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I've been shopping for an electric vehicle charger and have found some that are designed to be plugged into a household clothes dryer outlet. I think that the common dryer circuits are 30 amp and 50 amp (in the US). I believe that household mains circuits shouldn't use at more than 80% of the circuit's rated current capacity when the load is sustained over time. Almost all of the chargers I've looked at are 16, 32 or 40 amp chargers. So using a 40 amp charger on a 50 amp makes sense to me because it would running at 80% capacity.

I did manage to find a 24 amp charger which the manufacturer promoted as being ideal for 30 amp dryer circuits. I would have though that 24 amps would be a common charger value since it would represent 80% of the capacity for 30 amp circuits. But for some reason, the seemingly arbitrary values of 16 and 32 amps are the common charger capacities.

Now the problem I'm having is that, while 24 amps is an obvious value for chargers on a 30 amp circuits, I can't find one anymore. The closest I could find was a 26 amp charger (Amazon).

My question is about the 80% rule:

  • How critical is the margin for error?
  • Is using a 26 amp charger on a 30 amp circuit a bad idea because it would be running at about 87% capacity?
  • Is the extra 7% acceptable, or is 24 amps a critical value which I shouldn't exceed?
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  • Hi, radioshack guy! please don't accept an answer so quickly. H-RM's answer is a lot better than the one you selected. Sep 15, 2021 at 18:43
  • Suggestion from a BEV owner: drop the $$ and put in a NEMA14-50 outlet. Your car will limit the current flow to what the car is designed to handle, and with a 40-A (continuous) supply other or future users will be much happier with your charger. Sep 15, 2021 at 18:45

3 Answers 3

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There's no such thing as a NEMA 10-30 charger

That's because no reputable EVSE (charger) vendor, anywhere, at all, is going to say "Hey, come buy our EVSE that isn't grounded and doesn't connect to a ground". What could possibly go wrong?

The obsolete, dangerous NEMA 10 family connectors do not have ground wires. That third pin is neutral. Neutral is not ground. EVSE's have no use for neutral.

I realize you're seeing some cheap Chinese EVSE's Amazon... Amazon allows third party sellers (the eBay gang), who have no problem selling dangerous and illegal products. These things will start a fire, or kill you. They also don't provide basic safety protection all EVSEs must have - protection especially needed on a NEMA 10-30 socket!

Wiring major appliances without ground has been outlawed since the 1960s. Due to lobbying, dryers and ranges (ONLY) got a reprieve, but even they were outlawed for groundless wiring 30 years ago.

Effectively all dryer circuits in the US are 30 amps. Dryers need neutral (EVSEs do not), so dryers use either the modern NEMA 14-30 (grounded) or the obsolete and dangerous NEMA 10-30 (ungrounded). All dryers readily convert between the two plug types; changing cord/plug is a simple procedure. (it involves removing the ground strap responsible for bootlegging ground on the NEMA 10).

Unfortunately, idiots sometimes do not realize how easy it is to change a dryer cord, and have removed 14-30 receptacles and fit 10-30. In that case, the junction box or cable does contain a ground wire (or ground via metal conduit) and a conversion to 14-30 is easy. It's worth checking. If ground is not there, the Retrofit Ground rules allow just running a 10 AWG ground wire back to anywhere that has a 10 AWG ground back to the panel.

Note that the only reason dryers and ranges were permitted into the 1990s is the logic that dryers and ranges are very rarely unplugged, so the socket, which is simply not built for frequent use, isn't likely to break in a way that would create the most dangerous situation. Obviously if you're going to be changing plugs a lot, that logic doesn't hold up!

24A EVSEs are readily available... from normal suppliers

Many EVs max out at 24A, so their manufacturers obviously sell companion level 2 chargers which do the same, often identified as a 30A unit to "help" consumers. It's very typical for competent EVSE makers to design for the circuit ampacity, accounting for the 125% rule. Just a spot check of a popular EVSE maker, Clipper Creek, shows a considerable selection of 24A EVSEs:

  • HCS-30 and LCS-30 (both hardwired),
  • LCS-30P (your choice of NEMA 14-30, 14-50, or 6-50 plug).

