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#94734 - 08/10/05 10:32 PM A/C Feeder  
George Little  Offline
Joined: Jan 2004
Posts: 1,489
Michigan USA
If I have 3 air conditioning condensers on a feeder and the info on the name plates say Minimum circuit 36a. - What minimum size feeder conductor should I install and how should it be fused?

Each condensing unit has 2 motors and a crankcase heater.

George Little

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#94735 - 08/10/05 11:00 PM Re: A/C Feeder  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,141
Interesting question ... again. ;-)
Does voltage drop figure in here?
Are you going for the minimum that is legal (2 ga with 125 O/C) or what would give better reliability (1 ga and 150) both using 240.4(B).
I would want to use staggered timeouts on the compressor time delays so you wouldn't have 3 liquid slugged compressors in locked rotor at the same time on a power hit.

Greg Fretwell

#94736 - 08/11/05 07:53 AM Re: A/C Feeder  
George Little  Offline
Joined: Jan 2004
Posts: 1,489
Michigan USA
Greg- I would say that #2 and 125a OC Minimum.

Some would say #1 and it probably would be a better design choice for the reasons you stated. Some inspectors maintain that you add an extra 25% of the largest motor. Some inspectors will take the largest motor of an individual A/C unit and add up all the other motors and come up with a smaller conductor yet.

George Little

#94737 - 08/11/05 11:07 AM Re: A/C Feeder  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,141
I think we already covered that ground last time. Some might even argue that the 125% thing has been done 3 times in that engineered number on the HVAC label.
If the run is not excessive the #2 should be OK. The only caveat I would have is staggering the time out relays and that is really a design issue.

Greg Fretwell

#94738 - 08/11/05 06:38 PM Re: A/C Feeder  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Every A/C I've seen also state, on the nameplate, "minimum circuit ampacity."

Since this is one of the "exceptions" to generalbranch circuit sizing rules, an example would be a specification of "25 amp" minimum ampacity being provided by #12 conductors.

Now, if you were to use a single feeder, and tap off it to all three condensers, I would forget the "minimum circuit ampacity" and size the feeder according to "full load amps" and motor feed rules.

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