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I'm planning to purchase a portable indoor infrared sauna and locate it outdoors under my deck. The sauna will require a dedicated 120V 20A outlet. I'm wondering if it makes sense to install a multi-branched circuit rather than 2 separate circuits. (I would like an additional outlet so won't have to unplug the sauna anytime I needed power for other applications). This would require a twin 20A breaker at the main panel and 12-3 wire run in the crawl space to the receptacle near the sauna. My thought was if I were to decide to convert to a 240V 40A outlet in the future (ie upgrade to a 240V outdoor sauna), then couldn't I still use the existing wiring and simply change the twin breaker to a double pole?

Since this outlet will be located on the exterior of the house, I'm assuming that the receptacle should be GFCI, is this correct?

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Multi-wire branch circuits (MWBCs) must be on a 2-pole breaker, never ever a twin! Measure between the two hots, that must always read 240V. If it reads 0V, you are overloading the neutral.

What you want to do is easy enough, though. Run yourself an 8/3 or 6/3 cable. In the service panel, land it on a 20A 2-pole breaker.

At the other end, bring it into a large box such as a 4-11/16 square deep box at the location where you would want that outlet. Then bring into that box your two 20A subcircuits, on 12 AWG wire. All the whites get wirenutted together, then black #6/8 to one of the #12 blacks, and red to the other black. Continue the #12 cables on to your outlet locations.

If you ever fit the sauna, you'll need to abandon the MWBC. You can't put 15/20A receptacles on a 40A circuit unless you fit a subpanel here. However if you think you might fit a subpanel, run #6 wire. That will let you breaker for 60A, and a 60A-fed subpanel will easily support the sauna and the 20A circuits at once.

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    This is how I would suggest to do it also, so many folks try and save a few bucks even knowing there is a possibility of future updates I have redone 1 job 3 times for a customer the 3rd time it was what I had suggested the first time.(much like this plan) a little extra now saves time and $$$ later.+
    – Ed Beal
    Mar 8, 2019 at 20:19
  • OK, thanks! I'm thinking that I will just plan on two separate 120V 20A circuits and abandon the idea of a future 240V circuit! One outlet will be dedicated for the sauna and another available for other applications. What is the best approach, running two separate 12-2 romex or a single 12-3 romex from the main panel to the receptacle? Is GFCI receptacle recommended for this location?
    – Grant
    Mar 8, 2019 at 22:21
  • When I say "MWBC" that is the name of 2 circuits on a single 12/3. So that is fine, but it needs to be on a 2-pole breaker. Outside would require a GFCI somewhere, either at the breaker or at the first receptacle on each leg. I am NOT A FAN of putting GFCIs outside, because you must pay extra for an outdoor rated one, and they fail quickly anyway.
    – Harper - Reinstate Monica
    Mar 8, 2019 at 22:44
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As mentioned in the comment, you must use a two pole breaker for a multiwire branch circuit.

You won't be able to use the 12 gauge wire suitable for a 20A circuit with a 40A circuit. You could run oversized wire now and pigtail to connect receptacles, but when you upgrade to a 40A circuit, you will have to disconnect the receptacles.

Usually the best bet for future flexibility is conduit, not oversized wires / cables.

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    If the op ran large wire now he could still have a receptacle but will have to pigtail at the breaker and receptacle. But you are correct 12 gauge won't work as planned.
    – Ed Beal
    Mar 8, 2019 at 20:12
  • @EdBeal, Pigtail at the breaker because the larger wire won't physically fit into the 20A breaker?
    – JPhi1618
    Mar 8, 2019 at 21:50
  • Yes the larger wire would not fit in the receptacles. Edited to clarify.
    – batsplatsterson
    Mar 8, 2019 at 22:09

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