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#94686 08/08/05 07:01 PM
Joined: Jan 2004
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What size Service Entrance Conductors do you need for a 60a fused Service Disconnect when it is being used for an Air Conditioner. FLA of AC is 32a Max fuse 45a? (Tricky)


George Little
2017 / 2014 NEC & Related Books and Study Guides
#94687 08/08/05 08:53 PM
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I'll bite.
8ga copper?


Greg Fretwell
#94688 08/08/05 09:43 PM
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Greg- Look at 230.42(B) and 230.79(D) also consider the terminals. You might change your answer. [Linked Image]


George Little
#94689 08/08/05 09:58 PM
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I see this as a 230.79(A) "single branch circuit" with a "limited load"
FLA 32a x 1.25 is a 40a load. 316.16 says 8cu is OK.
45a is well within the circuit O/C device limits for a motor load.
I assume overload protection is in the HVAC equipment?


Greg Fretwell
#94690 08/08/05 10:14 PM
Joined: Mar 2005
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isnt this that trick question?
where #10 is rated for 35 amps??

#94691 08/08/05 10:48 PM
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Unless I losing it- The Service Conductors feeding the fused Service Disconnect need to be sized to meet the rating of the disconnect. Based on 230.42(B) for specific installations. That being the case, a 60a switch would be required to have 60a - 75° terminals could use #6THWN wire on the line side of the switch. On the load side the conductors could be smaller and fused accordingly. 75° terminals THWN #10 would work. (35a)


George Little
#94692 08/08/05 11:13 PM
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This is a 45a disconnect if you have 45a fuses in it. I had that out with NFPA in reference to 400a equipment with 250a fuses in it. (there was even an 1999 ROP written)
They said service conductors get sized to the fuse installed, not the maximum fuse that fits.

I still find myself back at 230.79(A).

10ga won't work because you need to use 125% of FLA since HVAC has to be called a continuous load. On a hot day (the top of the design range) a properly sized system should not cycle.

From a practical sense you are just protecting the conductors from the load and that is 32a (plus a 25% cushion) 40a.
Short circuit protection can be at least 175% of FLA in art440.


Greg Fretwell
#94693 08/08/05 11:28 PM
Joined: Jan 2004
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Greg - I probably could buy into your Service Entrance conductor thing- at least for the time bring, but the #10 is adaquate for the 32a AC. Air Conditioners are not considered continous loads. The wire is rated for continous loading and the 32a. FLA on the AC name plate has already calculated the 125% for largest motor and 100% of the other motor etc.

Edited to say 125% not 1.25%.

[This message has been edited by George Little (edited 08-08-2005).]


George Little
#94694 08/08/05 11:57 PM
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OK I agree if the "32a" is the "minimum circuit ampacity" on the nameplate but the terminals in the AC would still have to be labelled 75c to use 10ga.
I was confused when you said "FLA" and not minimum circuit ampacity from the nameplate.


(off topic)
There is no way I would say HVAC is not a continuous load. The manual J calc assumes continuous operation for the max design ambient.
Oversized AC units won't remove enough humidity and you get a clammy house.


Greg Fretwell
#94695 08/09/05 12:04 AM
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The more I read this the more I am still saying #8cu
The disconnect is 45a and needs a 45a conductor per 230.42(B)


Greg Fretwell
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