100A subpanel wire

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rszimm

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Tucson, AZ
So I know this has been hashed on for a while, but looking back through the history I can't seem to get a good answer. I'm adding a subpanel in my garage to power a current plug-in car and perhaps a future one, as well as maybe an outlet for a future welder. 100A subpanel even though I'll only be populating one 40A circuit for the current charger. Cable run is about 120feet.

I'm thinking Aluminum SE cable though the walls because, well, I'm cheap. Running over the roof (flat roof) with EMT is also a possibility, but Cu THWN is still a lot more costly than than Al SE. Plus, I'm in Arizona and I think I'd have to use a 0.58 correction factor if I went over the roof, so my Cu may actually end up being bigger than an Al cable run through the walls. For these reasons I'm leaning toward the SE cable and a bunch of drywall patches.

Problem is, for the life of me can't nail down what size SE cable I need. #2, #1 or #1/0?? I called the local building department and they told me they just always use the 90 degree column of 310.15(B)17 (yep, that's the one they pointed me to). So they'll accept a #4 all the way up to 115A, but I KNOW that they're misreading the code (or at least they're not fully understanding the other sections).

So 310.15(B)(6) indicates #2, but I'm pretty sure I can't use this because it's a subpanel and doesn't supply "all loads that are part of the dwelling unit". So my best guess is the 310.15(B)16 75 degree column which points me to #1. However, in the 100' run, there is a 3' section that runs through some insulation, so maybe I need to step all the way down to the 60 degree column, and then I'll need a #1/0.

Ultimately I just want it to be safe, and I'm going to be drawing a lot of energy through this daily (at least 20kWh daily with one car. Significantly more if we get a second fully electric), so I don't want a lot of transmission loss. If you guys point me to #1/0 I'll do that, but it's going to kill me pulling that donkey dong through the walls. I'm hoping #1 will suffice.
 
As long as the feeder is properly protected it is "safe".

I would not get real excited about the 100 A panel. I might be more worried about whether the existing service calculations are adequate. Start there. If you are really adding the kind of loads you are suggesting, you may have a totally different issue to deal with than the panel feeder rating.
 
petersonra said:
As long as the feeder is properly protected it is "safe".

I would not get real excited about the 100 A panel. I might be more worried about whether the existing service calculations are adequate. Start there. If you are really adding the kind of loads you are suggesting, you may have a totally different issue to deal with than the panel feeder rating.
Click to expand...

I don't think the main panel will be an issue. It's a 400A service that I upgraded from 200A last year. Actually it's a 200A/200A panel with the secondary 200A feeding all of the old main circuits, and the main 200A panel currently feeding just a pool pump. It's this basically empty 200A panel that I'm going to pull my 100A subpanel off of.

And to answer your comment jumper, I'm not sure how to do load calcs when I'm really speculating now as to what loads might be in the future. The only load I know of is a 30A circuit, but I think the Tesla charger actually requires a 100A circuit by itself, so I'm using that as my worst case. The chargers are starting to get smart about communicating with one another so that you can put two 100A chargers on the same circuit and they each communicate to make sure only one charger is working at once.
 
338.10 has caused numerous discussions especially since the wording has changed in the last 3 or 4 Code cycles so the "true" answer will depend on which Code cycle is in effect.
From you mentioning the amount of cable in insulation, I would assume '11 or '14 which would limit you tom the 60° rating, however, a rather well hidden exception located at 310.15(A)(2) will likely allow you to use the 75° rating due to the limited exposure to insulation.
 
Regardless of the sub panel rating, you can use whatever size wire you want provided that

a) it will carry the load you are putting on it
b) it has the appropriate OCPD in the main panel

If you're just going to have the 40A car charger, and you want to be really cheap, you could go with #6 on a 50A breaker. Your future car charger sounds like a big if, and if you add a welder, you could just run it when the car charger is off, yes?
 
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