EV Charging Sub-panel Questions

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dafish

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Project Manager
Hello all. Please forgive what may at first glance sound like DIY questions - they are not. I've become accustomed to looking before I leap, even when I'm hiring job outs. In this case I'm project managing contractors working on a sons garage update. I don't want to piss off my service guys, yet what I read and what I'm being told don't align. Hopeful mods will consider the safety aspect of this and not think I'm trying to DIY.

I’m going to have an electrician install a sub-panel in an attached garage for electric vehicle charging. But I'm in a small town and there is still a lot of "that's the way we do it" going on. As an example, I've had several electricians tell me they use #2 aluminum for 100 amp sub-panels. They don't seem to hear "EV charging", or consider ambient temperature correction and etc.. Mind you I'm prolly the one that's wrong, but that's why I'm here - Making sure I know what a safe and professional proposal needs to include.

All said and done I want to be able to deliver 96 amps at 240 volts for more than 3 hours of constant use. This seems to mean I require wire and connectors rated to 120 amps (125%). I also need to deliver 2 amps at 120v intermittently. I assume that’s as simple as saying I need wiring for 122 amps, with a feed breaker in the main panel of 100 amps. Correct so far?

It seems easy enough until I get to wire selection vs. ambient temps. The wire will go through a garage ceiling and ascend/descend insulated walls. Here in Illinois I suppose I should assume ambient temperatures could occasionally exceed 105f in the ceilings, but it’s not likely to break 113f.

Questions:
  • Since it will be installed in an insulated wall and ceiling, am I required to request wire ampacity rated to only 60C and then derate it by .71? I seem to have read I can use 3/0 aluminum rated to 75C and derate by .82 since I’m well over 10 AWG wire. Is this correct?
  • Do I have any other choices regarding acceptable temperature in insulated walls? 2/0 aluminum rated to 90c, if safe in walls and the ceiling, would also work and likely be less expensive and available. Is this allowed and safe?
  • Is there any rounding allowed here? I see 1/0 90c rated wire is rated at 135 amps. Temperature corrected that’s 117.45, and the 20% constant load de-rating puts me at 93.96. Awful darn close. If it was safe.
  • Regarding the whole “de-rating” thing, be it ambient temperature or extended operation, is the correct move here to simply install a 100amp 240v breaker in the main panel and fuse the sub-panel per it’s individual loads (de-rating again for the EV plugs)
If it matters, all “end point” runs will be in copper. This discussion is only about the feed between the main panel and the sub-panel.

Since I'm invariably wrong, here are the sites I've been using for reference.:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Ambient Temperature Correction

Finally, how hard would you push on this? Again, I don't want to piss my local providers off, there aren't lots, so it's helpful to know if I'm being picky or if these are things I need to politely and gently request I be humored on.

Guys, my sincere thanks to anybody that takes the time to help me keep this thing safe!

-d
 
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The reality is that the code is very conservative so it is unlikely you would have a problem even if it is not strictly speaking to code.

I think you are more or less on track although I don't think I would get real worried about derating for ambient temperature.

Just tell them you want a bigger feeder ran for possible future use.
 
For a 96A continuous load and 2A non-continuous load, you need:

1) Wire and breaker with a termination ampacity of 1.25 * 96 + 2 = 122A.
2) Wire with a conditions-of-use ampacity of 98A.

So if you want to use a 0.71 temperature correction factor, the 90C base ampacity for the conditions of use calculation needs to be 98/0.71 = 138A.

Specify 2/0 Al SER cable (say), and a 125A breaker.

Cheers, Wayne
 
wwhitney said:
For a 96A continuous load and 2A non-continuous load, you need:

1) Wire and breaker with a termination ampacity of 1.25 * 96 + 2 = 122A.
2) Wire with a conditions-of-use ampacity of 98A.

So if you want to use a 0.71 temperature correction factor, the 90C base ampacity for the conditions of use calculation needs to be 98/0.71 = 138A.

Specify 2/0 Al SER cable (say), and a 125A breaker.

Cheers, Wayne
Click to expand...

98A*071=70A temp correction
 
wwhitney said:
For a 96A continuous load and 2A non-continuous load, you need:

1) Wire and breaker with a termination ampacity of 1.25 * 96 + 2 = 122A.
2) Wire with a conditions-of-use ampacity of 98A.

So if you want to use a 0.71 temperature correction factor, the 90C base ampacity for the conditions of use calculation needs to be 98/0.71 = 138A.

Specify 2/0 Al SER cable (say), and a 125A breaker.

Cheers, Wayne
Click to expand...
2/0Al and a 125A breaker wouldn't comply with 240.4 for a 0.71 temperature correction factor. However, the correction factor would be 0.87 (not 0.71) for 105-113F ambient temperature, so no problem.
 
david luchini said:
98A/0.71=138A
Click to expand...

2c72d4d5d0dc3dcb2b2071dffd23fe8f.jpg

It says multiple not divide
In heat insulation is weaker les current can flow
 
My sincere thanks to all!!

