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I have a 90 amp breaker outside my house, which then runs to my breaker box within my garage. My A/C and range are on separated external breakers.

I want to install a 240 V charger for a Tesla within my garage and am seeing many conflicting requirements for setups.

Initially, I was going to switch the main breaker off and install a sub-panel within my garage which would house the breaker running to the 240 V charger. The input for this would run from my garage breaker box. After doing some more research, I am getting conflicting numbers on if my house setup can support enough current to charge my car.

When I look at the specs, the Tesla charger states it should be on a 60A breaker.

If I only have a 90A breaker coming into my internal breaker box, that is not enough current to run my house and my charger. I’m somewhat technical, but the second I need to reach out to the external breaker box I am going to hire an electrician.

Is there anything I should know to seem more knowledgeable on the subject before the electrician comes?

Is it as simple as adding an external breaker outside and running a new wire to my garage like my A/C and range are currently wired? Or will I need to run a new breaker (~200A) to my internal breaker with larger wire and split off from there.

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    The Tesla EVSE (like many others) can be configured to work on any size circuit from 60A all the way down to 15A. You’ll have to do a load calculation to see how much more load your service can support. How many miles do you actually drive / how many kWh does the vehicle use per day? Odds are you don’t need 60A - 15A or 20A is plenty for many commutes.
    – nobody
    Commented Apr 25, 2023 at 1:44
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    Can you get us photos of the service panel that has the 90A breaker and the other breakers? Commented Apr 25, 2023 at 3:40
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    Yes, can you get us photos of both the service panel and the subpanel please? Commented Apr 25, 2023 at 3:50
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    Please revise your post title to ask a clear, specific question. This is expected of users on all SE networks.
    – isherwood
    Commented Apr 25, 2023 at 13:10

2 Answers 2

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BIG problem with that service equipment

Here's the problem. You have a 90A breaker feeding your house, but you have other breakers to other large 240V loads. That is called the "Rule of Sux" I'm sorry, "Rule of Six" where you have up to six main breakers. That would be well and good... The problem is, when you add up all the breaker handles, it adds up to more than your service size that your service can safely handle.

What prevents overloads? Nothing, except the NEC Article 220 Load Calculation. That's pretty good, and that would work, except for one problem: later additions. Because when people add stuff later, nobody bothers to re-do the Load Calculation. They just dogpile on more loads until something catches fire.

And that means, it's likely your panel is already overloaded from past additions. It's worth doing a new NEC Article 220 Load Calculation just to see where you are. You compare this to the service size, which can be determined either by asking the power company, or looking at the size of the service entrance wires (weatherhead to meter to main breakers).

If your Load Calculation shows headroom available, you can provision that breaker/circuit size (rounding down) and you're all set. Select that breaker size when commissioning the Wall Connector, and it takes care of limiting the car to a safe rate. Although it's super dumb; it only charges at the rate you set.

Cheat #1: measure power used and adjust the EV on the fly.

EVs can charge at any rate, controlled dynamically "on the fly" by a signal from the wall unit (EVSE or Tesla Wall Connector). The Tesla unit is very dumb, like most of them, and the most it can do is share a fixed power allocation across multiple EVSEs.

However, Emporia (Or Myenergi in Europe) makes a home power monitor that clamps the main power wires coming into the house, and tells the EVSE how much power is being taken off the grid right now. The EVSE compares that to the safe limit you have configured, and tells the EV to take the rest. Very slick. To adapt an Emporia EVSE to your Tesla, you'll need the adapter provided with the car. Tesla will cheerfully sell you another one for like $30.

This may be a good answer for you, because it does an end run around the load limits, and allows you to run at very high power when it's available (which will be 90% of the time, and pretty much all night).

