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So I currently have a hard-wired 240V heater in my garage: Though I don't use the heater often, I'd like to have the option to use it so my thought was I could have a 3-way switch that would switch on power to one or the other, never both. Is this possible?

The 240V heater is on a 30amp breaker and is rated for 30amps so I'm pretty sure the wiring from heater to panel is to code.

In the photo you can see there is a switch to optionally cut off power to the heater and I thought that this could be replaced with a 3-way switch that could either be off, power hard-wired heater, or power a 240V EVSE receptacle that would have to be branched off of the box. For the receptacle, I was thinking since my charger can be optioned with a NEMA 6-30 plug, that a NEMA 6-30 receptacle would be correct since there's only 3 wires from panel to heater (to my understanding a NEMA 14-30 would require a fourth wire to be run from panel to box).

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  • Those heaters are heavy loads I would want something more than a 30 amp rated snap switch, could it be done yes I would want a switch rated for a load disconnect if you use a standard switch you will find out why fairly quickly.
    – Ed Beal
    Feb 5 at 23:04
  • ok... so you're saying that the existing snap switch may or may not be sufficient. what is the name for the switch that i'm asking about... is it a 3-way switch? what amperage rating should i be looking for?
    – Jeff
    Feb 5 at 23:35
  • The question is very unclear...are you trying to alternate between the heater and an EV charger? Ignoring that possibility, switching such a heavy load with a simple switch is dangerous as @EdBeal pointed out. The best best would be a heavy duty contactor operated by a low voltage (ie: 24v) transformer and switch or thermostat. Feb 6 at 1:36
  • @Jeff -- does this switch need to fit in the existing box? Feb 6 at 2:41
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Simplest option would be to change the heater input to cord&plug, and put the receptacle required on the wall, and plug in one, or the other.

For a fair amount of money as switches go, you can get a 30A rated DPDT (douple pole double throw) center-off switch that does what you ask. The one I could find was in the "very beefy standard switch" look and may be similar to what you have (Presumably DPST) if it's properly rated for the load. The one I found was rated for motor load disconnect use.

You may also be able to find that function in a "gray box on the wall with a big handle" format, but I struck out (I found plenty of disconnect switches, which are single-throw and don't do what you need.)

If you want to completely overcomplicate it (but it might actually cost less) there is the "use a contactor relay" method mentioned in comments. It strikes me as overcomplicated, and it might also be code-dubious as you'd be assembling a system from recognized parts (we hope - beware of Amazon, etc. for this sort of product,) rather than using a directly listed product.

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  • There are DPDT switches in the "Gray box on the wall" format (called "double-throw safety switches") -- GE's TC35321 is the cheapest I can find Feb 6 at 3:11
  • I figured there were, just had a hard time finding an example when looking for one.
    – Ecnerwal
    Feb 6 at 3:15
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Negative, Ghost Rider. EVSE's need a dedicated branch circuit

A branch circuit is defined as the stuff past the last breaker.

So extend the 30A circuit down to a subpanel. The subpanel has two 30A breakers. You must pinkie-swear not to turn both on at once. A physical interlock is not required. The breaker, which is rated SWD, entirely replaces the stupidly inaccessible switch.

So, I would run EMT metal conduit (a new craft to learn, but it handles ground for you) and 1/2" pipe will suffice. Easy to work with once you get the hang of it. And before you get the hang of it, it is trivial to undo/redo, it comes apart like an Erector Set. EMT conduit also satisfies the "protected from physical damage" requirements for wiring in garages. Now you don't have to bust out drywall.

A suitable location for a panel is somewhere it's not likely to be cluttered up with junk in the 30"x36" working space in front of the panel.

Inside the panel, install an accessory ground bar, and remove the green screw on the neutral bar, to un-bond it. That will protect you in case some numbnuts tries to put a 120V circuit there (which it can't support).

Cost would be around $25 for the subpanel, $10 each for 2 breakers, and pencil in $30 for the requisite EMT conduit, plus $30? the NEMA 6-30 recep.

Wiring it

With EMT, you simply use THHN wires -- all will be #10. They are available in a variety of colors, and many hardware stores sell it by-the-foot. You need at least 7" coming into a junction box.

Once the EMT conduit is finished, I'd run four #10 wires - red black orange orange. The switch goes away and is replaced with a 4x4 domed cover - you need the cubic inches in that box for 4 splices. Make sure the utility-side and heater-side ground wires are bonded to the box metal. Then the EMT carries ground down to the subpanel.

Red and black go from supply in the former switch box, to the main lugs of the subpanel. The subpanel gets two 30A 2-pole breakers in it.

Orange and orange go from the 2 hot wires going into the heater, to a 30A breaker in this subpanel. With American 240V with 2 hot wires, nobody cares which wire is which.

From the NEMA 6-30 recep for your EVSE, run a wiring method of your choice (again I like EMT because it carries ground and satisfies physical-damage resistance requirements) to the subpanel, where it lands on the other 30A breaker.

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  • wow this is great detail. Thanks
    – Jeff
    Feb 6 at 6:52
  • This is unsafe and wrong advice. You're advising him to run a 60-amp sub panel on a 30-amp breaker which is illegal (pinky swears won't pass inspection) The heater might require a dedicated branch circuit but the EVSE doesn't (maybe local jurisdiction) but residential EVSE is limited to Level 1 and Level 2 which come standard with plugs (like you recommended them install) which by nature cannot dictate a dedicate/individual branch as an outlet can't be a "one utilization equipment" as defined in the NEC.
    – Jason
    Feb 7 at 8:57
  • so is the better solution to convert the heater from hard wired to plug and install a receptacle?
    – Jeff
    Feb 7 at 12:58
  • or are you saying that by installing a receptacle the circuit is no longer “dedicated” so doing a heater hard wire to plug conversion might not be allowed?
    – Jeff
    Feb 7 at 13:04
  • @Jason -- no, the EVSE requires a dedicated branch circuit for the outlet feeding it -- it's right there in the NEC in 625.40. Also, even if pinky swears don't pass inspection, a breaker interlock certainly will. Feb 7 at 15:50

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