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430.22

Single Motor Branch-Circuit Conductor Sizing

MotorsNEC 2023CEC Equivalent: CEC Rule 28-106

Motors are assumed to be continuous-duty loads, meaning they can run for three hours or more without stopping. When a motor runs continuously, the conductors feeding it build up heat steadily. To provide a thermal safety cushion, the NEC says you need to oversize the wire by 25 percent above what the motor nominally draws. So if Table 430.250 says your 10-HP 3-phase motor draws 14 amps, you need conductors rated for at least 17.5 amps (14 times 1.25). You then go to Table 310.16 and pick a wire size that meets or exceeds that adjusted ampacity. An important detail: you always use the FLC from the NEC tables (per 430.6), never the nameplate value, as the starting point for this calculation.

When You Need This

  • Sizing branch-circuit wiring for a new motor installation — this is the very first calculation you do
  • Replacing a motor and confirming the existing wiring is adequate for the new unit
  • Designing a control panel layout where each motor gets a dedicated branch circuit
  • Preparing for an exam question that asks you to size conductors for a single motor circuit
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Key Points

1Conductors must have ampacity of at least 125 percent of the motor FLC from NEC tables
2Always use the table FLC (per 430.6), not the motor nameplate FLA, as the starting point
3This 125-percent requirement is specifically for continuous-duty motors — short-time or intermittent duty motors have different rules in 430.22(E)
4After calculating 125 percent of the FLC, select a wire from Table 310.16 that meets or exceeds that value
5Do not forget to also apply temperature correction and conduit-fill derating if conditions warrant it
6For the highest motor among multiple motors, use this 125 percent rule; other motors on the same feeder are added at 100 percent per 430.24

Common Mistakes

Multiplying the nameplate FLA by 125 percent instead of using the NEC table FLC value

Forgetting the 125-percent multiplier entirely and just selecting wire based on the raw FLC

Confusing the 125-percent conductor sizing rule with the overload protection sizing, which uses a different percentage and different current value

Not checking terminal temperature limits per 110.14(C) after selecting the wire

Exam Tip

The math is straightforward: FLC from table times 1.25, then pick a wire from 310.16. But the exam may combine this with derating. If the question mentions a hot environment or a crowded conduit, you need to apply derating AFTER the 125-percent calculation.

Frequently Asked Questions

The conductors between the VFD and the motor are typically sized at 125 percent of the motor FLC. The conductors from the supply to the VFD input are sized based on the VFD input current rating, which may differ from the motor FLC.

NEC 430.22 is specifically for a single motor on a branch circuit. If you are running conductors to supply two or more motors, you need to follow 430.24, which says to add 125 percent of the largest motor FLC plus 100 percent of all other motor FLCs.

Inline Tools

Motor FLC Calculator

Look up full-load current for any motor HP and voltage

Related Code Sections

430.6Determination of Motor Full-Load Current430.52Motor Branch-Circuit Short-Circuit and Ground-Fault Protection310.16Ampacity of Insulated Conductors

This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.