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#203883 10/29/11 01:12 PM
Joined: Feb 2002
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I don't know if this should be here or in a different forum. An EC was talking to me and ask me a question, Since Table 310.15(B)(6)allows #2 AL wire for a 100 amp residential service, can you use #2 AL on the load side of a generator that has a 100 amp main breaker on the generator?

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Tom Offline
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One answer would be no, this isn't allowed. 310.15 (B)(7) says it is allowable for the main feeder conductors and further states that the main feeder conductors are installed between the main disconnect and the panelboard that supplies ... Since there is no connection of the generator to the main disconnect, this isn't the main feeder, nor are these service entrance conductors.

The commentary in the handbook explains that the conductors must carry 100% of the diversified load, which these conductors will carry.

However, the last part of the section says that feeder conductors to a dwelling unit (it does not say main feeder conductors) shall not be required to be larger than their service entrance conductors. So, if this is a 100 amp service, I believe it should be allowable to use #2 Aluminum conductors.


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Harold:

Splitting hairs by using the feeders shall not be required to be larger than the service entrance conductors would allow this, but I feel the amperage rating of 310.16 table, 75degree col should dule here. And IF the 'rehab' card is pulled, '05 NEC prevails with no change to the forementioned. (My two cents)

Last edited by HotLine1; 10/30/11 07:35 PM. Reason: Removed a question

John
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I would only let them use 310.15(B)(6) if it was sized to the service capacity, not the generator.
If this is a house running on 100a from the utility, then it could use 100a to size the generator feeder.
I am still a little uncomfortable with 310.15(B)(6) when people continue to load up a service after the initial installation until they are actually cruising at close to 80% a good part of the time and tripping the breaker occasionally.



Greg Fretwell
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The first thought is, "If I can use #2 AL for a 100 amp resi service, as per 310.15(B)(6) then I should be allowed to use it as a service feeder from a generator feed to the house".


What would you say to them?

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Another thought. The EC runs a #2 AL feeder from a generator to the house but has a 90 amp. disconnect on the generator itself. Now is the install allowed to use the #2 Al from generator to house?



I hope you guys all know me by now. I am not being difficult, just being a "Devils Advocate". I get questions like these all the time and I like to put them out here so that I get some answers and the guys who question me can see your answers too.

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If he has a 90a disconnect, he meets the letter of the code (assuming THWN in pipe or some other 75c wiring method) but he will not be getting all he paid for out of the generator.


Greg Fretwell
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Greg,

I would agree that using a 90 amp. breaker would meet the code.

Now would you fail that person if he had a 100 amp breaker in there with #2 Al THHN feeding the house off the generator?

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Harold:

YES, it would be a violation. A 90 amp, 2 pole CB is available.



John
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I agree, it may not be a serious safety matter but it is still a code violation. How much more would it be to use #1 ... it is a trivial cost to be compliant. This is bordering on the same thing we are talking about on the fluorescent thread.
Again, if the whole house is running on a 100a service, I would pass it because you will usually not be using any more on the generator than you would use on utility power.
That still might not be true if the user is shooting cords off to his neighbors.


Greg Fretwell
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