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Breaker Size
- Thread starter C Ward
- Start date
Hello,We do recommend using a 30 amp breaker with # 10 wire.
I'm trying to understand how to wire up the Leeson 5 HP motor compliant with the NEC. I'm not sure the above is correct, has anyone else looked into this? My initial read through the NEC is below, but perhaps I am missing something.
Thanks, Wayne
Motors are subject to Article 430 of the NEC. According to 430.6(A)(1), the ampacity of conductors, switches, and over current protection devices is not based on the nameplate FLA rating but on the nameplate HP rating, which is then translated into an ampere rating according to Table 430.148. That table specifies 28 amps for a 5 HP motor at 220V-240V. I assume the Leesen motor used is a capacitor start induction run motor, so none of the exceptions to 430.6(A)(1) would apply. [If it is a shaded-pole or permanent-split capacitor motor, then the nameplate FLA can be used and most of what follows is incorrect.]
Then section 430.22 specifies that the branch-circuit conductors supplying a single continuous duty motor should be sized to 125% of the motor's rating as determined in 430.6(A)(1). So that would be 35 amps.
If the wiring method is NM cable (romex), then article 334.80 specifies that the ampacity should be determined using a 60C conductor temperature rating. Table 310.16 then tells you that for 30C ambient temperature and 60C conductor temperature, the ampacity of #10 copper is 30 amps and of #8 copper is 40 amps. So I believe that in NM cable, a #8 copper branch circuit is required.
If the wiring method is a form of conduit using conductors with a 75C rating, or is type SE cable composed of conductors with a 75C rating, then you could use the 75C column of Table 310.16 _if_ the motor terminals and the breaker terminals are rated for 75C. In the 75C column, the ampacity of #10 copper is 35 amps, so that would be OK. Does anyone know if the motor terminals of the Leesen motor are rated for 75C? I believe most residential load centers and breakers are rated for 75C, but this bears verifying.
I assume the Leesen motor has a thermal protector built in, so motor overload protection is taken care of.
As for the branch circuit overprotection device, section 430.52 indicates that for an inverse time breaker (which I believe most residential thermal/magnetic breakers are), the breaker rating can be up to 250% of the motor rating. So that would allow a breaker up to 70 amps for the motor circuit. But if a breaker of 30 amps does not suffer from nuisance trips due to inrush current, then there is nothing wrong with using a 30 amp breaker, and that seems preferable.
A motor controller is required to turn the motor on and off, which can be the breaker under some circumstances. Otherwise, 430.83(A)(1) says the controller shall have a horsepower rating not lower than the horsepower rating of the motor. I'm not clear on whether the 30 amp relay offered here is OK to use, or if you need a relay that is specifically labeled "5HP @ 240V".