I'd like to add a receptacle (for x-mas decorations etc) in the soffit of my house. I'd prefer flush-mounted so it's the least visible possible. It will be wired to a switch (timer) inside the house; I may end up running 14/3 to it so there's also constant power.
What type of box should I use? What type of cover? Any other requirements to meet NEC?
You'll be working in a "damp" location, so you'll want to prevent moisture from entering or accumulating within the box. Since there's not likely to be moisture above the box, you probably won't have to worry about protecting the back side of the box. Because of this, you can probably get away with any type of box. When I did this, I used a weatherproof box and conduit.
National Electrical Code (NEC) calls for a cover that is weatherproof when a plug is not inserted for damp locations, however, I always use an "in-use" style cover on outside installations. I used this weatherproof expandable in-use cover which accordions out up to 3 1/2" when in use, and keeps a relatively low profile when not in use.
Obviously, you'll need ground-fault circuit protection. This can be provided either by a GFCI breaker, or GFCI receptacle.
Assuming the outlet is either facing outward or facing downward, your major concerns are ensuring the box and the covering are weather-resistant. You also need to have this outlet on a circuit that's already GFI protected, or is itself a GFI outlet. Because of that, you can't really run 14/3 with a switched hot and a constant hot - it's has to be either all switched or all constant. If you put in a 2-gang box, you can have one duplex receptacle switched and the second duplex receptacle constant on, but both need to be GFI protected. Also make sure you replace insulation properly and get any required firestopping correct. The NEC says the enclosure must be listed (I don't have the exact verbage in front of me), which means you have to get an assemble that has the Underwriter's Laboratories stamp on it, and one that is listed for wet / damp locations. The standard plastic or even metal boxes won't pass muster.
