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I'm remodeling my detached 3-car garage, so decided to have some 240VAC circuits installed for EV charging and possibly running 240VAC tools should the need arise. I don't actually have immediate plans to buy an EV, but figure I may as well do this while I have the walls opened up, and the yard dug up. So, I paid an electrician to install a 90A subpanel in the garage. It currently has two 40A breakers, each feeding a NEMA 14-50 outlet. The outlets are connected to the subpanel with 6/3 romex in the wall. One run of the 6/3 romex is about 20 ft, and another is about 40 ft to feed an outlet on a divider between the garage doors. There is one more 20A 120VAC circuit, connected with 12/2 romex. There are 4-gauge individual conductors in underground conduit between the main breaker at the house and the sub-panel in the garage.

Since the install, I've been reading that the latest model Tesla's can charge at a max 48A, and that they recommend a 60A breaker for that. With the current setup as-is, it seems I'd be limited to 32A max charge current. The idea seems to be to use only 80% of the circuit capacity. So, I was wondering if I could just replace the 6/3 romex with 4/3 romex. The wire is pretty expensive, but the wall is still ripped open, so it's just a matter of removing the 6/3, pulling the 4/3 cable through and hooking it up, and then also updating the breakers. Some thoughts:

My understanding is that the NEMA 14-50 is rated for 50A, and therefore cannot be on a 60A breaker. I could just leave the 40A for now, or update to 50A until and unless I decide to remove the NEMA 14-50 and hardwire a Tesla charger in its place.

If I were to eventually hard-wire 2 Tesla chargers, and remove the NEMA 14-50 outlets, and replace both of the 40A breakers with 60A breakers, then I'd have two 60A and a 20A breaker on the 90A subpanel. That's obviously "over-subscribed", and I've had one electrician tell me I can't do that, but I see over-subscribed panels all the time. My understanding is that the Tesla chargers can talk to each other and share the capacity, so it seems to me that this configuration would allow me to charge 1 car at the max of 48A on either outlet, and if 2 cars are plugged in, both could charge at 32A each, or possibly event 1 at 32A and 1 at 40A, as long as the 20A circuit isn't being used.

The 6/3 romex that is currently installed shares some holes through studs with the 12/2 romex.

Some questions:

  1. Are there any issues if I just replace the 6/3 with 4/3, assuming I leave the breakers and NEMA 14-50 outlets as-is? a) Can the 4/3 romex share stud holes with 12/2 romex, like the 6/3? b) Any issues if I replace the 40A breakers with 50A?
  2. Assuming I replace the 6/3 with 4/3, are there any issues if I then replace both NEMA 14-50s with hard-wired Tesla chargers capable of 48A charge each, and upgrade the 40A breakers to 60A?
  3. Any issue if I had a 50A breaker with a NEMA 14-50A outlet near the sub-panel? I would do this just to run tools. So, if I do all the above, I'd have a 90A breaker in the main panel feeding the sub-panel with 2x60A, 1x50A, and 1x20A.
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    Harper will (most likely) show up later with an incredibly complete answer, but which will boil down to: Yes, you can do that, but use aluminum instead of copper and save a bunch. Commented Oct 26, 2021 at 16:14
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    Instead of buying and installing new wire, then waiting for that to become obsolete before you get an EV, run conduit instead. Make sure it is (over-)sized sufficiently. Then later you can feed the appropriate wire size.
    – blarg
    Commented Oct 26, 2021 at 17:28
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    @Rodo, the chargers are built into the vehicles in today's line of electric vehicles. What is often called a charging station is really an EVSE, Electric Vehicle Service Equipment. It communicates with the EV and will provide a current level appropriate to the source power and the charger onboard the EV. Future-proofing the circuit, if within the budget, is a good idea. Prices for wiring aren't likely to go down over time.
    – fred_dot_u
    Commented Oct 26, 2021 at 18:00
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    @Rodo - The feature appears to be called "Power Sharing" and only on the "Gen 3 Wall Connector". More info: tesla.com/support/gen-3-wall-connector-power-sharing
    – RobH
    Commented Oct 27, 2021 at 19:09
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    While that guy's adapter got every note but one correct, the blunder is biblical. 30A and 15A outlets on 50A circuit protection! It's already a 4-space panel, if a 30A and two 15A breakers were fitted, it'd be genius uninterrupted. Commented Oct 28, 2021 at 2:01

1 Answer 1

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TLDR: Forget it, you can't predict the future. Install large conduit, or access covers that are easily removed for maintenance. Then you can install what you need, when you need it.

Learn. THEN install.

Right now, your project has a serious case of "Get-there-itis". As a result, you have run roughshod over the most important step: Learning stuff.

Dedicate some evenings to talking to smart people about what you should be doing, so you can put the best stuff in the walls before you put up the drywall.

Honestly I still think "conduit, and figure it out later" is your best choice.

