Lanai Receptacles in a Salt-Spray Location: Wet-Location Rules and Box Fill
An electrician is adding two receptacles to a covered second-floor lanai on a home about 200 ft from the ocean on Oahu's windward side. The lanai has a roof but no screens, and the receptacles will be mounted on the exterior wall about 36 in above the deck, fully exposed to wind-driven rain. The plan is to feed from an existing bedroom circuit with 12/2 NM-B run through the wall cavity into a 4 in x 4 in x 1-1/2 in galvanized steel box with cable clamps; three cables will enter that first box (feed in, and two out to the second receptacle), and a GFCI receptacle will be installed in it. The homeowner wants the receptacles usable during rain, and the electrician has 4 x 4 x 2-1/8 in boxes on the truck.
1. Is this a damp location or a wet location, and what does that require of the receptacles, covers, and boxes?
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A roofed but unscreened lanai exposed to wind-driven rain is treated as a wet location where rain can contact the receptacle face; at an absolute minimum it is a damp location, and designing to the wet-location rules is the safe call.
Requirements:
- 406.9(A) and (B): 15/20 A, 125/250 V receptacles in wet or damp locations must be listed as weather resistant (WR). In a wet location the receptacle must be installed in a listed weatherproof enclosure that remains weatherproof whether or not the attachment plug cap is inserted - the old flat flip covers fail this test. Use an extra-duty "while-in-use" cover.
- 210.8(A): outdoor outlets require GFCI protection, so the GFCI receptacle (or a GFCI breaker) is required, not optional.
- 314.15: boxes in wet locations must be listed for the location, arranged so water does not enter or accumulate, and the mounting must not allow drainage onto live parts. A standard galvanized box with a gasketed while-in-use cover can satisfy this only if the cover/box combination is listed together; many installers use a listed weatherproof box assembly.
- The NM-B stays inside the dry wall cavity; only the raceway nipple or the box penetrates the weather barrier, and that penetration must be sealed.
2. Perform a box-fill calculation for the proposed 4 x 4 x 1-1/2 in box and state whether it is acceptable.
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Use 314.16(B) with 12 AWG at 2.25 cu in per conductor, and Table 314.16(A) for the box volume: 4 x 4 x 1-1/2 in = 21.0 cu in.
Count the contents:
- 3 ungrounded conductors (in, out, out) = 3 x 2.25 = 6.75
- 3 grounded conductors = 3 x 2.25 = 6.75
- Equipment grounding conductors: all of them count as one (314.16(B)(5)) = 2.25
- Cable clamps present: one allowance (314.16(B)(2)) = 2.25
- One GFCI yoke: two times the largest conductor (314.16(B)(4)) = 4.50
Total = 22.5 cu in, which exceeds the 21.0 cu in box. The 1-1/2 in deep box is not acceptable.
The 4 x 4 x 2-1/8 in box has 30.3 cu in, which is ample. Note that if the box were changed to a weatherproof box with no internal cable clamps and the cable entered through listed connectors, one 2.25 cu in allowance drops off - but the 21.0 cu in box still fails at 20.25 vs 21.0 only by luck, so the deeper box is the right answer.
3. What changes would you make for the salt-air environment, and how would you support and protect the installation?
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Salt spray plus trade-wind moisture is a corrosion environment. Galvanized steel boxes and standard zinc-plated straps will rust through in a few years at 200 ft from the ocean; use PVC, fiberglass, or listed corrosion-resistant boxes, stainless steel or hot-dip hardware, and corrosion-resistant straps.
For the wiring method, keep NM-B inside the wall cavity where it stays dry, and transition outdoors in PVC conduit with THWN-2 conductors rather than exposing cable. Seal all penetrations with exterior-rated sealant, and drill drainage or position boxes so water cannot stand in them (314.15).
Support: if cable is used in the cavity, secure it per 334.30 (within 12 in. of boxes and every 4.5 ft); if PVC raceway is used outdoors, follow 352.30 support spacing (3 ft for 1/2 in. to 1 in.) and add an expansion fitting on any straight outdoor run of more than a few feet, because UV-heated PVC expands and contracts and will pull out of couplings. Use only UV-rated (schedule 40/80 sunlight-resistant) PVC, and verify the receptacle is WR-marked on the face, not just in the catalog description.