Hey all - the recent severe weather has got me thinking about how I can keep my furnace and fridge going during an extended power outage, and I have a question for the experts...

Through a quirk of history, I have a standard single-gang box in my dedicated furnace circuit (15A 120V, high-efficiency gas furnace) which does nothing but house a wire splice. It's also conveniently located for a generator connection, so this got me wondering if there exists a transfer switch small enough to mount in the existing box and provide a connection for the wire from the generator.

Related question, what's the right kind of socket to make this kind of temporary connection? I've seen some with a standard female NEMA plug, but not sure if that's "approved".