Wire size question 310.12

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olly

Senior Member
Location
Berthoud, Colorado
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Master Electrician
If I set a 200A, 8-space, meter/main and then install a 100A breaker to feed the dwellings main panel, I get to use Table 310.12 to size the wire for the main panel so I would use #2 AL, according to 310.12 (A) and (B), which points me to Table 310.12. All good so far right?

Ok, If I also install another 100A breaker to feed another panel in the basement I would not be able to use table 310.12? Is that correct? In this case I would set a 90A breaker and use #2 AL from past experience, but I don't know why this allowance is only for the main panel or as code words it, conductors supplying entire load.

My question is: I don't know why I can't use table 310.12 for both feeders?

Would I be able to use table 310.12 at all if I installed both panels at the same time, and got it inspected all at once?

This is not something I am going to do. Just trying to understand the rule.
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olly said:
If I set a 200A, 8-space, meter/main and then install a 100A breaker to feed the dwellings main panel, I get to use Table 310.12 to size the wire for the main panel so I would use #2 AL, according to 310.12 (A) and (B), which points me to Table 310.12. All good so far right?
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Yes, if that main panel has all of the dwelling unit's loads, and the rest of the breakers in your 200A panel supply only loads not associated with that dwelling unit.

olly said:
Ok, If I also install another 100A breaker to feed another panel in the basement I would not be able to use table 310.12? Is that correct? In this case I would set a 90A breaker and use #2 AL from past experience, but I don't know why this allowance is only for the main panel or as code words it, conductors supplying entire load.
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If it's in the basement of the same dwelling unit for loads in that dwelling unit, now neither feeder supplies all of the loads of that dwelling unit, and so both breakers would need to be 90A.

However, if your service were 100A instead of 200A, and the service only supplied loads in the dwelling unit, then the service could be rated 100A with only #2 Al per 310.12. And then each of the two feeder could stay protected at 100A.

Cheers, Wayne
 
Load diversity is why the single feeder may be so rated and used.

In my opinion, two such feeders should each be allowed to be used the same way.
 
LarryFine said:
Load diversity is why the single feeder may be so rated and used.

In my opinion, two such feeders should each be allowed to be used the same way.
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Why?

A load diversity factor gets bigger (closer to 100%) when the number of loads gets smaller. With fewer loads, there's a greater chance that all or most of them operate simultaneously.

So what's wrong with saying that you only get to use the 83% factor when supplying all of the loads of a dwelling unit? If you're supplying some but not all of the dwelling unit loads, the diversity factor should be bigger than 83%.

Cheers, Wayne
 
So here's where I think it would make sense is if the load supplied by a panel could hit all the major requirements of a dwelling then it should. Cooking, sleeping, bathroom. If there's all the kitchen circuits, bedrooms, and a bath outlet then I think it should have enough diversity even if the disconnect could have say an ac unit or something like that. But that's not how it is and my inspectors even if they agreed with me would have to go by the way its written currently which is already nice to have and fair vs having no derating.
 
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