air handling unit conductor & breaker size

[shaun]

alright this is the question/statement of the day. My partner wanted me to do a calc for him on paper while he did it in his head. it was for an AHU motor that is rated @ 15hp, 208v, 3ph. I had to detrmine the amperage so he could get the right size conductor and breaker. So naturally I went to the PIE CHART. I=P/E or I=15hp/208. OK 15hp=11185.5w, so in turn 11185.5w/208v=I, which is 53.8amps. Well, I told him he needed a 60amp breaker with #6 copper THHN. he told me no, because he was running two conductors and he only needed a 40 amp breaker because of that! My question to you guys is what in the !@#$ does running two conductors have to do with that equation and if it does have something to do with why can't I find it in the NEC or even in my UGLY book? oh by the way it was his project and if it is FUBARed then it is on him as he stated!

 

[bob]

Re: air handling unit conductor & breaker size

Shawn
First of all the method you used was incorrect.
You get the motor amperage form table 430.150
which shows a 15 hp with 46.2 amps.
If the method you used had been correct you said
"OK 15hp=11185.5w, so in turn 11185.5w/208v=I, which is 53.8amps. You forgot to divide by the
sq. root of 3. 11185w/{208x1.73) = 31 amps.
Conductor size requires 46.2 x 1.25 = 58 amps or
# 4 cu thhn or equal. Breaker size is from table
430.52. NEC allows up to 250% of the FLA for breaker size. 2.50 x 46.2 = 115 amps max.
I have no idea what he means by usine two sets of
conductors. You need to quiz him for more info.

 

[caosesvida]

Re: air handling unit conductor & breaker size

I assume this will be running for more than three hours at a time, continuous load derating?

 

[bphgravity]

Re: air handling unit conductor & breaker size

Ohm's Law is very useful in theory class, direct current systems, and AC systems that are purely resistive. Once you start try to apply it to typical circuits and installations, it doesn't quite work.
Anytime you see motor, or A/C unit, welder, etc.. you need to remember Article 240.4(G). Unlike genral purpose circuits and the like, specific loads have specific rules. In many of the cases, conductor sizing, overload sizing, and ground-fault/short-circuit protection are figured independently.