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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
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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
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

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 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

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