Can I use a regular metal box outside with a weatherproof cover?

No. NEC 314.15 requires the box itself to be listed for damp or wet locations, not just the cover.

Do outdoor boxes need a drain hole?

NEC 314.15 permits drainage openings between 1/8 inch and 1/4 inch in the bottom of an enclosure, and requires the box to be equipped to prevent moisture accumulating inside.

What cover does an outdoor receptacle need?

NEC 406.9(B)(1) requires a weatherproof in use cover for 15 and 20 ampere 125 and 250 volt receptacles in wet locations, marked extra duty where supported from grade.

Are metal electrical boxes code compliant for outside installations?

Yes, metal boxes are permitted outdoors as long as they are listed for the location. Section 314.15 requires that in damp or wet locations, boxes, conduit bodies, outlet box hoods, and fittings shall be placed or equipped so as to prevent moisture from entering or accumulating within the enclosure, and shall be listed for use in damp or wet locations. Cast aluminum and cast iron FS and FD boxes with gasketed covers meet that description, as do listed weatherproof sheet metal enclosures.

What separates an indoor box from an outdoor one

A standard 4 inch square steel box with a raised plaster ring is not listed for wet locations. Its knockouts, seams, and unfinished edges let water in and hold it, and the zinc plating is not intended for direct weather exposure. The listed outdoor equivalents are cast metal FS and FD bodies with threaded hubs and gasketed covers, listed sheet metal Type 3R enclosures, and nonmetallic weatherproof boxes.

Section 314.15 also has an important sentence for boxes that could collect water. Approved drainage openings not smaller than 3 mm, or 1/8 inch, and not larger than 6 mm, or 1/4 inch, are permitted in the bottom of the enclosure. Listed boxes with drain fittings are permitted. A drilled hole in a Type 3R enclosure bottom is the traditional way to keep condensation from pooling.

Covers, hoods, and the in use rule

Section 406.9(B)(1) requires 15 and 20 ampere, 125 and 250 volt receptacles installed in a wet location to have an enclosure that is weatherproof whether or not the attachment plug cap is inserted. That is the in use, or bubble, cover. It must be listed and identified as extra duty for outlet box hoods installed in wet locations supported from grade, per the same section.

Section 406.9(A) covers damp locations, where an enclosure that is weatherproof when the plug is removed is acceptable. A porch receptacle under a deep roof overhang is a damp location. A receptacle on an open exterior wall is a wet location.

Corrosion and dissimilar metals

Section 300.6 requires metal raceways, cable armor, boxes, cabinets, elbows, couplings, fittings, supports, and support hardware to be of materials suitable for the environment. Ferrous metal parts must be protected against corrosion inside and outside by an approved coating, and 300.6(A)(1) permits ferrous metal equipment in concrete or in direct contact with earth only where provided with corrosion protection approved for the condition.

Section 300.6(B) addresses aluminum. Aluminum equipment embedded or encased in concrete or in direct burial applications must be provided with supplementary corrosion protection. In coastal or chemically aggressive environments, PVC boxes and PVC coated rigid conduit outperform bare cast aluminum by a wide margin.

Field takeaway

Use a box listed for wet locations, not a standard indoor steel box with a weatherproof cover screwed to it. Add a drain hole where water can collect, use extra duty in use covers on wet location receptacles, and pick the metal to suit the environment per 300.6.