Subpanel Feeder Capacity

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#1

Originally Posted By: roconnor
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".


NEC 334.80 would require 90C insulation on the wires (which would normally be a 75A capacity for #6 wire with say THHN insulation), but then goes on to state that ampacity shall be based on 60C insulation which results in 55A capacity. This is consistent with typical ampacity tables and manufacturer?s data.

You are permitted to derate the 90C value ? and if you look at say an ambient temperature rise of 86F to 104F for unconditioned space, this would result in a 68A cable capacity. But that NEC section says you still cant exceed the 60C capacity of 55A. Why restrict the ampacity like that?

I have heard that the wires may have a 90C rating, but that the jacket is only rated for 60C. Why then allow the higher insulation value to be derated if the jacket is only good for 60C ... and is this subpanel feeder really an issue?

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Robert O'Connor, PE
Eagle Engineering ?
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NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong

#2

Originally Posted By: Dennis Bozek
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icon_eek.gif Wow heavy RC and all them big words and terms icon_eek.gif


And that is a good question....one that I cannot even answer

My best answer(umm guess) to this would be that it is a house! Commercial application would be different...but in a house things are different. For instance we know that 14 gauge wire is good for 20 amps and 12 gauge is good for 25 amps, however, in a residential atmosphere it is 15 amps and 20 amps respectively. I guess they are really telling you that it's ok if a commerical building burns down but a house is not.

I had a inspector tell me once that he wanted to see 4 gauge wire going to a subpanel just in case someone wanted to change out that 60amp breaker to a 100amp breaker. There is nothing that says a subpanel has to be 100 amps nor would I have to change out the 6-3G romex to a 4-3G romex cable. So needless to say it did not get changed but I could see his reasoning for such...just like I can see the same with the NEC. It all has to do with protection and for whatever reason, a residential environment gets protected far more than a commercial building. So in response to your question...."Is the subpanel feeder really an issue", I would say no. 6-3G NM-B is fine for that sub feed. However I would put the breaker at a 50 amp versus a 60 amp.

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This information has been edited and reviewed for errors by your favorite resident sparky.

#3

Originally Posted By: jmyers
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Dennis,


Could it be that the romex, sheathed cable, is already derated since it in encased in sheathing. Versus the single wire that you would have to derate based on how many wires you place in what size conduit?

Joe Myers

#4

Originally Posted By: roconnor
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Its not derating on the number of wires, since there are not more than three current carrying conductors. I think the NEC rules seem to be limiting the overall cable rating only to keep the sheathing cool. Okay … maybe a bit much for a home inspection.


Bottom line is it's 3-wire #6 romex which is usually limited to 55A. But since the actual wires have more capacity and the NEC will allow rounding up to the next standard breaker size in a lot of cases, my call is that it's acceptable as an existing 60A subpanel feeder.

--
Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong