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I have two 20amp 120V circuits to my shed. One leg is for a 30A 120V RV outlet. Can I use the other leg with a SPDT for 240V on that same outlet?

The switch would flip from neutral to the other hot.

This is for occasional use to charge a Tesla. Each circuit has a separate 20A breaker from a 50A breaker in the basement.

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  • Is there room in the panel to swap one of the breakers for a 2-pole one? How many wires feed the shed --- do the two circuits currently share a neutral?
    – jay613
    Mar 22, 2023 at 13:15
  • Yes, I have plenty of room in the subpanel. There are 3wires 8g feeding the shed so yes both circuits use the same neutral...thanks!
    – Dan
    Mar 22, 2023 at 13:28
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    A picture of the subpanel (preferably with the deadfront - the cover panel - removed) would help a lot. While you have the deadfront off, take a picture of all the labeling inside that panel. You can edit the pics into your question.
    – FreeMan
    Mar 22, 2023 at 15:28
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    @Dan Wait. Do you have a subpanel in the shed or are you saying you have a 20A circuit running on 8/3 wire to the shed? I am confused. You can edit your question to add details. Also are you committed to ultra-cheap hillbilly-tier dangerous, or are you willing to spend sane amounts of money doing it safely? You do own a Tesla. Nothing is a barrier, we can definitely get you into level 2 charging regardless, and at sane cost. Mar 22, 2023 at 19:09
  • thanks, I will edit the question. I see there is going to be a bit more to the project then I first thought. It's my daughter's fault. And she only gets down from Minneapolis a few time a year. Just thought I could get some 240 for her. Just didn't want to put in a full blown charging station just for that. Stay tuned;-)
    – Dan
    Mar 23, 2023 at 12:49

2 Answers 2

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To answer the specific question: No, you can NOT use the same receptacle for 120V vs. 240V by changing wire function with a switch. But that doesn't mean you can't get the same result through other means.

Clarifying a bit based on my understanding of the original post and comments:

  • 50A panel in the basement
  • 2x20A breakers feeding 8 AWG wires to the shed, together with an 8 AWG neutral
  • Shed has a 30A 120V RV receptacle

Unknowns:

  • Are there any "regular" 15A/20A receptacles in the shed?
  • How much capacity is actually available in the 50A panel (a.k.a., load calculation)?
  • Are the 20A breakers single breakers next to each other, single breakers next to each other with a handle-tie, or actually a 20A double-breaker?

My hunch is that the reason 8 AWG wires (typically capable of 40A) are connected to 20A breakers is that there are additional "regular" receptacles in the shed, and 15A and 20A receptacles can only be on 15A (15A receptacles only) or 20A (15A or 20A receptacles) circuits. If that is the case then the existing 30A 120V RV receptacle is a problem, because that should only be on a 30A circuit, not a 20A circuit.

Some possibilities:

  • Subpanel

The ideal thing to do would be to replace the 20A breakers with 30A or possibly 40A (if the source panel has enough load capacity available) double-breakers. Use the 8/3 to feed a subpanel in the shed. This subpanel can include:

  • 30A 120V RV

  • 20A or 30A 240V Tesla (using a proper 240V receptacle, or a hardwired EVSE but based on "occasional", travel cable with proper receptacle probably makes the most sense)

  • 15A and/or 20A circuits for lighting and general purpose receptacles

  • 30A only

Replace the 20A breakers with a 30A double-breaker. Keep the 30A 120V RV receptacle. Add a 30A 240V receptacle for charging. Does not allow for any 15A/20A receptacles.

  • 20A only

Make sure (and rearrange if needed) the 20A breakers are either single breakers next to each other with a handle-tie or a double-breaker. Replace the 30A 120V RV receptacle with a 20A 120V receptacle - which will change what RV cable you use but not substantially limit usage because the circuit was limited to 20A already. Add a 20A 240V receptacle for charging. No switch is needed - multiple receptacles are OK as long as they are all the right types. Any existing 120V 15A or 20A receptacles are perfectly fine, as long as you don't exceed total 20A load.

