Subpanel for multiple electric cars

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Old 04-29-17, 09:17 PM
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Subpanel for multiple electric cars

Currently I have one electric car charger in my garage, and soon we'll have a second electric car so I'm planning my strategy for properly installing two chargers. I was hoping I could outline my current thought process and see if the group believes it to be sane or not.

My house has 200A service into a main meter/breaker panel outside, and then also has a 90A subpanel in the garage. These two panels are easily connected through the crawlspace, and running additional wire from the meter panel to any of the garage walls would be easily done through the crawlspace as well. My current car charger is just hardwired with 6-2 NM-B back to the outside meter panel and terminated with a 50A breaker (charger pulls 40A and the circuit is oversized for the continuous load).

The goal is to have two chargers, each on 60A circuits so they can each deliver 48A continuous.

My tentative plan is to install a second (small) 125A subpanel in the garage, which would be fed by a 125A breaker in the outside meter panel. Then in the subpanel I'd have two 60A breakers, one going to each charger.

My biggest questions revolve around what type of wiring to use for all of this. For the subpanel it looks like I could use 1/0-1/0-1/0-2 aluminum SER, which is rated at 120A 75C (no insulation contact in this situation). As far as I've read, I'd be ok rounding up from 120 to use a 125A breaker in this case. And since the aggregate loads from the chargers will only be 96A, then multiplying by 1.25x for continuous use is 120A - seems safe for those conductors and breaker. 2/0 SER seems like it'd easily fit the bill, but the Eaton CH2125 breaker I need to use seems to only accept up to 1/0 according to the catalog page.

For the conductors from the subpanel to the chargers I was planning on using 6-6-6 copper SER, which is rated at 65A 75C. It'd be nice to just use 6-2 NM-B but it seems to be limited to 60C and 55A.

I'd be curious to hear whether I'm going about this way differently than everyone else would. Thanks!
 
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Old 04-30-17, 02:06 AM
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Just to verify this is an attached garage, correct? Do you have electric heat? Electric water heater? Electric stove? Electric dryer?
 
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Old 04-30-17, 04:07 AM
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You can use a 60A breaker on 6/2 NM-B. There is no 55A breaker, so you use the next higher standard which is 60A (NEC 240.4 (B)) as long as the calculated load is 55A or less (sized for the conductor, not the OCPD). You are fine with 48A.

You just have to check your outdoor panel to ensure it can take the 125A breaker. Some have 100A per-stab ratings, so you would need a 4-stab 125A breaker, and those are not available for all panels.

You might be better off (and will definitely come out cheaper) just running a second 6/2 back to the panel with a 60A breaker, and changing out the 50A breaker for a 60A on the existing circuit.
 

Last edited by taz420; 04-30-17 at 04:25 AM.
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Old 04-30-17, 04:58 AM
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Are you actually wiring up "chargers" or just receptacles to plug in the car? I did one a few weeks ago for a Tesla, but all he needed was a receptacle at 50 amps, 4 wire, 240 volts. He plugged directly into the receptacle and then into the vehicle.
 
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Old 04-30-17, 01:28 PM
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Teslas have the "charger" built into the cord but are limited to 40A. Most likely he is installing a standard J1772 EVSE like the Clipper Creek HCS60.
 
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Old 04-30-17, 07:08 PM
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Trying to reply to everyone all at once here, and thanks a bunch for the suggestions!

This is an attached garage so nothing will be underground.

I used the term "charger" just to simplify things for anyone who wasn't familiar with EVs. These are actually two EVSEs, specifically Tesla Wall Connectors. They are hardwired (not plugged into a 14-50 outlet), and have adjustable current output. In my situation they will each be set for 48A draw.

In regard to the comment about using a 60A breaker on 6-2 NM, this seems problematic to me since 125% of 48A is 60A, which 6-2 is not rated for. I believe you have to size the entire circuit for 125% of the load, not just the breaker.

As far as the overall load calculation for the house, I still need to complete one. A previous electrician did quote a job for me and confirmed via a very thorough audit that I had room for the two 60A circuits in the future. I'll double check it though. But to answer your question: gas heat, dual zone electric AC, gas water heater, gas stove, electric dryer.

And finally, the reason I don't want to just run a second 60A circuit from the main panel is that it would leave me with no open spots on that panel. I'm toying with the idea of going solar, and it would be nice to keep a free spot on the main panel for that tie-in.

I think I addressed all of the comments.
 
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Old 04-30-17, 07:23 PM
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It also looks like my panel (and apparently all Type CH panels) have a stab rating of 200A. So am I understanding this correctly that if I used a 125A breaker like the CH2125, I'd have to make sure it's across the panel from a breaker no larger than 75A?
 
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