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

Plan the load and the location together

A subpanel job lives or dies on two decisions made before you pull a wire: how much load it carries, and where it sits. Calculate the feeder per NEC 220, size the equipment ground per 250.122, and pick the panel rating with headroom for the next circuit the homeowner will inevitably ask for. Undersized feeders are the number one callback on detached structure jobs.

For a remote subpanel feeding a garage, shop, or pool house, run the demand load with the actual continuous loads at 125% per 215.2(A)(1). Don't forget the 6 amp minimum receptacle load if the structure has any general purpose circuits. If the run exceeds 100 feet, voltage drop usually drives the conductor size before ampacity does.

Location dictates everything downstream: enclosure type, conductor insulation, conduit method, and grounding. A panel mounted in a finished basement is a different animal from one in an unheated barn or a pump house ten feet from a well head.

Feeder conductors and the four-wire rule

Since the 2008 NEC, feeders to a separate building or structure require four wires: two ungrounded, one grounded (neutral), and one equipment grounding conductor per 250.32(B)(1). The old three-wire feeder with a bonded neutral at the remote panel is gone for new work. Keep the neutral and ground isolated at the subpanel. Remove the bonding screw or strap. Every time.

Size the grounded conductor per 220.61 and the EGC per 250.122 based on the feeder overcurrent device, not the conductor. If you upsized the ungrounded conductors for voltage drop, you must proportionally upsize the EGC per 250.122(B). This catches a lot of inspectors' eyes on long runs.

• Two hots sized per load calc and voltage drop
• Insulated neutral, isolated from the can at the subpanel
• EGC sized per 250.122, upsized if the ungrounded conductors were
• Grounding electrode system at the separate structure per 250.32(A)

Wet, damp, and the difference that matters

NEC Article 100 defines three location categories: dry, damp, and wet. Damp is partially protected, like a covered porch or unheated basement. Wet is exposed to saturation: outdoors uncovered, below grade, in direct contact with earth, or inside raceways underground. Pick the wrong one and you'll be replacing equipment after the first hard rain.

Panels installed outdoors or in wet locations need a NEMA 3R or 4 enclosure per 312.2. NEMA 3R sheds rain but is not submersion rated. Mount it so the knockouts you actually use are below the live parts, and use listed wet location hubs or fittings on every top entry. A 3R can with an unsealed top knockout is just a rain collector with breakers in it.

Run conduit out the bottom or sides whenever possible. Every top penetration on an outdoor panel is a future water intrusion call. If you must enter the top, use a Myers hub, not a locknut and bushing.

Conductors and conduit in wet locations

Conductors in raceways in wet locations must be listed for wet locations per 310.10(C). THWN-2, XHHW-2, and USE-2 all qualify. Plain THHN does not, even though most modern building wire is dual rated THHN/THWN-2, check the print on the jacket before you pull it. Underground PVC is a wet location by definition per 300.5(B), even if the conduit looks dry when you open it.

Seal raceways that pass from a warm interior to a cold exterior with duct seal or listed sealing compound per 300.7(A) to prevent condensation from filling the panel. This is enforced more often than people expect, especially in cold climates where the temperature differential drives moisture into the can overnight.

1. Verify wet location rating on the conductor jacket print
2. Drill drain holes in the bottom of 3R enclosures only where the listing allows
3. Seal interior to exterior raceways at the warm end
4. Use listed wet location fittings, not indoor connectors with sealant

GFCI, AFCI, and bonding at the structure

Branch circuits leaving the subpanel still follow 210.8 for GFCI and 210.12 for AFCI. A garage subpanel feeding 125 volt 15 and 20 amp receptacles needs GFCI protection on those circuits per 210.8(A)(2). Pool, spa, and fountain equipment falls under Article 680 with its own bonding and GFCI rules that override the general requirements.

At the separate structure, drive ground rods or use the available grounding electrodes and bond them to the EGC bus per 250.32(A) and 250.50. The grounded conductor (neutral) stays isolated. This is where the four-wire feeder rule pays off: the fault current path back to the source is the EGC, not the earth and not the neutral.

Inspection-ready details that save callbacks

Label the feeder disconnect at both ends per 408.4 and 110.22. Mark the panel as a subpanel with the source location. Torque every lug to the manufacturer spec listed inside the deadfront, and document it if the AHJ requires per 110.14(D). Loose neutrals on a subpanel cause more weird call-outs than any other single failure.

Take a photo of the open panel with all conductors landed and labels visible before you close it up. When the inspector questions something six months later, or the homeowner adds a hot tub, that photo is worth an hour of your time.

Confirm working clearance per 110.26: 36 inches deep, 30 inches wide, 6.5 feet high, and clear access. In a wet or damp location, that clearance is non-negotiable, and storing pool chemicals or fertilizer in front of the panel is a code violation waiting to be cited. Build the job so the next person who opens the deadfront sees craft, not compromise.