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?