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I have a 20amp double pole 240v breaker suppling power to my 120v dishwasher and garbage disposal. They share the same circuit. It’s 12-3 awg and I have a access to this circuit before it runs to my appliances. I’m interested in know if I can add a junction box to tap into it to supply 240v 20amp to a nema 6-20 outlet. I would only charge my EV when in not running either appliance. It would be nice to throw a switch cutting off power to the appliances while in charging my EV. Thoughts or concerns I’m interested in hearing about it.

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  • I take it that the circuit has a grounding wire on it? Oct 23, 2023 at 0:37
  • What drives this choice? Are you out of breaker space or load calculation? All it takes is one slip-up to pop the breaker, and that's a headache.
    – KMJ
    Oct 23, 2023 at 6:38

2 Answers 2

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EV specialist. I applaud your thinking on charging at a sensible intermediate speed other than 50/60A. This sounds legit to me if you use a switch... however you will need a 2-pole GFCI breaker if you want a socket.

There's no quarrel with mixing 120V and 240V loads and outlets on a MWBC (shared neutral) as long as the breaker is honest 2-pole (2 independents, handle-tied, will not suffice). The problem here is 210.23(B)(2):

210.23 Permissible Loads, Multiple-Outlet Branch Circuits
(B) 15- and 20-Ampere Branch Circuits
(2) Utilization Equipment Fastened in Place. The total rating of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch circuit ampere rating where lighting units, cord-and-plug connected utilization equipment not fastened in place, or both, are also supplied.

NEC 100.1 Definitions.
Fastened in Place: Mounting means of equipment in which the fastening means are specifically designed to permit removal without the use of a tool.

Note that the code says nothing about the appliance being hard-wired vs. cord-and-plug: the deciding factor is whether it's "fastened". And the Code means "fastened or worse", so if the appliance does need tools to remove it, that also counts (disposal). If the disposal's nameplate says >10A, you lose. If the dishwasher is fastened in place, and takes >10A, you lose. And here, "lose" means you'll be required to use that switch.

The switch will need to be DPDT and 20A rated. Not a cheap piece of kit. But cheaper than the GFCI breaker, required since you are adding a socket to a garage.

What about eliminating the socket and running to a Wall Connector commissioned for 16A / 20A circuit? It would certainly be more of a class act - the "draping the cords through the dust and wet" routine is going to get old fast. That brings us around to

NEC 625.40. Each outlet installed for the purpose of supplying EVSE greater than 16 amperes or 120 volts shall be supplied by an individual branch circuit.
Exception: Branch circuits shall be permitted to feed multiple EVSEs as permitted by 625.42(A) or (B).

So yes, a hardwired EVSE will make the switch mandatory. Effectively, the switch is acting as an EVEMS per NEC 625.42. A very dumb EVEMS with a 2-digit price, but hey, if a $800 SimpleSwitch can do it, and that thing is literally a switch, then yes.

But the same rule applies to the socket, unless you can say with a straight face that it's for a kiln, welder, compressor, and sure, maybe an EVSE from time to time. It's up to the inspector whether that will pass a "sniff test".

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Yes, you can do this. Is it a good idea? Maybe not.

What you have is a Multi-Wire Branch Circuit or MWBC. This can be powered by two single breakers or a double-breaker. MWBCs used to be much more common in the past as a way to save on wire costs due to the shared neutral. They are less common today due to (a) requirements for GFCI (e.g., dishwasher and disposal which used to not require GFCI but do with recent code updates) and (b) concerns about when they are implemented incorrectly.

GFCI is an important piece of this. An MWBC used only for 120V can have GFCI after splitting the neutral, either as a deadfront for hardwired devices or with a receptacle for plug-in devices. But if it is used for 240V then you must use a GFCI/breaker if GFCI is required. GFCI has been required for a while for disposal and dishwasher (but not nearly as long as for kitchen countertop receptacles) and is now (subject to adoption of the latest NEC) required for most 240V circuits in garage and outside. That most is critical - more below.

With a proper 240V double-breaker, an MWBC is perfectly safe. When implemented using single breakers there are often problems. The breakers in a MWBC need to have common shutoff. Common shutoff can be done as a double-breaker or with a pair of breakers and a handle tie. In addition, if an MWBC includes any 240V receptacles (as you propose), it must have common trip, which is only provided by a double-breaker (you have that, so you're OK). Many older MWBCs don't even have the breakers (or fuses in my case before my panel replacement) connected to each other in any way, which can lead to an MWBC with both hots on the same leg and the possibility of an overloaded neutral and the inability to actually power any 240V devices. You are fine, but I am mentioning all of this because another person with an MWBC might not have a double-breaker and instead have possible issues.

The next possible issue is overloading. This is an issue at any time with any circuit that has multiple receptacles. For example, a circuit that has receptacles in 3 different bedrooms always runs the risk of being overloaded. However, for most circuits, most of the time in most houses, the risks are minimal because most circuits with large loads are:

  • Dedicated to a particular specific use, as is likely the case with your disposal/dishwasher circuit.
  • Dedicated to a particular room, such as kitchen, bathroom or laundry room - this allows easy monitoring of usage (e.g., don't use the toaster and the waffle maker at the same time).
  • Only have occasional usage. For example, with large power tools in a one-person shop, it is unlikely you will run a table saw and another large tool at the same time. (Dust collector is the exception since it can be used with any other tool, so put that on a different circuit and you're all set.)

EV charging changes things. A lot. It is a continuous load, but unlike dryer, water heater, HVAC, etc. it doesn't have to have a dedicated circuit. It is out-of-sight, out-of-mind - e.g., you plug in the car to charge when you get home and then go eat dinner. After dinner you run the dishwasher (not worried about the disposal - you use it for 10 seconds at a time) without even thinking about the car charging on another receptacle of the same circuit.

But technically you can do this. An MWBC can have 240V (6-15 or 6-20) receptacles as well as 120V (5-15 or 5-20) receptacles. I just don't think it is a good idea to use it that way for a large continuous load like EV charging.

In fact, I don't even like it used that way for disposal and dishwasher. Perfectly fine to use an MWBC, but I much prefer hardwiring the disposal and dishwasher. Of course, if you hardwire those devices then you can't use the MWBC for any plug-in 240V loads, assuming the dishwasher uses at least 1/2 of the circuit capacity.

So technically you are OK: double-breaker - good, only receptacles not hardwired - good.

But I am still very concerned about overloading, and there are other considerations as well.

Many EV chargers, and in particular the Tesla Wall Connector, are actually designed to be hardwired. Connecting many of those (there are likely some exceptions) plug-in violates the manufacturer's instructions. There are "travel chargers" designed to be plugged in, but the idea is exactly that, travel. You really should have a hardwired installation for nightly charging.

There is another advantage of a hardwired installation: GFCI. EV service equipment (chargers) already take care of GFCI. But current code (in many places, soon enough in others) requires GFCI for all 240V receptacles in garage or outside, but not for 240V hardwired devices that include GFCI (i.e., EVSE). So if you end up adding a new, separate, 240V circuit for EV charging then connecting it hardwired instead of plug/receptacle saves on the cost of a double-breaker GFCI.

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