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We are planning to put in a new circuit in our garage for an EV charger. The main panel (200A) is on the other side of the exterior of where we would put it, so it’s only maybe ~4 feet of cable needed. The only thing coming of the main panel is the 100A line to our solar/battery which ultimately goes to the subpanel that the whole house is on, so there’s already conduit running through the wall and into the garage. The EV we’re hoping to get can accept up to 48A so we’ll probably put in at least a 60A circuit, but we also want to future-proof a bit so we’re wondering if we should just put in an 80A circuit in case future EVs take more amps or we want to charge more than one. We figure that if we or a future owner wanted to just have a 14-50 outlet, it would be very easy to turn the junction box into an outlet.

Here are our questions:

  1. Can the new line run through the same conduit as the 100A line at least into the garage or do we need another hole in the wall?
  2. Would an 80A circuit be much more expensive than a 60A one?
  3. I’ve heard 80A circuits need a disconnect box, too. Is that accurate?
  4. Is this all really silly and we should just run a 60A line and call it a day.
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    What size/type are the conduit and wires in place now? Please note that in conduit, you do NOT want to run cable, you want to run individual wires. Cable does not work well with "conduit fill" (which is what that first question is about) and is generally miserable and annoying to get through conduit. Wires are better on both points.
    – Ecnerwal
    Jan 9 at 12:42
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    It's a 1-1/4" EMT conduit carrying 3 #2 AWG THWN-2 wires and a #8 AWG THWN ground.
    – levineds
    Jan 10 at 3:18

3 Answers 3

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The state of the art

The future trend in EV charging is toward "Share2" class charge ports (EVESE), such as the Tesla Wall Connector v2 and v3. This is designed for a whole stable of EVs to share one current capacity, and dynamically split the capacity amongst them. The ceiling on the shared ampacity is 100A under current designs. However this requires (really benefits from) a subpanel to distribute power to each EVSE.

You need a subpanel anyway, so might as well do it proper.

Here's the fatal flaw. At 60A circuit size, there are no affordable sockets. Therefore, the 48A continuous/60A breaker EVSE must be hardwired. Hardwired ones must have a disconnect switch in line-of-sight to the EVSE.

You can faff around with trying to buy a literal disconnect switch if you really want to, but the cheapest disconnect switch made is a subpanel.

A pitiful 2-space panel will suffice for a single EVSE, however, the cost differential for a larger subpanel is trivial (at least on total project cost). And the "regret cost" of needing more spaces later is much higher. Therefore, it is always "pennywise, pound foolish" to chintz out on subpanels. We advise "go big, really big". Even a 24 space panel has only about a $60 cost differential vs. a 2-space "disconnect enclosure".

The subpanel will also serve as the disconnect for a second EVSE should you go into the Share2 scheme.

For many types of EVSE, it is "power sharing" not "circuit sharing". E.G. the Tesla Wall Connector V2 can share up to 100A, but requires its own breaker/circuit per EVSE. And it is limited to 60A per EVSE (hence 60A breaker and 65A wire* may be used). So that means you need multiple 2-space breakers to serve as disconnects.

Wire or cable

If a suitable subpanel location is quite close to the main panel, the hands-down best way to plumb that is with non-flexible metal conduit such as EMT. That lets you throw in wires any size you want, assuming the conduit is big enough.

For very short runs I don't sweat the price of aluminum vs copper, it's more about availability. Certainly do not sweat aluminum's safety - that has never been questioned on feeders this large.

#2 aluminum is a commodity item (because it's used for 100A services to a dwelling due to a favorable derate, 310.15b7). For us mortals doing subpanel feeder, it's only good to 90A.

#4 copper is 85A for us. Breaker at 90A*.

For honest 100A you need #1Al or #3Cu.

How do we provision houses, anyway?

The number of breaker spaces in a house do not matter. The sum of all the breaker trips in a house do not matter.

Any house, when attached to the electrical grid, is the subject of a Load Calculation. This takes the square footage of the house (as a "catch-all" for general household lighting and small-appliance load, typically 3 VA (similar to watts) per square foot), certain receptacle circuits (kitchen and bathroom) at typically 1500 VA per circuit... a formula for certain large appliances, and straight nameplate data for other large appliances.

And common sense is applied: appliances are allowed to overlap if they won't be run simultaneously. (A/C and heat; wood shop and EVSEs; multiple tools in the wood shop when you have only one dust collector).

Even in subpanels, the operative language is "sufficient for the loads to be served".

Now the Load Calculation must be done at the time of the house's construction, as a condition of the permit. However it should be re-done when any large load is added, and EVSE's definitely qualify. If a permit was pulled to add the large load, this would be part of the permit process.

