100 amp sub panel

[jzelectric]

I’ve found multiple threads discussing this topic but I can’t seem to find a straight answer. If running SER cable from a main panel to a 100 amp sub panel, why does it seem like everyone uses 2,2,2,4 aluminum SER and not the 1,1,1,3 as per the 75°C column (breaker terminal rating) of 310.15(B)(16). Is this a misinterpretation of 310.15(B)(7) considering the sub panel does not carry the load of the entire dwelling?

 

[augie47]

Here, it's availability. 2-2-2-4 is readily available and 90 amps is normally sufficient for the load.

 

[LarryFine]

Here, it's availability. 2-2-2-4 is readily available and 90 amps is normally sufficient for the load.

True, but 90a is a standard breaker size.

 

[infinity]

Is this a misinterpretation of 310.15(B)(7) considering the sub panel does not carry the load of the entire dwelling?

I think that it a combination of that, what Augie stated about the availability of #1 AL SER and guys just not reading the codebook.

 

[augie47]

True, but 90a is a standard breaker size.

It may be "uncommon" but it is a 240.6 "Standard Size" Breaker.

 

[jzelectric]

Thanks guys. I thought maybe I was missing something, like some sort of special exception. The general consensus of people I asked locally (fellow electricians, inspectors, supply house staff) was to use 2-2-2-4 on a 100a breaker. The only way I could figure they were arriving at that conclusion was using the 83% rule, which didn’t seem to apply in this instance. I felt like the odd man out suggesting it would have to be 1-1-1-3 to use the 100a breaker. Also, given the lack of availability of the #1 it just furthered me second guessing myself.

 

[wwhitney]

One corner case is where the dwelling unit service/main feeder itself is only 100A, so it could be wired with 2-2-2-4 Al. In case that, downstream 100A feeders, even if not carrying the full dwelling unit load, need not be larger than the service/main feeder, so they can be 2-2-2-4 Al as well.

Cheers, Wayne

 

[electrofelon]

Thanks guys. I thought maybe I was missing something, like some sort of special exception. The general consensus of people I asked locally (fellow electricians, inspectors, supply house staff) was to use 2-2-2-4 on a 100a breaker. The only way I could figure they were arriving at that conclusion was using the 83% rule, which didn’t seem to apply in this instance. I felt like the odd man out suggesting it would have to be 1-1-1-3 to use the 100a breaker. Also, given the lack of availability of the #1 it just furthered me second guessing myself.

#2 SER to a sub with a 100A breaker has got to be one of the most common code violation out there. In fact, I do not recall EVER seeing a 90A breaker on #2 (unless it was me).