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    +1 for suggesting Emporia -- but even if this answer was mediocre, I would still vote a +1 just for "Rule of Sux". Commented Apr 25, 2023 at 4:14
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    Robocop (the original movie) had the 6000 SUX as their satirical 1980's "future car" (8.2 MPG!). 90 SUX seems appropriate here.
    – Machavity
    Commented Apr 25, 2023 at 12:30
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An electric vehicle can charge at home from as little as 12A @ 120V (the same receptacles you have for plugging in your TV, computer, toaster, hair dryer, etc.) to as much as 48A @ 240V. 48A continuous requires a 60A breaker. So that is the maximum that you can use for a Tesla (and many other) EVSE (technical term for a charger). But you don't have to supply it with 48A, so you don't need a 60A breaker.

In fact, most people can do just fine with a 30A or even 20A 240V circuit. Using Tesla as an example, sticking with the 3 & Y as they are the most popular (S & X use the same EVSE but need a bit more power, on average, to go the same distance), a 20A circuit (16A continuous) will add ~ 15 miles of range per hour of charging and a 30A circuit (24A continuous) will add ~ 22 miles of range per house of charging. That doesn't sound like much, but if your car sits for 10 hours at night, as most do most of the time (midnight shopping is the exception, not the rule, for most people) that gets you 150 to 220 miles of range each night. In other words, a 20A or 30A circuit will get you fully charged every night unless you drive a lot more than most people. There are exceptions for everyone, which is why there are Superchargers.

So the goal for most people should be to find 20A to 30A of capacity. We figure out capacity with a load calculation. This takes all of the stuff on a panel and determines how much it will typically use. This is much more nuanced than "add up all the breaker values". It includes square footage of the house, size and type of built-in cooking equipment, standard circuits for kitchen and bathrooms, clothes dryer (if electric), water heater (if electric), HVAC (maximum of heating or cooling), etc. A load calculation needs to be performed at each relevant level:

  • Utility Service - typical is 200A, but many older houses have less and some newer, larger, houses have 400A.
  • Main panel - this will usually have a breaker the same size as the utility service (sometimes 2 x 200A in the case of 400A).
  • Subpanel - if the EVSE will be fed from a subpanel instead of the main panel then all the devices served by that panel need to have a load calculation, which is then compared to the feed breaker (e.g., 90A) to that panel.

Based on "90A breaker for the breaker box in the garage" + "A/C and range on separate breakers", it sounds like you actually have a main panel either with the meter (meter main) or near the meter. You need to find out the main breaker size on that panel (if it has a main breaker, it could actually have up to 6 independent breakers without a single overall main breaker) and the utility service size. Let's say for the moment that this panel is either a 200A panel (and therefore 200A max) or a rule-of-six panel with a 200A utility feed. In either case, that would give you a total of 200A to work with. A load calculation for this panel should be done properly (i.e., calculate everything everywhere) but the baseline would be 90A for the 90A feed and the nameplate ratings (may be significantly less than the breaker sizes) for the A/C and range. Add those up. If it is at least 20A less than the 200A (or whatever it is - you must find out) feeding the panel then you are in great shape and you can the EVSE breaker in that panel.

If the main panel does not have sufficient capacity then you are likely out of luck unless you can make changes elsewhere in the house to free up capacity. There are ways to share power between devices, but that gets a bit trickier.

If the main panel has the capacity but doesn't have a physical space for an additional breaker or you really want to run the EVSE off of the subpanel then you have to do a load calculation on the subpanel. I am skeptical that you'll find 20A to spare in a 90A panel, but it is possible, particularly if you have gas water heating and/or gas clothes dryer.

Note also that with some (I believe with Tesla it is fine) but not all EVSE, you can change the configuration at a later time. So if you start with 20A because that's all the capacity you have available and later you do a heavy-up or make changes to other appliances to get more capacity then you can replace the breaker and the wire to increase the EVSE charge rate.

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    You can modify the charging rate at any time. Currently my Tesla is being charged from a 40 amp breaker. So my charge limit is 32 amps which gives you about 10% a hour for a long range model 3. The Model 3 or Y can charge at a max rate of 48 amps which would require a 60 amp breaker.
    – JD74
    Commented Apr 25, 2023 at 2:58
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    regarding your last paragraph: Changing the wire to the garage might be quite the hassle, but one can put in the big wire now, and still use a small breaker.
    – Christian
    Commented Apr 25, 2023 at 14:18

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