows the same NEC 430.52 rules. However, VFD manufacturers typically specify semiconductor fuses or Class J fast-acting fuses rather than inverse-time breakers. Semiconductor fuses have faster clearing times that protect the drive's input rectifier bridge. Using the wrong fuse type can void the VFD's short-circuit current rating (SCCR). Always check the drive installation manual for the specific fuse recommendation. The VFD fault codes guide covers the consequences of incorrect input protection sizing.

Step 4 — Size Overload Protection (430.32)

Overload protection is separate from the short-circuit protection sized in Step 3. The overload device protects the motor from sustained overcurrent due to mechanical overload, not from short circuits or ground faults.

NEC 430.32(A)(1) sizes the overload relay based on motor nameplate FLA (not the NEC table FLC used in Steps 1–3):

• Motors with service factor 1.15 or higher: overload trip at 125% of nameplate FLA. If nameplate reads 62A: 62A × 1.25 = 77.5A overload trip setting.
• Motors with service factor 1.0: overload trip at 115% of nameplate FLA. If nameplate reads 62A: 62A × 1.15 = 71.3A overload trip setting.
• Motors with temperature rise not exceeding 40°C: 125% of nameplate FLA (same as SF 1.15).

Overload relay types: bimetallic (Class 10, 20, or 30 trip curve), solid-state electronic (Allen-Bradley E300, Siemens SIRIUS 3RB, Eaton C440), and VFD electronic overload (built into every industrial VFD). Electronic overloads provide faster trip on phase loss and more accurate thermal modeling than bimetallic elements. The motor phase loss protection article covers why standard Class 20 bimetallic overloads are insufficient for single-phasing protection.

Step 5 — Disconnect Requirements (430.102, 430.109)

NEC 430.102 requires a disconnecting means for every motor:

• 430.102(A) — Controller disconnect: a disconnect must be in sight of and within 50 ft of the motor controller (VFD, starter, contactor). In an MCC, the MCC bucket disconnect satisfies this requirement.
• 430.102(B) — Motor disconnect: a disconnect must be in sight of the motor location. If the controller is not in sight of the motor, a separate disconnect at the motor is required. Common configurations: fused disconnect at the motor, non-fused disconnect at the motor (when the MCC provides the fused disconnect), or a cord-and-plug connection for portable motors.
• 430.109 — Disconnect type: the disconnect must be rated for the motor HP, voltage, and locked-rotor current. For motors over 2 HP at 300V or less, a motor-circuit switch (horsepower rated) is required. A general-purpose switch is acceptable only for motors 2 HP and under.

Complete Sizing Summary — 50 HP, 460V Motor Example

• Motor FLC (Table 430.250): 65A
• Branch circuit conductor (430.22): 65A × 1.25 = 81.25A → 4 AWG copper THHN (85A)
• Overcurrent protection — breaker (430.52): 65A × 2.5 = 162.5A → 175A inverse-time breaker
• Overcurrent protection — fuse (430.52): 65A × 1.75 = 113.75A → 125A dual-element time-delay fuse
• Overload protection (430.32, SF 1.15): 62A nameplate × 1.25 = 77.5A overload trip
• EGC (Table 250.122, 175A OCPD): 6 AWG copper
• Disconnect (430.109): HP-rated motor circuit switch, 50 HP 480V minimum

Common Article 430 Errors in the Field

• Using nameplate FLA instead of table FLC for conductor and OCPD sizing — this is the most common error. The NEC specifically requires Table 430.250 values for Steps 1–3.
• Confusing overload protection with overcurrent protection — the 175A breaker does NOT protect the motor from overload. The overload relay (sized at 77.5A) provides that protection. Both devices are required.
• Misapplying the "next standard size up" rule — NEC 240.6 permits rounding up to the next standard size only for motor branch circuit overcurrent protection per 430.52. This exception does not apply to feeder overcurrent protection or non-motor branch circuits.
• Missing the second disconnect — if the motor controller is not within sight of the motor (common when the MCC is in a separate electrical room), a second disconnect at the motor location is required per 430.102(B). Many installations miss this requirement.
• Undersizing the EGC for VFD circuits — Table 250.122 sizes the EGC based on the overcurrent device rating, but VFD circuits benefit from a larger EGC to handle high-frequency common-mode current. The VFD wiring guide recommends a dedicated insulated copper EGC in addition to metallic conduit.

NFM Consulting's SCADA and industrial controls team designs NEC-compliant motor branch circuits and VFD installations for industrial facilities, including motor control center specification, protection coordination studies, and commissioning across Texas.