NEC Article 430 Motors: Sizing, Overloads & Disconnects

Article 430 is one of the longest and most complex articles in the NEC, and it shows up heavily on both journeyman and master exams. The math-based questions, conductor sizing, overcurrent device sizing, and overload sizing, all follow specific formulas with specific tables, and getting the sequence right is what separates a passing score from a miss.

This guide covers the five areas that appear most frequently on the exam: FLC tables, conductor sizing, overload protection, branch-circuit overcurrent protection, and disconnecting means.

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The Critical Rule: Use the NEC Table, Not the Nameplate

Before diving into the sections, there's one rule that governs almost everything in Article 430 and is frequently tested on its own:

NEC 430.6 states that for the purpose of determining conductor ampacity, the overcurrent device rating, and the ampere rating or setting of motor control devices, the values in Tables 430.247, 430.248, 430.249, and 430.250 shall be used instead of the motor nameplate current rating.

The nameplate current is used for overload protection sizing only. For everything else, conductors, breakers, fuses, use the NEC table FLC value.

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NEC Motor FLC Tables

Table 430.250, Three-Phase AC Motors (Most Common on Exams)

Selected values from Table 430.250:

Table 430.248, Single-Phase AC Motors

Selected values from Table 430.248:

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430.22, Conductor Sizing: Single Motor

Conductors supplying a single continuous-duty motor must have an ampacity of not less than 125% of the motor's full-load current as listed in the applicable table.

Formula: Minimum conductor ampacity = Table FLC × 1.25

Worked Example: Size conductors for a 10 HP, 460V, three-phase motor.

1. Look up Table 430.250: 10 HP, 460V = 14A
2. Multiply by 125%: 14 × 1.25 = 17.5A
3. Select conductor with ampacity ≥ 17.5A → 12 AWG copper (rated 20A, per Table 310.16 at 60°C)

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430.24, Conductor Sizing: Several Motors

Where a feeder supplies several motors, the conductor ampacity must be at least:

125% of the largest motor's FLC + sum of the FLC of all other motors

Worked Example: A feeder supplies three 3-phase motors: 10 HP/460V, 5 HP/460V, and 3 HP/460V.

FLC values from Table 430.250:

• 10 HP/460V = 14A (largest)
• 5 HP/460V = 7.6A
• 3 HP/460V = 4.8A

Minimum ampacity = (14 × 1.25) + 7.6 + 4.8 = 17.5 + 12.4 = 29.9A

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430.32, Overload Protection

Continuous Duty Motors Over 1 HP

Separate overload protection must be sized per NEC 430.32(A). The nameplate current is used for overload sizing (this is the exception to the "use the table" rule).

Key exam point: The nameplate is used for overload sizing. The NEC table is used for conductor and overcurrent device sizing. These are two different values.

If the overload device sized at the above percentages is not sufficient to start the motor or carry the load, it can be increased to a maximum of:

• 140% of nameplate FLC for motors with SF ≥ 1.15 or ≤40°C rise
• 130% of nameplate FLC for all other motors

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430.52, Branch-Circuit Short-Circuit and Ground-Fault Protection

This section, along with Table 430.52, is one of the most tested in Article 430.

The branch-circuit overcurrent device must not exceed the values in Table 430.52. Unlike conductor sizing, this protection device rating is applied to the table FLC (430.6).

Table 430.52, Maximum Rating or Setting

If the calculated value does not correspond to a standard device size, the next higher standard device may be used.

If the calculated size is still not sufficient to start the motor, higher percentages are permitted up to:

• Nontime-delay fuse: up to 400%
• Time-delay fuse: up to 225%
• Inverse time CB: up to 400%

Worked Example: Table 430.52

Size the dual-element time-delay fuse for a 5 HP, 230V, single-phase motor.

1. Table 430.248: 5 HP, 230V = 28A
2. Table 430.52, single-phase, dual-element: 175%
3. 28 × 1.75 = 49A
4. Next standard fuse size down: 45A (or next up: 50A, use next higher standard size per 430.52)
5. 50A dual-element fuse

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430.62, Motor Feeder Short-Circuit and Ground-Fault Protection

The feeder overcurrent device protecting several motors shall not exceed the largest branch-circuit overcurrent device for any motor supplied, plus the sum of the FLC ratings of all other motors on the feeder.

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430.102, Disconnecting Means Location

Two separate disconnect requirements exist:

430.102(A), Controller disconnect: A disconnecting means must be located in sight of the motor controller.

430.102(B), Motor disconnect: A disconnecting means must be located in sight of the motor and the driven machinery.

"In sight of" is defined in Article 100 as visible and not more than 50 feet distant.

Exception: If the disconnect cannot be located in sight of the motor (locked-rotor, dangerous location), the disconnect can be out of sight if it is capable of being individually locked in the open position.

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430.109, Type of Disconnecting Means

For motors rated 600V or less, the disconnecting means must be one of:

1. A motor-circuit switch rated in horsepower
2. A circuit breaker
3. A molded case switch
4. A manual motor controller with a "Suitable as Motor Disconnect" marking (for motors 2 HP or less, 300V or less)

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430.110, Ampere Rating of Disconnecting Means

The disconnecting means must be rated at least 115% of the full-load current rating of the motor.

Use the table FLC (430.248 or 430.250), not the nameplate.

Worked Example: What is the minimum disconnect rating for a 7.5 HP, 460V, three-phase motor?

1. Table 430.250: 7.5 HP, 460V = 11A
2. 11 × 1.15 = 12.65A
3. Minimum disconnect rating: 15A (next standard size)

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430.83, Controller Rating

Motor controllers must be rated in horsepower and must be rated at not less than the horsepower rating of the motor at the applied voltage.

Exception: a general-use switch with an ampere rating at least twice the full-load current of the motor can be used as a controller for stationary motors of 2 HP or less at 300V or less.

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Article 430 Calculation Quick Reference

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Related Reading

• NEC Article 220, Load Calculations
• NEC Article 240, Overcurrent Protection
• NEC Article 440, HVAC Equipment
• Electrical Math Study Guide
• Start Your Practice Test