The NEMA 6 socket family is 2 hots and a ground (no neutral). A dryer can't use it, but an EVSE can. The 14-30 is the universal donor, and since you'll be sharing the socket, stick to that one.

24A chargers are marketed (by the big guys) precisely because of the NEC 125% rule. China doesn't have a rule like that, which is why you don't see it offered in the cheap crud sold online.

But for the record, no - 26A devices require a 35A or 40A circuit. But no 26A EVSE is allowed on the North American power grid in the first place, because they are not designed properly, safe, nor UL-listed.

Amps are just a software setting anyway

Many EVSE's allow you to program/set the allowed amperage on switches on the unit. So many 32A+ EVSE's can simply be "dialed down" to 24A.

If this doesn't make sense to you, here is a technology briefer on how EVSE's work. The upshot is the EVSE isn't a charger at all, just a referee between the AC power grid and the charger on the car. Part of its job is telling the car how much it can draw, which it does with a pulse signal. The pulse can say anything.

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  • Tesla sells a NEMA 10-30 plug for their Mobile Connector: shop.tesla.com/product/nema-adapter-bundle Is it okay for some reason, or are you saying Tesla isn't reputable in this case? Jan 1, 2022 at 6:48
  • @JosephSible nobody's perfect. Elon Musk has sufficient mastery of product quality to make space rockets reusable, for Pete's sake. However, Musk is a notorious scofflaw (ignoring COVID lockdown rules in California for instance) so it wouldn't surprise me if it's not UL listed. Or perhaps it is - remember, EVSE's contain GFCI. Jan 1, 2022 at 21:02
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The numbers are what they are, for reasons:

  • 16A = 80% of a 20A circuit.
  • 24A = 80% of a 30A circuit.
  • 32A = 80% of a 40A circuit.
  • 40A = 80% of a 50A circuit.

Some chargers will make use of an existing circuit and/or existing wiring. For anything new, the wires themselves are generally less than the labor costs involved. The result is that you end up with:

  • 16A/20A: If this is a 120V circuit, most houses already have plenty of them. But if it is a 240V circuit, not so many lying around "extra" (though easy enough to install). The important thing though is that they use 12 AWG wire, and an existing spare 120V circuit can often be easily turned into a 240V circuit as long as no neutral is needed. Therefore common at the low end.
  • 24A/30A: Yes, this is common as a dryer circuit/receptacle. However, most people with these are using them for dryers! Swapping plugs is not as easy (for a bunch of reasons) as with a standard 15A or 20A receptacle, and certainly not very convenient to be switching every time you want to charge vs. dry clothes. Therefore not very common.
  • 32A/40A: This is a nice jump up - double the 16A/20A low-end charger. Once you are running a big circuit, not a big deal to put in larger wire to support 40A instead of 30A. Therefore very common, as long as you have the power available.
  • 40A/50A: Even better, but more of a chance that you may just not have enough power available.

The end result is I would expect a lot of 16A, relatively easy installation possible in almost any home - and 32A and 40A, much more work but once you are installing a full new circuit, go for high capacity. But not much 24A as it really only makes sense if you are repurposing a not currently used dryer receptacle.

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Code calls chargers a continuous load so no matter what your plans are 80% of the rating is the max by NFPA 70 or the national electric code.

When it comes to dryer circuits in the US they are 30A there are 30/50 receptacles but it would be a code violation of every residential dryer I have installed or wired to be on a 50 amp circuit (yes there are some commercial electrics that are larger).

As far as can you go to 87% Are you going to get a permit? can you afford a lawsuit if the building burns down and someone is injured?

I guess you get the idea that unless the mfg states it can be on that size circuit (and if listed or certified by a recognized 3rd party) no there is no wiggle room.

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