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. For VFD-fed motors, Part X (430.122) and the VFD wiring guide modify the line-side conductor and fuse rules.

The critical distinction in Article 430 that causes the most sizing errors: conductor sizing and short-circuit/ground-fault 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.

VFD / adjustable-speed drive (ASD) exception: when the motor is fed through a VFD, NEC Article 430 Part X (especially 430.122) modifies the conductor rules — line-side conductors are sized from the drive rated input current, not motor table FLC. See the VFD wiring and NEC 430 guide. The steps below are the across-the-line / conventional motor-branch sequence.

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: maximum 250% of motor FLC. 50 HP example: 65A × 2.5 = 162.5A. Per 430.52(C)(1), select the largest standard rating from 240.6(A) that does not exceed 162.5A → 150A breaker. If 150A will not allow the motor to start, Exception 1 permits the next higher standard size → 175A. Further exceptions raise the percentage ceiling only when documented starting issues remain.
• Dual-element time-delay fuse: maximum 175% of motor FLC. 65A × 1.75 = 113.75A → largest standard size not exceeding that is 110A. Exception 1 allows the next higher standard size (125A) only if 110A will not start the motor.
• Instantaneous-trip breaker (MCP): Table 430.52 starts at 800% of motor FLC for many Design B applications; exceptions allow higher settings (commonly up to 1300% for Design B, and higher for some Design C/D / premium-efficiency cases) when needed to start the motor without nuisance trip. Confirm the Design letter and the current NEC Table 430.52 / exceptions. MCP trip settings are not rounded like 240.6 fuse/breaker ratings. MCPs are used in MCC combination starters where the overload relay provides the running (time) protection. Example at 800%: 65A × 8.0 = 520A instantaneous setting — enough for typical Design B locked-rotor (~6×) while still clearing faults.

The NEC intentionally allows motor branch short-circuit devices much higher than conductor ampacity (for example a 150A or Exception-1 175A breaker on an 85A conductor) because motor starting current is typically 5–7 times running current. The overload relay (Step 4) provides the running overcurrent protection. This two-layer scheme is unique to motor circuits and is a frequent source of confusion — including the myth that you always “round up” from the Table 430.52 percentage.

VFD / ASD Systems (Part X — do not skip)

Do not size the VFD line side as if it were a plain motor branch under 430.22 / 430.52 alone:

• Line conductors (430.122(A)): ampacity at least 125% of the drive’s rated input current (nameplate / manual), even if the connected motor FLC is smaller.
• Motor leads (430.122(B)): generally 125% of motor FLC per 430.6, unless the drive is listed and marked “Suitable for Output Motor Conductor Protection.”
• Short-circuit device on the drive input: must satisfy the drive’s listed SCCR fuse class/type and rating (semiconductor, Class J, Class CC, Type gR, etc.). Using a “430.52-sized” inverse-time breaker that is not on the manufacturer’s SCCR schedule can void the listing. Table 430.52 still matters for motor-branch concepts, but the drive manual governs the input device.

Full cable, shield, and filter details are in the VFD wiring guide. Fault codes from wrong input protection are covered in the VFD fault codes guide.

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 from the motor controller (VFD, starter, contactor). NEC Article 100 defines “in sight from” as visible and not more than 50 ft distant. In an MCC, the MCC bucket disconnect satisfies this requirement.
• 430.102(B) — Motor disconnect: a disconnect must be in sight from the motor location. If the controller is not in sight from the motor, a separate disconnect at the motor is required (with limited exceptions). 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 / 430.110 — Disconnect type and rating: use a type listed in 430.109 (for example an HP-rated motor-circuit switch, molded-case switch/breaker, or other permitted devices). Ampacity of the disconnect must be at least 115% of motor FLC per 430.110(A). General-use snap switches are limited (commonly stationary motors 2 HP or less and 300 V or less under 430.109) — confirm the subsection that matches the installation.

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 max → 150A inverse-time breaker (does not exceed 250%); Exception 1 → 175A only if 150A will not start
• Overcurrent protection — fuse (430.52): 65A × 1.75 = 113.75A max → 110A dual-element time-delay; Exception 1 → 125A only if needed to start
• Overload protection (430.32, SF 1.15): 62A nameplate × 1.25 = 77.5A overload trip
• EGC (Table 250.122): 6 AWG copper for a 100–200 A OCPD (covers both the 150A base and Exception-1 175A cases)
• Disconnect (430.109 / 430.110): HP-rated motor-circuit switch (or other permitted type), ampacity ≥ 115% of FLC, 50 HP / 480 V class

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 short-circuit device (150A or Exception-1 175A breaker in the example) does NOT protect the motor from overload. The overload relay (sized at 77.5A) provides that protection. Both devices are required.
• Always “rounding up” from Table 430.52 — the base rule is do not exceed the table percentage; Exception 1 to 430.52(C)(1) allows the next higher 240.6(A) size only when the smaller device will not start the motor. That exception does not apply to ordinary feeder or non-motor branch-circuit OCPD sizing.
• 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.