How much power can I supply for an EV charging circuit?

Question

I would like to install an EVSE. The main panel is in the garage and I should have no problems running conduit a few feet to the location where I plan on mounting the charger. I will make sure I get the correct breaker, wire size, and receptacle according to the specifications on the charger I end up getting. But before I buy a changer, I am curious to know how much additional power I can get from this panel. Obviously I can't just add up the capacity of all the breakers because they are already much greater than the main breaker. I suppose I should do a load-calc, but I have no idea how this is done. Do I need to look at the data plates on the back of all my appliances? This seems like it could be a lot of work.

If I just eyeball the loads, I guess that this service and panel should be able to supply another 30 amps without any problem.

I suppose that if I run the oven, all the burners on the range, the microwave, the AC, the dryer, a few hairdryers, and charge my car at the same time I might pop the main breaker, but this seems not likely for our family.

Answer 1

TLDR: Get a load-sensing energy monitor + EVSE pair, and allow them to automatically adjust your EV charge rate on the fly to protect your house from overload. EV charging is NOT a fixed rate, and can dynamically - well here, watch it happen in this video. Go to 29:30 to watch the auto-rate-change, though the whole video may be of value.

DO NOT rely on smart meter data of past usage.

First, that data isn't accurate. It is there to set billing tariffs, it is not for the purpose of safely sizing wires and services. Suppose you start a 24-amp electric dryer at 8:40 and run it til 9:20. It's only 20 minutes in each hour window, so it reports as an 8-amp peak for 2 hours. Really? An 8-amp dryer, so we can wire it with 16 AWG (10 amp) wire for that dryer?

Obviously no -- you need to look at equipment nameplates.

And even so, it presumes that your family will never change their energy use in the future, when they certainly will - particularly seasonally.

Even worse, if you have solar, that "usage" data will be poisoned by the solar production. It's nice when you get solar, but you can't provision circuits on the assumption that it will be there.

A NEC Article 220 Load Calculation is the right way, and not that hard.

I suppose I should do a load-calc, but I have no idea how this is done. Do I need to look at the data plates on the back of all my appliances? This seems like it could be a lot of work.

That's just naysaying talk. No, it's not that hard. You only need to do that for fastened-in-place loads and only fairly large ones (e.g. not lights or bathroom fan, not range hood, and not refrigerator - those are too trivial to bother measuring.) So we're down to water heater, range, built-in A/C, furnace, dryer and a few other luxury items.

Just grab a worksheet off the web from any municipality (i.e. government page) - there are a lot of DIY clickbait sites full of "article marketing" - badly written articles stuffed with keywords to spoof Google. They used to pay Bangladeshis a penny a page to write them, now they use ChatGPT.

Here's one from Clark County NV, they all look about the same:

• 3 VA per square foot of "habitable space" (not garage/utility rooms). This is a "catch-all" to cover hard-wired lighting and any portable, plug-in consumer 120V toys like blender, toaster, hair dryer, PCs, electric blankets or whatever. From here on out, we are only concerned with huge or fastened-in-place loads (having a plug does not exempt them).
• 1500 VA for each of the two kitchen general-use receptacle circuits
• 1500 VA for the laundry room circuit (which covers everything plugged into that 120V circuit: washer and gas dryer)
• 4500 VA (almost always) for the water heater
• 5500 VA (usually) for the electric dryer
• A funny formula for ranges and ovens, see NEC 220.55.
• Heating load >OR< A/C load, only the larger one
• Ignore small-draw loads like refrigerator, freezer, range hood, bathroom fan, etc. Refrigerators only take about 1 amp (120 watts) when they're running. On average it's more like 40 watts. Freezers slightly less.

Then your formula gives you some favorable markdowns; these are to account for load diversity (you're not likely to run everything maxed out at once).

EV charging does NOT get ANY discount. It is the hardest "hard load" in your entire house, as it goes many hours at hard max rate. (limited to 80% of breaker trip).

Yeah, so like 12 paragraphs to tell you it's not that hard. Great, Harper.

Or, skip all this stuff and use an energy management system.

Emporia already makes a home energy monitor, the VUE. Their EVSE product is UL-listed to communicate with the Vue and tell the EVSE how much "headroom" there is on the service right now. The EVSE will then dynamically adjust car charge rate to keep your power draw within the limits of your service.

This is a "silver bullet" for people who want the max possible charge rate for vanity, but also don't want to burn their house down.

I know the product selection in this category is slim, but it really is the silver bullet and stupidity avoider.

A case study in doing it wrong

I've a Tesla Model-X with a Level 2 charger rated at 48A, to which is connected to a 60A breaker circuit. In the summer when my A/C is running, sometimes the main breaker trips. I simply just limit the number of Amps the car is allowed to pull via the app. Mostly I just leave it set at 25A, as my car mostly always charges overnight. If I need a quick charge for whatever reason, I just drive to the closest supercharger.

