If I'm reading NEC 430 Part III correctly, a 5 HP, 230 V, 3-phase motor with 12.8 A full-load current needs a 16 A circuit (125% of full-load current). So, 12 ga wire with a 20 A breaker should do. The 12 ga THHN wire has an ampacity of 30 A, derated for 7-9 conductors makes it 21 A, so that should be more than adequate, right?

I’m looking to run service for this dust collector. It quotes 12.2 FLA, but looking at a Leeson motors list, I found 12.0, 12.8, and 15 FLA for various 5 HP 3-phase motors.

But my service is 208 V 3-phase. Does that change anything?

  • How are you planning to run a 230V motor on a 208V supply? 230V sounds like an odd number for a 3-phase. In Europe, phase-to-neutral is 230V, and phase-to-phase (in a three phase supply) is 415V. In the US, I believe phase-to-neutral is 115V, and domestic supply comes with two phases (at 180⁰) where phase-to-phase is 230V. If you had three phases where phase-to-neutral was 115V, then phase-to-phase would be 208V - but I'm not sure how you'd get that in the US. Aug 28 '19 at 11:46
  • What size conduit?
    – JACK
    Aug 28 '19 at 12:47
  • The conduit is 3/4”
    – Rick
    Aug 28 '19 at 13:13
  • 1
    We really need to look at the motor faceplate. Some motors are fine with the lower voltage some will not run. 12 awg wire is limited to 20 amps but there is an exception for in code for motors that allow for a larger breaker to be used so that won’t be a problem here. We do need the motor face plate to know for sure – As far as 230 3 phase that is what used to be used years back before we switched to 208.
    – Ed Beal
    Aug 28 '19 at 13:54

When selecting conductors for motors in the US, you do NOT use the motor nameplate FLA, you use values from a table in the NEC, in this case table 430.250. The value in that table for 5HP @208V is 16.7A, so you use 125%, ergo 20.9A. The 12ga 30A rating you quote is for 90C, you can only use that value when de-rating, so assuming you are correct on the de-rate factor (I'm not checking your work), you are still fine.

The only thing you use the nameplate FLA for is in selecting the Overload Protection.

  • Oh, I see, that defines the wiring requirements, but the circuit breaker is defined in 430.32(A)(1), is that right? The note is “to protect a motor from an overload, the motor nameplate full-load cur- rent is used to select the overload protection rather than the full-load current values from tables 430.248 through 430.250, which are used to select the feeder and branch-circuit wiring.”
    – Rick
    Aug 28 '19 at 20:30
  • 2
    The overload protection device (OL Relay) is what they are referring to in 430.32. For the Circuit Breaker, you use 430.52, and the subtle tell in the chart there is that they use the term "Motor Full Load Current", not "Motor NAMEPLATE Full Load Current", which means that for selecting the OCPD, you use the table 430.250 values as well.
    – JRaef
    Aug 28 '19 at 20:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.