Field guide: installing a subpanel, coastal considerations (edition 6)

Why coastal subpanels need a different approach

Salt air eats hardware. A subpanel that runs twenty years inland will pit, corrode, and fail in five on the coast if you spec it like a Kansas job. Galvanic action between dissimilar metals accelerates fast within a mile of the shore, and the AHJs in barrier island towns know it. They will fail you for things a suburban inspector waves through.

This guide assumes you are pulling a feeder from an existing service to a detached structure or a remote part of the same building, within roughly one mile of saltwater. The code references apply everywhere. The hardware choices are where coastal work diverges.

Enclosure and rating selection

NEC 312.2 requires enclosures in damp or wet locations to be listed for the environment and mounted with at least 1/4 inch airspace from the wall. On the coast, treat anything outdoors or in an unconditioned crawlspace as wet. A NEMA 3R painted steel can will rust through the back panel before the breakers fail. Spend the money on stainless or fiberglass.

Type 4X is the minimum you should bid for any exterior coastal install. 316 stainless beats 304 in chloride exposure, and fiberglass beats both for longevity if UV protection is rated. Verify the listing covers continuous salt fog, not just splash.

• NEMA 4X stainless (316 preferred) for exterior wall mounts
• NEMA 4X non-metallic for piers, docks, and direct salt spray zones
• Avoid NEMA 3R painted steel within 1500 feet of mean high water
• Stainless mounting hardware only, never galvanized lag bolts into pressure-treated lumber

If the existing service panel is rusted at the bottom knockouts, replace it before you tap a feeder off it. A new subpanel fed from a dying service is a callback waiting to happen.

Feeder sizing and conductor protection

Feeder ampacity follows NEC 215.2 and the conductor tables in 310.16. For a typical 100A subpanel feed, 1 AWG copper THHN/THWN-2 in conduit, or 1/0 AWG aluminum XHHW-2, gets you there with margin. Coastal humidity does not derate copper, but it punishes terminations.

Run the feeder in PVC Schedule 80 underground per NEC 300.5, with a minimum cover of 18 inches for PVC at residential branch circuits and 24 inches for feeders. Above grade, PVC coated rigid (PCRMC) is worth the premium where exposure is direct. EMT corrodes at the couplings within a year or two on the windward side of a building.

1. Pull four conductors: two hots, one neutral, one equipment grounding conductor (215.6)
2. Never use the conduit as the EGC on a coastal run, the threads pit and lose continuity
3. Use NoAlOx or equivalent on every aluminum termination, including the lugs you think are pre-treated
4. Torque to the listed value with a calibrated wrench, then mark the screw head

Grounding and bonding at the subpanel

This is where coastal jobs get failed most often. The subpanel neutral must be isolated from the equipment ground per NEC 250.142(B). Pull the bonding screw or strap, verify the neutral bar floats, and land the EGC on a separate ground bar bonded to the enclosure.

If the subpanel feeds a separate building or structure, NEC 250.32 requires a grounding electrode at that structure. On the coast, a single 8 foot rod often will not give you a usable resistance reading in sandy soil. Drive two rods at least 6 feet apart, or use a Ufer if new concrete is going in. Supplemental ground rings buried in damp sand around foundations actually work well here.

Coastal soil resistivity swings hard with the tide and rainfall. A reading of 25 ohms in August can climb past 100 in February. Do not rely on a single rod, ever.

GFCI, AFCI, and surge protection

NEC 210.8 keeps expanding. As of the 2023 cycle, GFCI protection is required for nearly every 125V through 250V receptacle outlet up to 50A in dwelling unit garages, accessory buildings, outdoors, kitchens, basements, laundry, and within 6 feet of any sink. If your subpanel feeds a detached garage, workshop, or boathouse, plan for GFCI breakers in the subpanel itself rather than wrestling with downstream devices in a wet box.

NEC 230.67 requires a Type 1 or Type 2 SPD on dwelling services. While that article addresses services, coastal lightning density makes a Type 2 SPD at the subpanel a cheap insurance policy. Lightning grounds through the path of least resistance, and a salt-laden ground plane is a low-resistance highway straight into your panel.

• GFCI breakers for all 15A and 20A circuits feeding outdoor or wet locations (210.8)
• AFCI per 210.12 for habitable rooms in dwellings, including remote structures used for sleeping
• Type 2 SPD at the subpanel, 40kA minimum surge current rating
• Whole-structure SPD if the subpanel feeds sensitive equipment like HVAC inverters or well pumps

Final inspection prep

Coastal AHJs look for three things first: enclosure rating sticker visible, EGC isolated from neutral, and conduit penetrations sealed. Carry duct seal and a tube of NSF-listed sealant on every job. Open knockouts and unsealed LB fittings will fail you faster than a torque issue.

Label the feeder breaker at the main service with the subpanel location and the subpanel itself with the source per NEC 408.4. Take a photo of the inside of the panel with the deadfront off before you button it up. When the homeowner calls in three years asking why a breaker is tripping, that photo is gold.