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#27757 - 07/31/03 09:00 PM A/C circuit sizing  
CTwireman  Offline
Joined: Feb 2002
Posts: 840
Connecticut, USA
I know this is a "dead horse" topic, but I'm still confused about it. This subject seems to be a source of mass confusion in the electrical trade.

So, as an example, here is an a/c unit nameplate (from an 2 ton Trane unit):

Min circuit ampacity: 15 amps

Min fuse/breaker size: 20 amps

Max fuse/breaker size: 25 amps

Line running amps: 11 amps

Does this mean that the unit can be fed by #14 protected by a 20 amp breaker?

Or #12 on a 20 amp breaker?

Do you size the conductors by the "minimum ampacity," and size the breaker by "minimum fuse/breaker"? [Linked Image]

One thing I do understand is that in this situation, the breaker only provides short-circuit protection, as the motor is protected by a thermal overload device.


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#27758 - 07/31/03 10:37 PM Re: A/C circuit sizing  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
In this case the code would permit #14 on a 25 amp breaker.


#27759 - 08/01/03 12:53 AM Re: A/C circuit sizing  
elecbob  Offline
Joined: Aug 2002
Posts: 141
A/C unit: 120/208 V 3 phase
Nameplate reads:
18.2 amps FLA.
Min. circuit: 31.5 amps
I had to replace the 30 amp disconnect and #10 wire with a 50 amp disconnect (40 amp disconnect not available) and run #8 wire.

#27760 - 08/01/03 06:52 AM Re: A/C circuit sizing  
Redsy  Offline
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Were you using NM Cable?
If not, it seems you were in compliance with 310.16, which lists #10 as rated for 35 amps.
As far as having to upsize the disconnect, I think you need to know the Locked Rotor Current so you can back-calculate using Table 430.151(B).

#27761 - 08/01/03 10:25 AM Re: A/C circuit sizing  
elecbob  Offline
Joined: Aug 2002
Posts: 141
Thanks Redsy. I was going to pull 4#10's but the inspector said I needed #8 for the 31.5 amps. I was a little suprised to say the least. He is coming out for an inspection on some other gear next week. I will ask him again before I do the pull.

#27762 - 08/01/03 05:01 PM Re: A/C circuit sizing  
Redsy  Offline
Joined: Mar 2001
Posts: 2,056
Bucks County PA
He may be thinking about the asterisk at the bottom of Table 310.16, which refers to 240.4(D).
However, 240.4(D) refers to 240.4(G), which permits particular Articles to specify conductor ampacity selections.

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