Also, keep in mind it's a moving target. 5 years ago, nobody would have foreseen the circuit-sharing EVSE's. And nobody ever, ever says "No need for flexibility, nobody's going to invent anything new in electrical".

EVSEs run at 80% of circuit limit max. (not to be confused with breaker trip)

You see "125% derate" all over Code. What that means, especially for a dedicated circuit for an EV, is the EV's actual load cannot be more than 80% of the circuit ampacity.

An example: you have 55A wire. The maximum EV charge rate is 80% of that (44A). Not 48A.

the 6/3 romex.... it was good for 55A ... found that 60A breakers were typically used.

I saw someone say elsewhere that 6/3 can get pretty warm at 48A,

Note that 55A breakers are not made. In that case, you are allowed to "round up" to the next available breaker, which is 60A. This does not make the circuit 60A and you cannot run a 48A charger. You cannot plan to use those extra amps, so you still must run a 44A charge rate at most.

Wire ampacity, and copper (what a waste)

Yeah, as people allude, I'm not a fan of copper. Aluminum is perfectly capable as a large feeder.

replace the 6/3 romex with 4/3 romex. The wire is pretty expensive

Many people heard horror stories pertaining to aluminum wire in the 1970s, but didn't really validate those stories. They pertained to small 15-20A branch circuits, which were being put on terminals properly tested for copper wire only. (turns out copper terminals don't play well with others). Also, we only learned this in the 2000s but it turns out screw torques matters on all screws! (and electricians always torqued large feeder lugs). Some say "I use copper because I don't want a dissimilar metal problem" - yeah, the lugs in the panel are made of aluminum, because aluminum lugs do play well with others.

However, use of aluminum wire depends on the device's terminals being labeled for aluminum wire! It also helps if they are rated for 75C thermal.

Wire (and terminals) have a thermal rating: generally 60 degrees C or 75C. The lowest thermal rating, and the metal, decides the ampacity of the wire.

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The first thing you notice is that NM and UF cables must run at a lower ampacity because of an inferior insulating jacket. This is not a disadvantage for 15-30A small circuits, since 240.4(D) disallows use of the 75C thermal column for those.

So above 30A, people usually cross over to other wire types. Your installer used THWN wire, because NM isn't allowed outdoors and UF above #6 doesn't exist. Hardly anyone pushes it up to #4 NM, I barely knew the stuff existed.

You do not have a 90A subpanel. The individual wires in the conduit are good for 75C, now look in that column, see the red highlight? You have an 85A sub panel. If your contract specified 90A, they will need to change out your wire.

Oversubscription works for general-use circuits only

The idea of "oversubscription" (breakers on each phase add up to more than phase ampacity) is based on general household use, and the nearly zero probability that all circuits will be maxed out at once.

This is all captured in the standard load calculation for a house, which allocates living/bedroom circuit amps based on the square footage of the house. Code doesn't care if you have 1 circuit for lighting, living, bedroom and basement receptacles, or dozens. Same load calc either way. And that explains the oversubscription you see.

That's obviously "over-subscribed", and I've had one electrician tell me I can't do that, but I see over-subscribed panels all the time.

However, when you are engineering something specific that will run simultaneously or continuously, you must use actual numbers for your loads. Suppose someone ran 20A circuits for 16A Bitcoin miners. (80% rule). Each would need the full 20A provisioned to it, in addition to the house's normal load calculation. So if the latter was 73A, plus five miners per phase totaling 100A/phase, that requires 173A service.

Your two 48A EVSE's will run simultaneously. The 125% derate puts them to 60A, times two, giving 125A to be provisioned just for them. That ain't gonna happen on your 85A circuit.

Fortunately, Tesla and CrippleCreek have answers for that.

You seem confused about how circuit sharing works

Which is understandable, because it's a completely novel concept in residential electrical design.

With circuit sharing, multiple EVSE's sit On. The. Same. Circuit. This is an exception to the "dedicated circuit per EVSE" rule and is authorized by UL, via the approved instructions. That means full circuit ampacity wire must be run to each of them, normally one expects them to be "daisy-chained" though any tree topology is also allowed.

They are configured master-servant. The master EVSE is told the ampacity of the entire circuit they share. It then coordinates with the other EVSE's to assure that, together, they do not draw more than 80% of that ampacity. So for instance on a 55A circuit, the master EVSE is told 44A.

The EVSE's then use whatever proprietary or patented, but UL-approved, technology to guarantee that the sum of ampacity draws being authorized by the EVSE's does not exceed the program value (e.g. 44A).

One could run a separate breaker'd circuit to each EVSE in the sharing network, but it would be a waste of wires and breakers if one did.

This is the fundamental problem with your strategy. You want full flexibility to adapt to emerging technologies, but you also want to "finish" the garage (why???), entombing the utility spaces so flexibility is impossible.

The ONLY answer to what you want is large conduit, or some sort of removable panel system to give you access to walls.

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