Can THHN be used outside?

Only if it is dual rated THHN/THWN-2. Plain THHN is a dry and damp location insulation, and NEC 300.9 makes the inside of an outdoor raceway a wet location.

Do outdoor receptacles need a bubble cover?

Yes. NEC 406.9(B)(1) requires an enclosure that stays weatherproof with a plug inserted, which means an in use cover, plus a weather resistant listed receptacle.

Is NM cable allowed in a wet location?

No. NEC 334.12(B)(4) prohibits NM cable in wet or damp locations. Use UF or conductors in a raceway.

What does the NEC require for electrical circuits in wet locations?

Article 100 defines a wet location as an installation underground or in concrete slabs or masonry in direct contact with the earth, a location subject to saturation with water or other liquids such as vehicle washing areas, and an unprotected location exposed to weather. That last clause is the one that catches people. Anything outdoors and not under a roof is a wet location, and the inside of any raceway installed in a wet location is also a wet location per 300.9.

Conductors and cables

Section 310.10(C) requires insulated conductors in wet locations to be moisture impervious metal sheathed, of a type listed for use in wet locations such as RHW, TW, THW, THHW, THWN, THWN-2, or XHHW, or of a type listed for the purpose. Plain THHN is a dry and damp location insulation, so the dual rated THHN/THWN-2 that most supply houses stock is what you actually want in a raceway outdoors. NM cable is prohibited in wet locations by 334.12(B)(4). Type UF is permitted for underground and wet locations per 340.10.

Section 300.9 is the rule that surprises apprentices. Where raceways are installed in wet locations above grade, the interior of those raceways is considered a wet location, so the conductors pulled through an outdoor conduit run must carry a W rating even if both ends are inside a dry building.

Boxes, fittings, and drainage

Section 314.15 requires boxes and conduit bodies installed in wet locations to be listed for use in wet locations and to prevent moisture from entering or accumulating within the enclosure. Approved drainage openings not smaller than 3 mm and not larger than 6 mm in diameter are permitted in the field. Section 312.2 requires cabinets and cutout boxes in damp or wet locations to be weatherproof and mounted with at least 6 mm of air space between the enclosure and the wall.

Fittings matter as much as the box. Set screw EMT connectors are not raintight and are prohibited outdoors, so use compression fittings listed as raintight per 358.42. Section 225.22 requires raceways on exteriors of buildings to be arranged to drain and be listed or approved as suitable for use in wet locations.

Receptacles and GFCI

Section 406.9(A) requires receptacles in damp locations to have an enclosure that is weatherproof when the receptacle is covered, with the attachment plug cap not inserted. Section 406.9(B)(1) requires receptacles in wet locations to have an enclosure that is weatherproof whether or not the attachment plug cap is inserted, which is the in use bubble cover. Section 406.9(B)(1) also requires 15 and 20 ampere, 125 and 250 volt receptacles installed in a wet location to be listed weather resistant type.

Section 210.8(A)(3) requires GFCI protection for dwelling unit outdoor receptacles, and 210.8(B)(4) covers outdoors in other than dwelling units. Section 210.8(F) requires GFCI protection for outdoor outlets supplying HVAC and similar equipment.

Field takeaway

Use THWN-2 or UF, listed wet location boxes with a drain hole at the low point, raintight compression fittings, a weather resistant receptacle in a listed in use cover, and GFCI protection. Arrange the raceway so water runs out rather than pooling in a low box.