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I'm adding a 240V circuit and receptacle to my garage for wood working tools. I really only need a 15A circuit, but want to leave myself extra room for upgrades without running new wires later. I also thought it would be good to be ready for EV charging in the future, so more amps seems better.

I know a 6-20R will accept a 6-15P, but will a 6-30R?

Or should I look into a 14-30R for the EV compatibility and can I safely use some kind of adapter for my 6-15P tools?

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  • This seems like a case of only looking at pictures, but not knowing how big the plugs really are. The 30A plug is just massive compared to the 15A. Check out this video to see someone working with one (disclaimer: I watched video muted, and have no idea if he knows what he is doing - example of plug size only).
    – JPhi1618
    Dec 5, 2018 at 16:56

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No. The backwards compatability of 20A sockets with 15A plugs is a "one-off" because of a very special exception in Code.

It does not apply to any other combination of ampacities at any level.

Generally you want the breaker to be a reasonable near match to the appliance, so if the appliance has an internal problem, the breaker will trip before the appliance's line cord starts a fire. 30A is just too large to protect a common appliance cord.

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  • Just from diagrams of NEMA receptacles and plugs it appears (to me) that a 6-30R would accept a 6-15P. Is this so? If it would, then 95% of people would do so and not think twice about it. Do 240 V appliances generally have instructions which state the recommended breaker size and do reasonable people (who are not scofflaws) generally ignore those recommendations?
    – Jim Stewart
    Dec 5, 2018 at 13:18
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    No, a 6-15P will not fit in a 6-30R. The pins are in the same general arrangement, but the spacing and pin size is too different. The way "reasonable" people handle these situations is they install the receptacle needed by the appliance, which then mostly dictates the wire size and the breaker size. The next most common method (and potentially more dangerous) is they find/make a "cheater cord" to connect their appliance to an existing receptacle of a different type.
    – longneck
    Dec 5, 2018 at 13:54
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If you want to prep for future needs, there are a couple good ways to do it.

  1. Run conduit rather than cable, with room for additional circuits in the future. Run conduit large enough for wiring for whatever you might want to do in the future. Conduit is the best way to future proof.

  2. Cable to accommodate a future subpanel in the garage. Determine the wire size needed or your future additions. Run cable of that size from the panel to a junction box where you'd locate a future garage subpanel. Pigtail that cable with #12 wire to a 20A breaker in the main panel. Transition to #12 in the junction box, and run the #12 cable to the 20A receptacle you need now. This is a good way to go if you're fairly certain what your future needs will be.

I think both of these alternatives are superior to bastardizing your receptacle.

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  • What kind of conduit would be best for an inexperienced person to install? I understand that metal conduit is the best, but vinyl or flexible plastic is much easier to install. How hard is it to pull wires through flexible plastic conduit?
    – Jim Stewart
    Dec 7, 2018 at 13:51
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    @JimStewart - PVC conduit IMO, hands down - no special skill to install it, just use factory bends. Its ugly but functional. IMO flexible is best used just for short lengths that really need to be flexible.
    – batsplatsterson
    Dec 7, 2018 at 23:32
  • I'm planning conduit anyway in the garage, I'll just leave room for adding later. Is there a good table or something to tell me how big the conduit needs to be, say to fit 2 12awg NM cables for the 240v and 120v at 20A, and leave room for whatever guage I need later say for 50A?
    – Matt
    Dec 11, 2018 at 17:07
  • @Matt - there are tables to use if all the conductors are the same size, but not when you're mixing. If you search the terms "conduit fill calculator" you'll find online tools that will help with this. You might want to try a couple and make sure they give you the same or close to the same answer ... some are better than others. Remember no harm in going a little larger than the minimum code will permit.
    – batsplatsterson
    Dec 11, 2018 at 17:35

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