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Others have asked similar question, but I wanted to put in my setup and get any advice/critiques. Adding two ChargePoint chargers to my garage. New car charges at 11Kw onboard charge (with 240v, that’s 45.8A), so I’d like to set the charge points to max 48A to take advantage of this although not needed for my other car at this time. Chargers are continuous pull, that means 1.25x 48A = 60A rated circuit.

Going to do two separate runs from the circuit panel. First is 45 feet and second is 75 feet. Going to use the following for each charger: 2 hots- 6 AWG THHN/THWN-2 copper 19 strand cerrowire 1 ground - 8 AWG THHN/THWN-2 copper 19 strand cerrowire

Both runs, even in the unfinished basement ceiling, in 1” conduit. These will be two runs of 1” conduit (schedule 40, 90C rated) so there are only 2 load carrying wires and the one ground per conduit run.

The 1” conduit is rated at 90c, and the 6 AWG THHN/THWN-2 copper 19 strand cerrowire is rated at 75A at 90c (55A at 60c).

The actual question: Since the wire is rated 75A at 90c, and the 1” conduit is rated at 90c with no amperage adjustment needed for just 2 load wires, I think I can use a 60A breaker and set this to 48A. In fact, I think I could go up to a 75A breaker and set the charger to 60A if I wanted to in the future. Am I missing anything? I have seen a lot of posts about 6 AWG being rated only to 55A (I guess at 60C), but since my conduit is rated at 90C, it seem like I should be able to use the 90c amperage of 70A.

Thanks ahead of time for any advice and answering a very similar to question to other posts! I just wanted to put in my specific setup and see if I am doing anything wrong.

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    Can you confirm that you did a load calculation to see if there is enough spare power?
    – crip659
    Jan 1 at 14:44

2 Answers 2

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I'd really like to advise you to please watch Technology Connections' superb video on home EV charging, and the section at 28:15 seems written for you. I hear this kind of "huge charging" plan from EV novices a lot, and obviously, so does Alec.

It seems like either you believe EVs can only charge at one specific speed (if the above didn't put that right, see Alec's other video on how EV charging works), or you're a "Fastest Charge Possible" type personality. It's nice for travel, where you need 0-100% overnight, but it doesn't make any practical sense for real world EV charging at home.

You can really do one circuit and use Power Sharing with equipped EVSEs (not the ChargePoint; honestly we're not big fans of it since it provides so few options for provisioning power to the EV). A single 48A circuit is going to be plennnnty for 3 cars with Power Sharing, given that it's unlikely all 3 are going to need 150 miles of recharge in the same night.

I am skeptical about your panel having 120A of service capacity free in it. A Load Calculation must be done on the panel and service. We see a lot of people "freestyling" their Load Calcs based on hopes, wishes, and formulas their AHJ or insurer would not approve. Sacramento's worksheet accurately captures NEC 220.82, the most appropriate for EV charging. EV charging is a crushing load on a panel, and must not be underestimated. That's why less is better! Anyway, if there are Load Calculation issues, we have a variety of options to get you plenty of charging (despite the Load Calc) if you need it. Come on back and ask questions about that if applicable.

New car charges at 11Kw onboard charge (with 240v, that’s 45.8A)

That's not how that works. They're giving you the sales number, or a presumption that the voltage is 230V. Actually, the EV spec does not specify volts or watts, but rather amps. In the US or Canada, you'll have honest 240V, so 11.56 kW nominal.

The charge rate, in amps, is decided by the EVSE not the EV. I guarantee you they did not program the car to artificially limit the charge rate to 45.8 amps. It supports up to a given ampacity, but that will be a "round by EV standards" number such as 24, 32, 40 or 48 amps.

2 hots- 6 AWG THHN/THWN-2 copper 19 strand cerrowire 1 ground - 8 AWG THHN/THWN-2 copper 19 strand cerrowire

That's fine in conduit, but the ground can be 10 AWG.

Both runs, even in the unfinished basement ceiling, in 1” conduit. These will be two runs of 1” conduit (schedule 40, 90C rated) so there are only 2 load carrying wires and the one ground per conduit run.

There's no Code necessity to run 2 conduits. If you put both circuits in the same conduit, you would require an 80% derate from the highest temp permitted for the wire, which is 90°C. For 6 AWG wire that's 75 amps. An 80% derate off that puts you at 60 amps, which means it's fine to run both circuits in the same conduit.

That sucker is going to get HOT (whether it's in 1 conduit or 2). After all, you're running it almost at 75°C limit, so it's hardly a surprise if it runs at 75°C. The PVC conduit may sag or deform from all this heat; I would use EMT. It will also cool better. Some people find this heat coming off the conduit objectionable, but like Alec says in the video, the Fastest Charge Possible (tm) comes with headaches you hadn't bargained for. That's the least of them.

In fact, I think I could go up to a 75A breaker and set the charger to 60A if I wanted to in the future. Am I missing anything?

Does not work that way, in 2 dimensions.

