What is the failure mode of making an adapter to convert a 240 V outlet to 120 V

Question

I have an EV charger that uses a NEMA 14-50 outlet with 6 AWG back to a 50 amp breaker. The outlet is in a location where there are no nearby 120 V outlets and it would be convenient to occasionally to have 120 V there instead.

Functionally it is possible with just a dryer plug, a 20 amp duplex outlet, and a box to create an adapter to convert (in the functional sense, not physics) the 240 V outlet into 120 V.

Edit for clarification: I'm not talking about making a permanent hard wired change to my home's wiring. I'm talking about a stand alone adapter that I can unplug the EVSE and then plug in this adapter on an as needed basis and then switch it back. In this way it's impossible for both to be used simultaneously.

One safety failure of the adapter is if a device pulls more current than its own cord (or internals) can handle but less than 50 amps because it wouldn't trip the breaker and potentially cause a fire. Is there anything else I'm missing? I saw some inline circuit breakers on Amazon for up to 250 VAC 20 amp. Would that be a way to fix the safety issue or am I thinking about it too simplistically? What is wrong with adding these sort of circuit breakers to the outlet?

Another clarification: I'm not interested in adding a subpanel for a few reasons.

1. It requires a permit
2. It is more expensive even setting aside the permitting cost
3. It's cumbersome on the wall
4. It would then allow the car to charge while also using the 120 V at the same time. In doing so, it may trip the 50A breaker since the car pulls quite a bit of current unless. I recognize that I could reduce the charging rate but I don't want to.

Answer 1

You can't just slap up a subpanel anytime you want to split a 240V plug. However, with a NEMA 14, you can -- because it has all the right wires to feed a subpanel.

So, that's it. You convert the 6/3 cable run into a "Feeder" to the subpanel.

As feeder, 6/3 cable is rated for 55A. They don't make breakers that size, so the feed breaker (in your main panel) gets bumped to 60A.

Eaton CH and Square D QO have perfectly compact 8-space panels, and that's the way to go if you are cramped for space. Otherwise if you match your main breaker's type, you can reuse that 50A breaker.

You don't need a main breaker in this panel, unless the building is detached from the house (and then you do). If so, either choose a panel with a main breaker, or those 8-space panels can have a main breaker added via backfeeding.

Install a 50A breaker to feed that EVSE socket. Then install as many 15A or 20A breakers as you please to supply receptacle loads. You can safely use two 120V circuits at the same time while charging. Realistically due to the 80% rule, your EVSE will actually pull 39A and your random power tools will pull 12A per leg. So you're at 51A which is not a challenge for the 6/3 or the 60A feed breaker.

You can't overdo this. If you're really going to town and running the saw, dust collector, compressor for blowing sawdust away, shop vac for your apprentice to pick up sawdust, etc. -- then the EV is certainly not in the garage at this time!

Answer 2

Technically, yes you could do it and the cemeteries are probably full of people who tried it. There are adapters out there that would do the job similar to the one below but as you stated as a safety concern, there's no protection for the items plugged into it. These are not a good idea. Many sold online are not approved for use in the USA. If it's just for occasional use, use an occasional extension cord from an existing 120V outlet.

Answer 3

Code and liability issues come to mind, I'm pretty sure the instructions for your charger say to use on a dedicated circuit.

The UL (or other NRTL) Listing is only valid when installed and used as instructed in the instructions that were submitted with the device for testing. The NEC requires compliance with the listing.

I can't really think of at instance that adding a fused tap sharing a circuit would actually damage your charger or car being charged, but I am not an electrical engineer or lawyer, so I personally wouldn't be making second guesses.

Answer 4

The biggest problem, as you noted, is current. If you put a 15A or 20A device, or even worse, a 15A or 20A receptacle, on a circuit that is designed (and breakered) for 50A, then if there is an in-between fault - e.g., 30A for a significant amount of time - the circuit breaker will not trip but the device, receptacle, etc. could easily burn up (in the literal sense of the word, taking your house with it).

You may say (as many have before): I'll put in this receptacle, and I'll be there when I use the device, so I can make sure nothing goes wrong and pull the plug if something goes wrong. That might work for you. But then you have a guest or someone fixing something in your garage, or the next owner of the house, and they don't know about the non-standard setup and plug in something that turns out to have a fault, and go away for lunch and come back to greet the fire department. Unlikely, but it could happen.

There is a solution. Assuming the key problem is: I don't want to run 50' of wire around the house and through the walls just to add a convenience outlet. You can install a SUBPANEL. This will take the 50A connection as an input and provide outputs to the EV and to one or more circuits for 120V devices/receptacles (and even other 240V), all breakered appropriately. It will cost a little more to start with, but be safe and code-compliant, and not require running a new cable back to the main panel.

There is one other possibility if you really don't want to install a subpanel. You may be able to hook in to the lighting circuit. Residential lighting circuits are typically 120V, 15A or 20A, and can (generally) have receptacles on the same circuit.

With any of these options, keep in mind that garage receptacles generally require GFCI protection, either at the breaker or at the receptacle. You may have existing receptacles in the garage that don't have GFCI, but if you add a new receptacle then that is usually required.

Answer 5

What you are proposing is VERY non-code compliant and dangerous. For a few more dollars, do a small sub-panel that provides proper over-current protection for the EV charging outlet as well appropriate breakers for 120v outlets. There are tons of Q/A posts here on sub-panels. You'll thank me later!