Commercial Exhaust Fan

[joebell]

I need to replace wiring to 2 commercial hood fans on a roof. The wiring in place now is romex with SJ cord. The motors are 2 speed 120 volt and there are 2 HP ratings on each 3/4 and 1/3. Am I correct in assuming the HP are directly related to the speed? Also when size the conductors and breaker for this I will also assume I use the higher of the 2 ratings.

How do I size the conductors and breaker for this? Under 430.248 the FLC is 13.8 amps and 430.22 says this should be a continuos load so I need to apply 125% which brings it the ampacity to 17.25 so each fan should be wired with #12. Currently these fans share a single 20 amp breaker.

The fans are also thermally protected so I do not beleive overload protection is required. As for the breaker I look to table 430.52 I could size the breaker to 250% of the FLC. This gives me 34.5 amps and I am allowed to go to the next higher breaker size which according to 240.6 is 35 amps. Do they even make this size breaker in a single pole?

The speed control for the fans is a 3way switch to switch the speeds I know this can't be right considering they are the 15 amp residential variety any thoughts on how to rectify this issue as well?

Please let me know if this is correct or if I missed a step.

Thanks in advance
Joe

 

[benaround]

Joe, 430.22(A) that 125% is for the conductor ampacity not to add to the

motor amps. So, looks to me that a 20a circuit to each 13.8a motor would

work. Do they make a 20a, HP rated, three way switch ? If they do I guess

that would be compliant.

 

[LarryFine]

Joe, as a side note, I'd suggest asking the users of the fans whether they really need both speeds. I do the wiring on a fair amount of commercial-kitchen fire-suppression retro-fits, and I've not had anyone who wants to keep the two speeds or the occasional variable-speed control. They're happy with high speed.