Yes, you're missing the labeling on your load center / panel, which limits you to 75C both inside the enclosure and at terminals. UL approved that labeling as part of giving the panel a UL listing. If you want to run the 90C thermal numbers, you need either an industrial 90C panelboard, or to pigtail with 75C wire into an external enclosure rated 90C, then use 90C splices. At both ends. That costs more than the next size of wire. I would not play games like that unless I was boxed in by an underground conduit constraint.

Even so, "60 amps" is not a commonly supported charging rate/speed, so finding EVSEs with that speed would be an adventure. The next size up is generally 80A / 100A circuit. In that case you're not going to worry about the 6 AWG THHN wire (which is cheap anyway, under a dollar a foot) and you'll just go straight for #1 aluminum to the mandatory disconnect that is required above 48A, and then #3 copper the last 2 feet to the EVSE.

I have seen a lot of posts about 6 AWG being rated only to 55A (I guess at 60C)

They're referring to NM and UF type cable, which has inferior insulation. UF is stated directly in Table 310.16, and NM is set in NEC 334.80.

it seem like I should be able to use the 90c amperage of 70A.

That works inside the conduit... that's why you can derate 4 wires in the conduit. Does not work in consumer-tier load centers rated 75C.

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Wire/Breaker Sizing

In most circumstances, wire/breaker sizes based on a standard ampacity chart use either the 60 C column for NM-B cable or the 75 C column for wires in conduit and certain other types of cable. The 90 C column doesn't factor in to most setups, with the exception of derating of multiple circuits in the same conduit. In other words, ignore the 90 C column.

Which means that generally speaking for a 60A breaker/48A actual charge (80% derate) you use 6 AWG copper wires in conduit or 4 AWG aluminum wires in conduit. However, in addition to any local rules regarding aluminum wires, if the EVSE requires copper (as many do) then you must use copper. From a practical standpoint, that means one of three possibilities:

  • Copper the entire distance if < 30A/24A or a relatively short distance - because for smaller sizes it doesn't make that much of a difference in cost and for larger sizes the splices get expensive.
  • Aluminum most of the distance and then a subpanel. A subpanel allows you to add extra circuits (e.g., lighting, 120V receptacles for tools, etc.). The catch is that a subpanel requires a 30" x 36" open space in front of it, which may not be practical in many setups.
  • Aluminum most of the distance and then a disconnect switch. A 60A disconnect switch costs $20 or less, satisfies the requirement for a disconnect if actually needed for any reason, and is perfect for bridging aluminum to copper for EVSE because EVSE (like typical outdoor AC units) doesn't use neutral, so no neutral to have to bridge aluminum to copper.

Load Sharing

It is critically important to manage your loads sensibly. It may not matter much for you right now, but long-term this will be important to the electric grid and, if/when peak demand pricing makes its way down to residential billing, to your wallet. Assuming your load calculation allows 60A total, you should configure your EVSEs to talk to each other and share that 60A/48A actual. Meaning if only one car is charging, it can have the full 48A. If two are charging and they both have low battery charge levels, then they each get 24A. If one has 80% and the other has 40% then the 80% might get 12A and the 40% get 36A. And so on. The technology to do this is readily available and is really, really important for anyone with multiple EVSE.

Load Calculation

Last, but absolutely not least, is a Load Calculation. First is on your entire service. Depending on how big your house is, how many appliances are gas vs. electric, any big electricity hogs (bitcoin mining, "extra" lighting, tankless hot water (please don't do that!), resistance electric heat, etc.) you may have 100A available, you may already be oversubscribed, or anything in between. If you really want 60A/48A but only have 20A available (or less!) then a heavy-up may be in order. But there are other ways of managing charge rates based on total usage, so if it comes down to that then there may be other options.

Keep in mind that a Load Calculation can be at multiple levels. For example, if you have 400A/320A service to 2 x 200A panels and one of those has a 100A feed to the garage then you really have to do 3 Load Calculations:

  • Entire service
  • The 200A panel that feeds the garage subpanel
  • The 100A garage subpanel

Using this example, if the Load Calculation is 300A total with 150A to each 200A panel then you may be able to move a few loads from one panel to the other so that the panel feeding the garage subpanel has more headroom. If that panel has enough capacity but the 100A feed is full then you may be able to upgrade that feed. Etc. Exact specifics vary a lot.

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  • Thank you, this was extremely helpful. I’m in conduit and so will use 75C colum and keep it 60A circuit breaker and set the EV charger at 48A max. Just to verify, it is ok for me to just run the copper 6 AWG THHN/THWN-2 from my main box to the charger? I wasn’t planning to run aluminum out to a garage subpanel simply because the garage is already adequately wired outside of the EV chargers and the run to the main box is very easy. But, I wanted to verify it is fine to just run the 6awg copper in conduit directly from main box to EVSE and skip the subpanel. Thanks!!
    – John
    Jan 2 at 2:25
  • Absolutely OK. The subpanel is for one of two purposes: to be able to use aluminum to save money (assuming the charger can't take aluminum directly) and to wire additional circuits (which you said is not an issue). But watch that load calculation... Jan 2 at 2:47

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