Field guide: installing a subpanel, wet location considerations (edition 1)

Plan the feeder before you cut a single knockout

Subpanel work goes sideways when the feeder gets sized last. Start with the load calc per NEC 220, then pick conductor size from 310.16 at the correct termination temp column (75C for most modern lugs, but check the listing). Don't forget the 83% rule under 310.12 only applies to dwelling service or main feeder, not every subpanel run.

For a 100A subpanel feed in a detached garage, you're typically pulling 1 AWG copper or 1/0 aluminum SER if the run is short and the terminations are 75C rated. Voltage drop matters more than people admit, especially on long runs to outbuildings. Keep it under 3% for the feeder per the informational note in 210.19(A).

Pull the permit. Inspectors in wet-location jurisdictions almost always want to see the feeder path on paper before you trench.

Bonding and grounding at the subpanel

This is where most failed inspections happen. At a subpanel, the neutral and ground bars must be isolated. Remove the main bonding jumper or bonding screw per NEC 250.24(A)(5) and 408.40. The neutral floats; the equipment ground bonds to the enclosure.

If the subpanel is in a separate structure, you need a grounding electrode system at that structure per 250.32(A). Two ground rods 6 feet apart, or a Ufer if you have access to the footing rebar. The equipment grounding conductor still runs with the feeder. No more three-wire feeders to outbuildings, that allowance was removed in the 2008 cycle.

If the inspector pulls the cover and sees a green screw still threaded into the neutral bar, you're done. Pull it before you energize, every time.

Wet location ratings, what the label actually means

NEC 100 defines wet, damp, and dry locations. Outdoor, below grade, in direct contact with earth, or in unprotected locations subject to saturation, that's wet. A covered porch with horizontal rain exposure counts as wet, not damp. Read 312.2 for cabinets in damp or wet locations.

For a subpanel mounted outdoors or in a wet location, you need a NEMA 3R minimum, often 4 or 4X for coastal or washdown environments. The enclosure rating is only half the job, the conductors entering it must maintain the rating. That means listed wet-location hubs, Myers hubs on top entries, and sealing locknuts where required.

• NEMA 3R: rain, sleet, ice formation. Standard outdoor residential.
• NEMA 4: hose-directed water. Commercial washdown, exterior industrial.
• NEMA 4X: same as 4 plus corrosion resistance. Coastal, chemical, food service.
• NEMA 6P: prolonged submersion. Rare for subpanels, but specified in flood-prone areas.

Conductor selection for wet locations

Per NEC 310.10(C), conductors in wet locations must be listed for wet use. The common ones: THWN-2, XHHW-2, USE-2 (underground only, not allowed inside structures past the point of entry). THHN by itself is dry only. Most modern building wire is dual-rated THHN/THWN-2, but check the print on the jacket before you pull.

Conduit in wet locations is treated as a wet location interior per 300.5(B) and 300.9. That underground PVC run from the main to the detached garage is wet inside, period. Use wet-rated conductors the entire length, even where the conduit emerges into a dry basement, until you transition at the first accessible point.

Sunlight resistance is its own rating. Above-grade exposed conductors need to be listed sunlight resistant or installed in conduit that blocks UV. SER cable run up an exterior wall without a sleeve will degrade in a few seasons.

Sealing, drainage, and the details inspectors check

Water gets in. Plan for it to get out. Bottom-fed conduits should have a drain fitting or a weep hole at the low point. Top-fed conduits need a Myers hub or a listed raintight fitting, never a standard locknut and bushing on a top knockout in a wet location.

1. Use listed wet-location fittings on every penetration above the bottom of the enclosure.
2. Add duct seal or listed sealing putty at conduit ends entering heated spaces, per 300.7(A), to stop condensation migration.
3. Slope underground conduits slightly away from the structure so groundwater doesn't pond at the entry.
4. Verify the cover gasket is intact and seated. Replace it if it's brittle.

Condensation inside a 3R panel in a temperature-swinging environment is real. A southern-exposed panel in a humid climate will sweat. That moisture finds the lowest point, usually the neutral bar, and you'll see corrosion in two years if the conduit isn't sealed against air migration.

On coastal jobs, I spec 4X stainless and stainless hardware throughout. Galvanized hinges seize in 18 months within a mile of saltwater. The upcharge pays for itself the first time you don't have to replace an enclosure.

GFCI and AFCI considerations at the subpanel

Wet-location subpanels often feed circuits that need GFCI protection per 210.8. If the receptacles downstream are accessible and on a wet-location-served subpanel, consider GFCI breakers at the panel rather than dead-front receptacles in the field. Easier to reset, easier to test, and keeps the wet-location box simpler.

AFCI requirements per 210.12 still apply to dwelling-unit branch circuits served from the subpanel. The subpanel being outdoors doesn't exempt the bedroom circuits inside the structure it feeds. Check the 2023 NEC adoption status in your jurisdiction, the AFCI scope expanded again.

Label the panel directory legibly, in pen or printed, not pencil. Inspectors check this. So does the next electrician, who might be you in five years on a service call you don't remember.