Field guide: installing a subpanel, wet location considerations (edition 2)

Plan the load and the location together

Before you touch a knockout, run the math and walk the site. A subpanel in a damp basement, detached garage, or pool equipment shed lives a different life than one in a dry utility room. Calculate the feeder per NEC 220 Part III, confirm the panel rating covers continuous loads at 125%, and note every circuit that will land on a GFCI or AFCI breaker. Wet location work usually means more two-pole GFCI breakers than the apprentice expects, so size the cabinet accordingly.

Walk the route from the service to the subpanel and flag every transition between dry, damp, and wet locations as defined in NEC 100. A conduit that leaves a heated basement, runs under a porch, and terminates in a detached garage crosses at least two of those zones. Each transition changes your fitting list, your conductor insulation, and sometimes your wire size.

Field tip: count the GFCI two-pole breakers before you pick the panel. A 20-space loadcenter fills up fast when half your circuits need GFCI protection and you also need a main breaker plus space for future.

Pick the right enclosure for the environment

NEC 312.2 requires enclosures in damp or wet locations to be listed for the use, mounted with at least 1/4 inch airspace between the enclosure and the wall, and weatherproof. A standard indoor loadcenter in a NEMA 1 box does not belong on the outside of a detached garage, even under a soffit. Use a NEMA 3R rated panel for outdoor and wet, or a NEMA 4X if the location sees washdown, salt air, or chemical exposure.

For pool, spa, and irrigation equipment subpanels, verify the listing covers wet location and confirm the deadfront and breaker interlock work after the gasket is seated. Some 3R panels ship with knockouts on the top, which violates NEC 312.2 if used in a wet location without an approved hub. Bring the conduit in from the bottom or the back whenever the listing allows.

• Indoor dry: NEMA 1, surface or flush mount.
• Damp, covered exterior: NEMA 3R minimum, 1/4 inch standoff.
• Wet, exposed to weather or washdown: NEMA 3R with bottom entry, or NEMA 4X.
• Pool and spa equipment rooms: confirm listing per NEC 680.

Feeder, grounding, and the four-wire rule

For a subpanel in a separate building or structure, NEC 250.32 requires a four-wire feeder: two ungrounded, one grounded (neutral), and one equipment grounding conductor. Isolate the neutral bar from the enclosure, remove the bonding screw or strap, and land the EGC on a separate ground bar bonded to the cabinet. The 2008 edition closed the old three-wire allowance, so any new install or major rework follows the four-wire rule even if the existing run is older.

Ground electrodes at the second structure are still required per NEC 250.32(A). For a detached garage or shed, that usually means two ground rods 6 feet apart, or a single rod with a documented 25 ohm reading, bonded to the EGC at the subpanel. Do not use the neutral as a substitute. Auxiliary electrodes do not replace the EGC, they supplement it.

Conduit, conductors, and water management

Any raceway that transitions from outdoors or underground into a heated interior is a condensation factory. NEC 300.5(B) and 300.7(B) require conductors in wet locations, including the interior of underground raceways, to be listed for wet locations. THWN-2, XHHW-2, and RHW-2 all qualify. Plain THHN does not. If you pull THHN/THWN-2 dual-rated wire, you are covered, but verify the print on the jacket before you energize.

Seal the raceway against moisture migration. A bushing on the outside is not enough. Use a listed duct seal or expanding sealant at the building entry, and slope underground runs so condensate drains away from the panel. On a 3R subpanel mounted outside, drill a small weep hole at the lowest point of the cabinet only if the manufacturer's instructions allow it. Otherwise, rely on the gasketed door and bottom entry.

Field tip: pull an extra 12 inches of conductor at the bottom of any 3R subpanel and form a drip loop inside the cabinet. Water that wicks down the jacket falls off the loop instead of running into the lugs.

GFCI, AFCI, and the wet location circuit list

NEC 210.8 governs GFCI requirements and the list keeps growing each cycle. For a wet location subpanel, expect to provide GFCI protection on receptacles in garages, accessory buildings, outdoors, crawl spaces, unfinished basements, boathouses, and within 6 feet of sinks or wet bar work surfaces. NEC 210.8(F) covers outdoor outlets serving dwelling units, including the HVAC disconnect that used to be exempt. Many AHJs now require GFCI protection on the feeder itself when it serves a pool or spa panel under NEC 680.

AFCI requirements under NEC 210.12 still apply to most dwelling unit branch circuits even when the subpanel is detached. Do not assume a workshop or finished room over a garage skips AFCI. Read the occupancy classification first.

1. List every branch circuit and mark GFCI, AFCI, or both.
2. Confirm breaker availability in the panel manufacturer's catalog before ordering the panel.
3. Check load side neutral isolation, GFCI breakers need a dedicated pigtail to the neutral bar.
4. Test every GFCI and AFCI with a calibrated tester at final, not just the built-in button.

Inspection ready checklist

Before you call for inspection, walk the install one more time. Confirm working clearance per NEC 110.26, label the disconnect with the source per NEC 408.4, and verify the feeder OCPD at the main panel matches the conductor ampacity after derating. Wet location installs fail inspection most often on missing standoffs, wrong conduit fittings at the building entry, and a forgotten bonding jumper at the second structure ground rods.

Document the calculated load, the conductor type, and the grounding electrode method on the panel directory or in a permit packet. An inspector who can see your math signs off faster, and the next electrician who opens that cabinet in 15 years will thank you.