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#94686 - 08/08/05 07:01 PM Service conductors  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
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

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#94687 - 08/08/05 08:53 PM Re: Service conductors  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,123
Estero,Fl,usa
I'll bite.
8ga copper?


Greg Fretwell

#94688 - 08/08/05 09:43 PM Re: Service conductors  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
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 Re: Service conductors  
gfretwell  Offline


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Joined: Jul 2004
Posts: 9,123
Estero,Fl,usa
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 Re: Service conductors  
watersparkfalls  Offline
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Joined: Mar 2005
Posts: 220
Washington...Not DC
isnt this that trick question?
where #10 is rated for 35 amps??


#94691 - 08/08/05 10:48 PM Re: Service conductors  
George Little  Offline
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Joined: Jan 2004
Posts: 1,489
Michigan USA
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 Re: Service conductors  
gfretwell  Offline


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Joined: Jul 2004
Posts: 9,123
Estero,Fl,usa
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 Re: Service conductors  
George Little  Offline
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Joined: Jan 2004
Posts: 1,489
Michigan USA
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 Re: Service conductors  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,123
Estero,Fl,usa
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 Re: Service conductors  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,123
Estero,Fl,usa
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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