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  • ok, sounds like I should update my subpanel. Yes there are a couple lights and outlet one one of the circuits. I will change the breaker I use for the RV to 30A and then add another 30A for the charging circuit. Separate outlets for them in the garage. Right?
    – Dan
    Mar 22, 2023 at 14:31
  • More complicated than all of that. Before going any farther, please clarify: What size is the panel in the basement (the main breaker on it)? What size are the breakers feeding the 8 AWG wires to the shed? Is there a subpanel in the shed? If there is, does it have a main breaker or disconnect? Mar 22, 2023 at 14:54
  • It is more than just "on 30A for RV and a separate one for charging". The RV is 120V (hot/neutral), the charging is hot/hot (generally doesn't actually need neutral). They can share a 30A double-breaker (two hots) circuit if done right. Mar 22, 2023 at 14:55
  • subpanel is outside the house and I put an 8g wire to the shed with separate 20A breaker on each hot. Got a 100A panel in the basement and a 50A out to the subpanel (used to be for a spa). So I could put just one 30A breaker in the subpanel to the shed then. Each leg should be good for 30A each...ya I have a junction box in the shed with a main disconnect.
    – Dan
    Mar 22, 2023 at 16:25
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TLDR: You can easily and safely get the charge speed you want, charging your Tesla at 150 miles per 10 hours - 3 times the speed of the 120V charging that you tried. You can do it with 12/3 wire, a 20A 240V breaker and the socket at bottom. You just need the correct circuit and dongle.

30A is overkill and I wouldn't waste money chasing 30A. But feel free if you want to; just do it right.

There is no limit to the number of receptacles on a 15-30A circuit

You don't actually need a 30A circuit, but you absolutely can have a 30A circuit that has both a TT30 outlet and a NEMA 14-30 outlet! You simply need to use a 2-pole (240V factory handle-tied) breaker.

"That was easy"

Wait. One of those is 120V and the other is 240V. Can they mix?

YES! 120V outlets and 240V outlets can be mixed on a circuit, as long as the amps match. (30A on 30A; 20A* on 20A etc.) Doing so makes it a "MWBC" (not really important what that is) - the important part is, it needs a factory-made 240V breaker with a feature called "common trip". As long as you have the correct breaker, you're all set.

See the bottom for a factory made, UL listed socket that mixes voltages.

They make a correct socket for ANY application! Really.

You're already familiar with the TT30. Meet the rest.

enter image description here

Above are 120V and 240V sockets for 15A to 20A.

enter image description here

Above are 240V sockets at 30A. Do not use the one in the middle. If you're wondering where the 120V/30A socket is, you already met it. It's the TT30. Despite its slant eyes, it is safe.

Why does it matter? If you use the correct socket for the voltage and breaker, then you won't kill anyone or set anything on fire (or have annoying breaker trips).

The car comes with a "travel unit" EVSE that is intended to live in your trunk and be used for opportunity charging "on the road", which is why they give you the plugs you'll find on the road e.g. the large RV socket (that's An RV socket not an EV socket!) However, Tesla makes 8 of them and will sell you any for around $40.

enter image description here

Note they don't make a NEMA 6-30 - use the NEMA 14-30 instead.

However the NEMA 6-20 is perfectly adequate for your needs. It will provide 3 times the charging speed of the 120v charging that you tried and weren't happy with.

Circuit breaker (and wire) MUST match the socket

On all circuits, the breaker must be an exact match for the socket size. So 30A breaker = 30A socket. Don't ever put a TT30 on a 20A circuit ever again. If you want to install a 120V/20A circuit, use the NEMA 5-20 which is made for that, and then make a short "dongle cord" to adapt from 5-20 to TT30. Keep the dongle in your trailer so you can plug in anywhere. (actually you can just buy that at RV stores).

  • The only exceptions are that 15A sockets are allowed on 20A circuits - that's because all 15A appliances and sockets are certified to not fail dangerously on 20A circuits. Also an exception for 40A circuit but nevermind that.

This thing exists, by the way.

Notice something special about it? That's right. The sockets are different. And now that you have the decoder ring, you can see one is 120V and the other is 240V.

enter image description here

You feed that with 12/3 cable off a 240V "2-pole" breaker with "common trip". Now you can charge your RV and your Tesla at the same receptacle - in fact they can be plugged in at the same time since they use different sockets. (just don't use them At the same time).

Now do you see why it's important that the right socket be used for the right job? When you use sockets properly, you never need to worry about plugging the wrong thing in and killing someone / starting a fire.

If you must do something improper, do it in a short "cheater cable" that is 6" long - that way people who pick it up look at it and go "well hey, this is a cheater cable! I better be careful how I use this!"

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