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#94734 08/10/05 11:32 PM
Joined: Jan 2004
Posts: 1,507
G
Member
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 08/11/05 12:00 AM
Joined: Jul 2004
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G
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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 08:53 AM
Joined: Jan 2004
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G
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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 12:07 PM
Joined: Jul 2004
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G
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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 07:38 PM
Joined: Jan 2005
Posts: 5,329
Cat Servant
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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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