# subpanel feeder sizing --- again



## macmikeman (Jan 23, 2007)

mikewillnot said:


> I regularly run 100-amp and 125-amp sub-panels, and sizing the aluminum feeder cable (SER) seems to be getting more complicated, and for some reason has me staring blankly today.
> 
> We're officially on 2014 here, but transitioning soon (they say) to 2017, and all the inspectors are waving that around. I don't even own a 2017 at this exact moment, though I just ordered one.
> 
> I usually use #1 SER for 100-amp panels and #1/0 for 125. It's never in insulation, with the possible exception of an occasional foot or two. Am I doing something wrong?



I know you are probably sizing based on the overcurrent protection means and not the panel rating, but just in case........ It doesn't matter what size panel you use as long as the overcurrent protection is not larger than the panel rating. 
So you can use a 200 amp rated panel with a 100 amp main breaker for it, as long as the sub feeder is rated for 100 amps also. I always just use 1/0 for 100 amps now since it is readily available around here and #1 is not.


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## mikewillnot (Apr 2, 2013)

I am using 125-amp rated main lug panels. 

I am concerned about sizing the SER correctly for the overcurrent protection, which in these cases is either 100 or 125. 

(I know...some people might say that's doing it backwards).


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## HackWork (Oct 2, 2009)

If the subpanel is feeding an entire dwelling unit, such as an apartment, then you can use #2 for 100A. But if it's just a random subpanel, then #2 is only good for 90A, which is what I use. If you really need that extra 10A, then you would have to use #1. I have never installed a 125A subpanel so I don't know the size off the top of my head.


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## mikewillnot (Apr 2, 2013)

HackWork said:


> If the subpanel is feeding an entire dwelling unit, such as an apartment, then you can use #2 for 100A. But if it's just a random subpanel, then #2 is only good for 90A, which is what I use. If you really need that extra 10A, then you would have to use #1. I have never installed a 125A subpanel so I don't know the size off the top of my head. ...


Not for an entire dwelling. 

They're for EV chargers (continuous loads), plus whatever other typical garage loads there might be... sometimes for 2 or more chargers -- so the additional amps are important, especially with some models of car.


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## HackWork (Oct 2, 2009)

mikewillnot said:


> Not for an entire dwelling.
> 
> They're for EV chargers (continuous loads), plus whatever other typical garage loads there might be... sometimes for 2 or more chargers -- so the additional amps are important, especially with some models of car.


Then just use the table, your sizing of #1 for 100A and #1/0 for 125A is right.


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## A Little Short (Nov 11, 2010)

You just need to size it off of whatever your feeder breaker is. If that breaker is less than 100A then don't worry about the panel being rated for 100A. Size the wire to the breaker feeding it.


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