That's the classic scenario that'll burn your house down. This person is foolishly betting the house on the main breaker working perfectly, apparently unaware these devices are built with a great deal of tolerance to avoid nuisance trips.

A big red flag: your main breaker should NEVER trip unless a flashy-bangy problem just happened, like a dryer shorting out internally. If the main breaker is tripping from intentional overload, it's not overloading a little... it's overloading a lot. This is a great way to burn your house down. I expect an epidemic of this from fools with EVs wanting "the biggest charge possible" without knowing what they are doing. This is a threat to our freedom becasue heavy-handed legislation is likely to follow.

No one should be using "the app" to limit charging. If only 25A is safe to use based on the house's Load Calculation, then the Tesla Wall Connector should be re-commissioned at 25A instead of 60A.

Just because you've gotten lucky so far doesn't make it safe. Electrical safety relies on defense in depth, and you're not allowed to defeat/bypass all the layers except one.

I realize there's a testosterone-driven competition to have a faster EVSE than the next guy... but you're playing with fire. (of course that's part of the game, isn't it, just like a lot of "jackass" type activities we see on Tiktok.)

Honestly for the guy who's "just gotta have it" for vanity reasons, that's where you have energy monitor-based products where they are doing demand side management to let your car take full power during the moments the house has it to spare -- which is honestly, over 90% of the time. House loads (other than EV charging) are really spiky - most loads cycle on/off intermittently, often automatically... and the Load Calculation has well-developed science to let you exploit that without creating danger.

Answer 2

There are online load calculators, such as this one.

If your power company provides downloadable usage information, that can be a useful guide as well. Typically those provide either hourly or 15-minute information for up to a year. 15-minute data is best, as that is the standard for metering. They may provide kW (peak or average demand) or kWh (actual usage). What you are looking for is peak demand in Amps:

• 15-minute kWh - divide by 0.06
• hourly kWh - divide by 0.24
• 15-minute kW - divide by 0.24
• hourly kW - divide by 0.24

On the one hand, true peak demand would be best, but you will likely (in residential simple reports) only get usage or average demand. But since various parts of code effectively account for short-term moderate increases, this really isn't that much of an issue.

Ideal is to do both a load calculation and an analysis of the past year's usage and use whichever number is higher.

Once you figure that out, you will know how much headroom you have for EV charging. EV charging is a continuous load, so it is derated. For example, if you use a 20A circuit then you will actually only charge at 16A, 40A at 32A, 50A at 40A, etc. The breaker has to be sized based on the capacity of the wire, even though you will typically set the EVSE for 80% of that capacity.

You have a 150A main breaker. That is a little unusual - more common these days is 200A, and the panel may well be rated for 200A, so I suspect that is based on the utility service and/or the feed wire from the meter to the panel - i.e., you can't upgrade that without utility consent and making sure everything is just right.

The good news is that you have at least some gas appliances (presumably water heater, at least part of the kitchen equipment, furnace), small A/C (20A) and not a lot of extras, so my guess is you could install 30A or 40A without much of an issue.

But keep in mind that most people don't need more than 20A or 30A @ 240V to keep up with daily EV usage by charging through the night.

Also, do not install a receptacle. Install a hard-wired EVSE, whether Tesla or another brand. There is nothing gained by using a receptacle if you are going to leave the EVSE plugged in 24/7. If you actually need a high-powered receptacle for occasional other use (RV, welding, etc.) then you can install that on a separate circuit as you have plenty of space left in your panel. A receptacle adds another point of failure and in the latest NEC code GFCI is required for all garage receptacles including high-power 240V circuits which were previously exempt. Since EVSE includes GFCI circuitry, a hard-wired EVSE circuit is exempt from this rule.

Normally aluminum wire is worth considering for large loads. However, most (if not all) residential EVSE are not rated for aluminum wire, so use copper. Since you only have a short run that is not a big cost.

Answer 3

Every domestic EVSE can be programmed to limit the amount of current they allow the car to draw.

Smarter EVSE also have current probes that hook around the feeds of the panel to let the EVSE dynamically limit the charging current and this means it doesn't participate in the load calculation.

You are in luck given that your panel is right next to the place where you would put the EVSE. So the surcharge in bumping up the size of the wires to the EVSE is minimal.

So you can put in the max wire size and breaker but then limit the current at the EVSE if it would overburden your main breaker.

Answer 4

You probably have a smart meter installed by your power company. Take a look at it and there should be some reading on it dealing with current KW. Multiply that by 1000 and divide by 240 to get your approximate amps. This will vary during the day so take some reading at different times. There are devices out there to measure kilowatts but most don't do the 240V appliances. Look at your AC, range and dryer's nameplates for amperage. At just a quick glance, it doesn't appear you'll have any problem as it appears you've got a gas hookup for some large appliances and a 150 Amp main. You'll probably be charging the car in the late evening when loads are usually down. Also, don't install the EVSE until you know which car you'll be buying. The technology and efficiency are changing daily and you'll want to get the charger your car manufacturer recommends.