I just bought a new stationary 5HP air compressor, and I need to know what wire gauge I should use for it. Relevant compressor details:

  • 40 Amp draw
  • Magnetic starter
  • Length of run from power box to air compressor is 25 feet
  • This calculator might help. See NEC ARTICLE 430 — MOTORS, MOTOR CIRCUITS, AND CONTROLLERS for more info, or consult the owners manual of the compressor. – Tester101 Mar 23 '12 at 16:36
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    Don't forget the subpanel! – Jay Bazuzi Mar 23 '12 at 17:45
  • Check Full Load Amperage FLA on the motor. Then base wire size on that. Remember if this is off a subpanel, include the length to the main service panel and make sure the wires are appropriate size. – Tester101 Mar 23 '12 at 23:07
  • A model number might help if you need exact calculations. – Tester101 Mar 23 '12 at 23:08
  • @Tester101 I was under impression you base your wires on the FLC and overload protection on FLA. – Gregory Furmanek Oct 17 '17 at 5:06

Since it's on a 40 amp breaker I would use #8 THHN. 5HP is only 3750 watts (1 HP = 750 Watts (rounding up)) so you are only pulling less than 18 amps when running. Start up amps will be around 36 amps (rule of thumb) so that should be covered by the 40 amp breaker. You can also use @Tester101 suggestion also.

If this is an expensive piece of garage furniture you might make sure that magnetic starter has heater protection and phase drop protection, which will even work on single phase which will shut down the motor if one of the legs drop out. Heaters are over current protection. This sort of depends on the compressor because the starter could cost more than the compressor.

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  • 1
    +1 for #8. I might do #6 to a subpanel (depending on what else you'll be running off the panel), then #8 to the compressor outlet. – Tester101 Mar 30 '12 at 16:23

The question is missing a few pieces of data to properly calculate wire gauge. Assuming your motor is a 230V 5HP motor your FLC (NEC table Table 430.248) should be about 28A and as such you may be able to use 10AWG copper wire.

On the other hand if the motor runs on 120V the FLC would be 56A and as such you need to run wire for 60A. Assuming your equipment is designed to handle 75ºC you may need 6AWG wire.

The wire is sized based on FLC (code book). Your over load protection should be sized based on FLA nameplate on the motor. Although I suspect a compressor should already have overload protection.

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  • The title of the question states it's a 230V motor, so yes, your 10AWG should be correct – ThreePhaseEel Oct 17 '17 at 11:42

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