The beauty thing of an EVSE is that the ampacity is a "soft" (well, actually, "firm") setting. If you run a Load Calculation and find you only have 45A to spare for EV charging, then commission the EVSE to <=45A. This setting is done either in DIP switches or a very special and hard-to-access WiFi network. (this is a UL requirement to keep casual consumers from spiking their charge rates and setting their house on fire).

This is particularly helpful with multiple EVSEs and Share2 method. If you have three EVSE's and 50A to spare, rather than giving each EVSE 16 amps, you can give them all 50A to share, and they will "share on the fly". You can hasten one EV's charge by telling the other EV to defer charging for awhile.



* say what??? There is no wire with a 60A ampacity, it's all 55A or 65A. You are not allowed to plan to use more than actual wire ampacity, so a 48A/60A EVSE needs honest-60A wire, so 55A will not do and 65A wire must be used.

However if you have an odd size of wire like that, and you do not plan to use more than the honest ampacity (e.g. 85A) then you round up to the next available breaker (e.g. 90A). This is not license to plan to use those extra 5 amps!

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  • So, if I'm understanding correctly, regardless of circuit size, there needs to be a disconnect that you can get at from the garage, and the breakers in a subpanel would qualify. It's a very small garage so wall space is at a premium so I would still probably go with a "smaller" panel. I know the sum of breakers is allowed to exceed the total amapacity, but is that limitless? Can we have 260A of breakers (we plan to add one more 60A circuit eventually) in a 200A MSP. If both chargers need their own 60A circuit, what is the point of the Share2? Why not just have two independent chargers then?
    – levineds
    Jan 10 at 3:40
  • @levineds The sum of the breakers doesn't matter; what matters is the actual (derated) draw of the things that will run simultaneously. Share2 lets you declare three 60A EVSE's to be 80A total instead of 180A total. (or just dial in the number that equals the headroom on your service). Jan 10 at 6:17
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    @levineds It's hard to envision a garage wall so full that you couldn't make room for a 24-space panel today. The small difference in cost today will be more than made up for when you go to add something else and discover that your economical little panel from today is full. Unless, of course, your main panel still has plenty of free space in it, in which case, do as you wish!
    – FreeMan
    Jan 10 at 18:27
  • @levineds I added a section talking about how Load Calculations are done. FWIW, all panels >8 space are 14-1/4" wide x 3.5" deep, trapped between wire bending space regs and the width of a stud bay. They differ only by height. I too have found myself stuck at 8 spaces due to dimensional issues. I have two FPE 6-space panels that are about 6x9", double-stuffed with 12 breakers each; I have no replacement option for them as they are boxed in by 480V panels. Jan 10 at 19:55
  • @Harper-ReinstateUkraine, I'm reading that a disconnecting means is required for greater than 60A or 150 V to ground (625.43). I believe that a 48A EVSE hardwired to a 240V 60A circuit avoids this requirement. Am I misreading the code here?
    – Stanwood
    May 8 at 2:50
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Whether you need another hole in the wall and a second conduit depends on how full the conduit you have now is. That depends on the size and type of the conduit and the size and number of wires in it now.

In general, if you need a local disconnect, you are probably better off simply running a sub-panel, because a sub-panel is more generally useful and if you use a main-breaker panel, there's your shutoff/disconnect. Cost is similar to or even less than "just a disconnect" because of volume.

If your main is 200 and your house runs off of a 100 A sub, unless you have not mentioned something you have 100A free to run a sub-panel here.

There is some cost difference between an 80 A breaker and a 60A breaker. For the aluminum wire you should be using (unless your charger specifies copper, and then that only needs to be from the last point of connection to it) the cost difference is very small for ~4 feet of wire. But do install the parts first and then carefully measure for the wire, including needed bending space, so you don't come up 6" short. With a sub-panel in the garage, any later upgrade is made very simple.

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  • We have a goal of someday adding another 60A line for a hot tub, but otherwise we really only have the 100A house sub right now, does that complicate things?
    – levineds
    Jan 10 at 3:42
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    @levineds Not really, EVSE ampacity is a "soft" (well actually "firm") setting and it can be changed by repeating the commissioning procedure with a new ampacity value set. Thus, you can dial the EVSE ampacity to match the surplus ampacity in your house's Load Calculation. Jan 10 at 19:22
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Your charger documentation will say what is the largest breaker you can use.

You can use larger wire than is required, larger wire costs more.

Using wire is phyically too big to connect to the breaker will make life difficult.

As the run is so short replacing the wire and breaker if upgrading won't be too expensive.

Install conduit large enough for the 100A circuit, but only install the 60A

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  • 60A breakers are odd ducks. They are the same price as a 15A breaker, but they use the same frame as larger breakers, and so, take the same wires as larger breakers. As such, 60A breakers have ridiculous headroom on wire size. Jan 10 at 0:47

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