Ampere rating for #2 AWG SER

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jeff48356

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Livonia, Michigan
Consider a major addition to a house, consisting of several rooms, including a laundry room and a separate electric water heater. If I install a 100A subpanel to be fed with #2 AWG SER cable, can I use a 100A breaker to feed that? Or would I need to use a 90A? Just looking at the ampere ratings in 310.15 for #2 AL.
 
I keep a 90A breaker on the truck to install for the inspection, then replace it with a 100A afterwards.

Code only allows you to use #2 for 100A if it’s feeding the entire dwelling unit.
 
KaBoom! said:
I keep a 90A breaker on the truck to install for the inspection, then replace it with a 100A afterwards.

Code only allows you to use #2 for 100A if it’s feeding the entire dwelling unit.
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Why do you do that?? Do you also take out the 15 amp breakers and put in 20's, and might as well take off the nail platest and TR receps while you are at it....
 
electrofelon said:
Why do you do that?? Do you also take out the 15 amp breakers and put in 20's, and might as well take off the nail platest and TR receps while you are at it....
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I do it because it’s a silly code.

Nail plates make sense. There is logic behind it. As do TR receptacles and 15A breakers.

The code is supposed to be able safety, and what I posted is not unsafe at all, and you know it.

I’ve used #2 Al code compliantly for a thousand 100A services and sub panels. The idiotic notion that if that sub panel isn’t serving the entire dwelling unit, such as an apartment inside the house, then it can’t use #2 is simply not something that I am going to entertain.
 
jeff48356 said:
Consider a major addition to a house, consisting of several rooms, including a laundry room and a separate electric water heater. If I install a 100A subpanel to be fed with #2 AWG SER cable, can I use a 100A breaker to feed that? Or would I need to use a 90A? Just looking at the ampere ratings in 310.15 for #2 AL.
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What code cycle are you under? Will the SE cable run through thermal insulation? Depending on the answers you might be limited to the 60° C ampacity of 75 amps.
 
Code only allows you to use #2 for 100A if it’s feeding the entire dwelling unit.[/QUOTE]

It was in old code book?
What cod #would you apply to services. I know there was table in the past but I can not find it anymore.



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infinity said:
310.15(B)(7) now uses the 83% rule.
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Yes, that's just what I was going to mention. You can apply 83% of the ampere rating of the conductors only when being used for services, because they are a short run from the meter to the panel, and don't run through any areas that could change the ampacity of the conductors. So 100A if used for main services, or 90A if used for a subfeed. But either way, the homeowner would never draw anywhere close to 90A in the addition, so I wouldn't have a problem using a 90A rather than 100A.
 
KaBoom! said:
I do it because it’s a silly code.

Nail plates make sense. There is logic behind it. As do TR receptacles and 15A breakers.

The code is supposed to be able safety, and what I posted is not unsafe at all, and you know it.

I’ve used #2 Al code compliantly for a thousand 100A services and sub panels. The idiotic notion that if that sub panel isn’t serving the entire dwelling unit, such as an apartment inside the house, then it can’t use #2 is simply not something that I am going to entertain.
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90 is the ampacity of #2 ser. I guess we can all start making up our own ampacities now. I'm going to start applying the 83% to 15 amp circuits. I'LL DECIDE!
 
electrofelon said:
90 is the ampacity of #2 ser. I guess we can all start making up our own ampacities now. I'm going to start applying the 83% to 15 amp circuits. I'LL DECIDE!
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LOL, maybe we can all do that. Why not remove all of those silly and expensive AFCI breakers after the inspection too. :roll:
 
infinity said:
LOL, maybe we can all do that. Why not remove all of those silly and expensive AFCI breakers after the inspection too. :roll:
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KaBoom! said:
Why wouldn't​ you do that? AFCI breakers are worthless.
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electrofelon said:
There are many codes I do not agree with for sure. But as a professional there is a set of rules that I have to follow. Keep in mind this is a professionals only code forum too.
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AFCI's are snake oil but are still required by code, but if going to decide what part(s) of a code your going to follow/comply with what value is the code? One is not a professional then, and no different then someone hired off Craigslist who says they are good with 'lectric.
 
jeff48356 said:
You can apply 83% of the ampere rating of the conductors only when being used for services, because they are a short run from the meter to the panel, and don't run through any areas that could change the ampacity of the conductors.
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I don't think that's the reason

If main disconnect is outside may not be short run and may run through insulated areas
 
KaBoom! said:
I do it because it’s a silly code.

Nail plates make sense. There is logic behind it. As do TR receptacles and 15A breakers.

The code is supposed to be able safety, and what I posted is not unsafe at all, and you know it.

I’ve used #2 Al code compliantly for a thousand 100A services and sub panels. The idiotic notion that if that sub panel isn’t serving the entire dwelling unit, such as an apartment inside the house, then it can’t use #2 is simply not something that I am going to entertain.
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If apartment inside the house meets art 100 definition of dwelling unit, you can use the alternate ampacity allowed by 310.15(B)(7) for the feeder to it.

At some point someone has determined the load diversity for an entire dwelling allows for lesser ampacity.

I will agree often times such a feeder within a dwelling often won't ever see 100 amps, but you can't just decide when you want to follow what code says and when you don't (well most of us can't, or won't subject ourselves to the potential liability should we not follow code).

Where I buy QO breakers 80, 90, and 100 amp are all the same price and typically in stock - so no point in installing a 90 and then changing to a 100 later anyway from cost perspective.
 
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