Retirede:
If my math and standards are correct just asking for 125 amps does seem like a good plan, and it keeps me from calling anybody out. Thanks for the hint. Now to be sure I understand a few details..

David:
Am I to read your comments as to say "yes, you can use 90c allowable wiring in insulated area's, and that's why .87 is the ambient correction factor?

All:
1) I get a sense ambient temperature correction isn't done frequently. Am I being needlessly anal on this?
2) I think I'm hearing I should tell him "No please" if he wants to install #2, or even #1. Correct?
3) If he has a roll of 2/0 on the truck it seems all feel he's good to go.
4) If he doesn't, but does have 1/0, I think I'm hearing that's acceptable. Is that correct?
5) I think I'm being told to ask for a 125 amp breaker in the main. Again, WTF do I know, but doesn't that violate the intent of NEC 210.20a reserve for the EV? Aren't I supposed to down-rate the breaker in the main?

I meet with an electrician tomorrow, so this is really helpful. Thanks!

-d
 
1) Yes; no, although I wonder if your assumption of ambient temperatures of 105-113 is too conservative. Actually it doesn't affect the wire size, so it doesn't matter.
2) Yes, assuming Al.
3) Yes.
4) No, that's not correct. 1/0 Al has a 75C ampacity of 120A. So after your 2A non-continuous load, that would only support a continuous load of (120-2)/1.25 = 94.4A. That would allow 1 EVSE to be 48A (60A circuit) and 1 to be 40A (50A circuit), but does not allow 2 EVSEs @ 48A (60A circuit).
5) You are misreading 210.20(A). For a 96A continuous load, that section requires a breaker of at least 120A.

Cheers, Wayne
 
Wayne:

Thank you. I see you know why designing to 96 amps. Nicely done sir. 113f too conservative? Ok, you got me. I was guessing, sorta, but a fast search suggested attic temps were tyypically 10f over ambient. I don't know of many days, if any, we hit 103f so I used 113f. I try to take advice though, so I'll look at it again with the next temperature up.

Is 90C rated Al wire allowed in insulated walls? It seems like David Luchini used the 90C temperature factor, so..

RE: NEC 210.20A. It's not so much I'm mis-reading it (well, who knows), it's that I don't see how the end-user is prevented from exceeding the de-rated limits of the circuit. Lets say an 80amp F-150 and a 32 amp Pacifica. That won't blow the 120amp fuse, but its sure as heck over the spirit of 210.20.

Thanks again!

-d
 
dafish said:
113f too conservative?
Click to expand...
I don't know, as I don't know your location, just wondering. And by "conservative" I meant "higher than necessary."

dafish said:
Lets say an 80amp F-150 and a 32 amp Pacifica. That won't blow the 120amp fuse, but its sure as heck over the spirit of 210.20.
Click to expand...
The EVSE has a maximum current setting that it communicates to the car. So if your feeder is designed for 2 x 48A maximum EVSEs, the EVSEs need to be set to 48A each. When you plug in your 80 amp capable F-150, it will only charge at 48A.

If you want the option to charge at 80A when there's just one vehicle, but also cap the total charging rate at 96A across both EVSEs, you'll need a model of EVSE that supports that feature with a communication wire between EVSEs.

Cheers, Wayne
 
Wayne:
Thank you again. So yea, it's sort of on the operator to not screw up. Interesting.

Anybody:
Is 2/0 90C AL wire allowed in insulated walls?
 
electrofelon said:
I would go 1/0 SER on a 125 and call it a day.
Click to expand...
Derating questions aside, that would be fine for a 96A continuous load (120A 75C ampacity) but not with the additional 2A non-continuous.

[Which is excessively conservative on the part of the NEC, in my opinion, as 98A is no problem for wire with a 120A ampacity, and 125A is fine for protecting 120A ampacity wire, and 125A >= 125% * 96 + 2.]

Cheers, Wayne
 
Larry:
Thank you. Assuming he used SER rated AL wire, which I think is 90C rated, as are the breakers, I think I see:

2/0 is 150 amp at 90C
Derates by .87 to 130.5 amp capacity

Load is 96 x 1.25 + 2, or 122.

Look clean, seems to handily support a 125 amp breaker. Is there more I need to consider asking for? I'm assuming a competent electrician, but one perhaps not aware of all codes.

Net:
For the tomorrow meeting my intent is to request:
Subpanel with 125 amp service using no less than 2/0 AL wire rated to 90C.
Homeline 125 amp 2 pole breaker in a GE 125 amp 8-14 spaces panel.


Electrofelon:
Yep, I get it, and appreciate ambient correction isn't heavily considered. But it's my son and his entire family, and for $100 of wire screw it. He and I agree we don't cut corners on safety or quality of install. Heck this entire thread is about making sure it's done safely.

Admins:
Thanks for allowing me the privilege of asking all these questions. I'm sure there are DIY's asking similar, so I'd wondered.
 
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