11.1 Motor Branch Circuits & Conductors

Why Motor Branch-Circuit Conductors Matter

The NEC treats motors differently from general branch circuits because motors draw a steady full-load current (FLC) and a much higher starting inrush. The Calculations part of the TX Journeyman exam (26 questions, 110 minutes) regularly tests NEC 430.22 single-motor conductor sizing and 430.24 multiple-motor feeder sizing, so you must know the Table 430.250 values and the 125% multiplier cold.

NEC 430.22 - Single Motor Branch-Circuit Conductors

NEC 430.22 requires branch-circuit conductors for a single continuous-duty motor to have an ampacity not less than 125% of the motor's full-load current (FLC). The FLC used for conductor sizing comes from Table 430.250 (3-phase AC motors), Table 430.248 (single-phase AC motors), Table 430.247 (DC motors), or the motor nameplate - but for conductors, disconnects, and short-circuit/ground-fault protection, you use the Table value. The nameplate FLC is used only for overload (430.32) sizing, which is covered in Chapter 7.

The 125% factor accounts for the continuous heating of a motor under sustained load.

Table 430.250 - 3-Phase AC Motor FLC (excerpt, copper conductors)

Table 430.248 - Single-Phase AC Motor FLC (excerpt)

NEC 430.24 - Multiple Motors on a Feeder

When a feeder supplies two or more motors, NEC 430.24 requires the feeder conductor ampacity to be not less than 125% of the largest motor's FLC plus the sum of the FLCs of all other motors on the feeder, plus the ampacity of any non-motor loads. The "largest motor" is the one with the highest FLC, not necessarily the highest horsepower - a 5 hp 230V motor (15.2 A) is larger by FLC than a 3 hp 460V motor (5.6 A).

Worked Example - 5 hp, 230V, 3-Phase Motor

Size the branch-circuit conductors for a 5 hp, 230V, 3-phase continuous-duty motor.

Step 1 - FLC from Table 430.250: 15.2 A

Step 2 - Apply 430.22 (125%):

• 15.2 A x 1.25 = 19.0 A

Step 3 - Select conductor from Table 310.16 (75C Cu column):

• 12 AWG Cu THWN = 20 A

Step 4 - Check 240.4(D) small-conductor rule:

• 240.4(D) caps 12 AWG copper at 20 A.
• 19.0 A <= 20 A, so 12 AWG Cu THWN is acceptable.

The 430.22 exception (which allows a reduced factor for certain continuous-duty motors with documented alternate loading) does not apply to a standard continuous-duty motor here, so the 125% multiplier stands.

Key Distinction: Table FLC vs. Nameplate FLC

Mixing these up is one of the most common exam errors. When a question says "size the conductors," reach for Table 430.250, not the nameplate.

Worked Example - Two Motors on a Feeder (430.24)

A feeder supplies a 10 hp, 230V, 3-phase motor (FLC 28 A) and a 5 hp, 230V, 3-phase motor (FLC 15.2 A), plus a 20 A non-motor lighting load. Size the feeder conductors.

Step 1 - Identify the largest motor FLC: 28 A (the 10 hp motor).

Step 2 - Apply 430.24:

• 125% of largest motor FLC: 28 A x 1.25 = 35.0 A
• Sum of remaining motor FLCs: 15.2 A
• Non-motor load: 20 A
• Total = 35.0 + 15.2 + 20 = 70.2 A

Step 3 - Select conductor from Table 310.16 (75C Cu):

• 6 AWG Cu THWN = 65 A (too small)
• 4 AWG Cu THWN = 85 A (>= 70.2 A, acceptable)

Result: 4 AWG Cu THWN. The 125% multiplier applies only to the largest motor; the other motor and the non-motor load are added at 100%.

Conductor Correction and Adjustment - 310.15

The 430.22 125% ampacity is the minimum before environmental factors. After selecting a conductor from Table 310.16, you must still apply correction for ambient temperature (310.15(B)) and adjustment for more than three current-carrying conductors in a raceway or cable (310.15(C)(1)). Motor circuits often run in hot mechanical rooms and in crowded conduits, so both factors appear on the exam.

Correction (310.15(B)): Table 310.16 ampacities are based on a 30C ambient. Where the ambient exceeds 30C, multiply the table ampacity by the Table 310.15(B)(1) correction factor for the conductor temperature rating. A 12 AWG Cu THWN-2 conductor taken from the 90C column (30 A) in a 40C ambient is derated to 30 A x 0.91 = 27.3 A. You may use the 90C column for derating only when both the conductor and its terminations are 90C-rated; the final ampacity still cannot exceed the 75C (or 60C) termination-limited value.

Adjustment (310.15(C)(1)): When more than three current-carrying conductors share a single raceway or cable, apply Table 310.15(C)(1): 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50%. A neutral carrying only the unbalanced current of a 3-phase, 4-wire wye circuit with linear loads is not counted; with nonlinear loads (variable-frequency drives, LED drivers, harmonics), the neutral is counted as a current-carrying conductor.

The 430.22(A) Exception

The general 125% rule has a narrow exception. For wound-rotor motors and for motors on variable-duty or short-time-rated service, 430.22(A) Exception No. 2 permits a reduced conductor ampacity where the duty cycle does not continuously load the motor. This exception does not apply to a standard continuous-duty squirrel-cage motor. Reach for it only when the question explicitly names wound-rotor, variable-duty, or short-time-rated service.

Largest Motor for Feeder OCPD - 430.62

NEC 430.62 sets the feeder short-circuit and ground-fault protection for a multiple-motor feeder: the OCPD rating is based on the largest motor short-circuit protection (from 430.52) plus the sum of the FLCs of all other motors on the feeder. The largest motor is again the one with the highest Table FLC. This differs from the 430.24 conductor calc: the conductor uses 125% of the largest plus 100% of the rest, while the OCPD uses the largest motor 430.52 percentage plus 100% of the rest.

Equipment Grounding Conductor - 430.126

The equipment grounding conductor for a motor circuit is sized from Table 250.122 based on the rating of the motor-circuit overcurrent protective device (the short-circuit device per 430.52), not on the conductor ampacity. A 60 A breaker protecting a motor circuit requires at least 10 AWG copper EGC per Table 250.122. Candidates often try to size the EGC from the conductor ampacity rather than from the OCPD rating - a consistent trap.