Field guide: installing a subpanel, coastal considerations (edition 4)

Sizing the feeder for salt air realities

Coastal subpanel jobs start with the feeder, and the feeder starts with honest load math. Run the calc per NEC 220, then look at the conductor table and add one size. Salt air pits lugs and oxidizes strands faster than anything inland, and a slightly oversized conductor buys you margin when the terminations start to degrade in year three.

Aluminum is fine when it is sized right and torqued right. SER from the main to a detached structure is common, but if the run crosses any exterior space within a mile of surf, copper THWN-2 in PVC pays for itself. NEC 310.10(C) covers wet location ratings, and every underground or exterior conduit run is treated as wet inside per 300.5(B).

If the homeowner says "we get spray on the windows in a nor'easter," treat the whole site as a wet location, not just the obvious exterior runs.

Grounding and bonding at a separate structure

This is where coastal jobs get messed up most often. A subpanel in the same building as the service uses the four-wire feeder with the neutral isolated and the EGC bonded to the can. A subpanel in a detached structure follows NEC 250.32: four-wire feeder, isolated neutral, separate grounding electrode system at the second structure.

The grounding electrode system at the detached building is not optional and not a formality. Drive two ground rods 6 feet apart minimum per 250.53(A)(2), or prove 25 ohms or less with one. In sandy coastal soil you will rarely hit 25 ohms with a single rod, so plan on two from the start and skip the meter dance.

• Four-wire feeder: two hots, neutral, EGC
• Neutral isolated from the enclosure at the subpanel
• EGC bonded to the can
• Separate GES at the detached structure (250.32(A))
• GEC sized per 250.66 to the new electrode system

Enclosure selection and mounting

NEMA 3R is the floor for any exterior coastal install, and even then you are looking at a 5 to 10 year service life on a painted steel can within a half mile of the water. Stainless 4X is the right call for direct exposure. If budget rules out stainless, specify a non-metallic 3R rated panel and verify the interior is rated for the conductor type you are landing.

Mount with stainless hardware. Galvanized lags will weep rust streaks down the siding inside a season, and the homeowner will call you back for cosmetics that have nothing to do with the electrical. Use a 1/4 inch standoff or a corrosion barrier between the enclosure and any dissimilar metal siding to prevent galvanic action.

Working clearance per NEC 110.26 still applies outdoors. 36 inches deep, 30 inches wide, 78 inches headroom. Do not let a deck rebuild or an HVAC condenser steal that space later. Document it in the photos you leave with the invoice.

Conductor protection and terminations

Every termination on a coastal job gets antioxidant compound, copper or aluminum, no exceptions. NEC 110.14 requires terminations be made per the listing, and most modern lugs are listed with the use of an oxide inhibitor regardless of metal. Torque to spec with a calibrated tool, log the values, and re-torque at the one-year callback if the customer agrees to a service visit.

Conduit penetrations into the enclosure get sealed. Use a listed duct seal or an electrical putty at the bottom of the can to keep insects and moist air out of the gutter space. Drain holes at the lowest point of any exterior raceway are required by 225.22 and 230.53 logic, and they should be open, not plugged.

Take a phone photo of every torque wrench reading before you close the cover. When you come back in three years for a callback, you will be glad you did.

GFCI, AFCI, and surge protection

The 2023 NEC pulled GFCI requirements into more circuits than most electricians track day to day. For a coastal subpanel feeding a garage, workshop, or accessory dwelling, expect GFCI on all 125V through 250V receptacles 50A or less per 210.8(A) and (B), depending on location. AFCI per 210.12 still applies to the dwelling-unit circuits the subpanel serves.

Type 1 or Type 2 SPD is now required at the service per 230.67, and a Type 2 at the subpanel is cheap insurance against the surges that ride in on coastal utility lines. A nearby lightning strike on a salt-laden ground plane behaves differently than the same strike inland, and panels without SPDs in these areas show clear damage patterns over time.

• SPD at service: required, 230.67
• SPD at subpanel: best practice, especially within 1 mile of coast
• GFCI per 210.8 for all qualifying receptacles
• AFCI per 210.12 for dwelling unit circuits

Inspection and handoff

Coastal AHJs vary wildly. Some want to see the GES driven before backfill, some want torque logs, some want photos of the bonding at the detached structure. Call the inspector before rough-in, not after. Confirm whether they want the neutral disconnect at the subpanel verified visually or with a meter.

Leave the homeowner a one-page sheet: panel location, feeder size, breaker schedule, SPD model and replacement indicator location, and a recommended re-torque interval. On the coast, three years is reasonable. Five years is the outside edge before you start seeing termination issues that could have been caught early.