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

Sizing the feeder before you cut anything

Subpanel work goes sideways when the feeder is undersized for the load calc, not when the install is sloppy. Run the calc per Article 220 before you pull a single conductor. Continuous loads at 125%, demand factors where they apply, and don't forget the largest motor at 125% per 430.24 if you're feeding shop equipment.

Feeder ampacity has to match or exceed the calculated load and the OCPD protecting it. NEC 215.2(A)(1) sets the floor. For a typical 100A subpanel feed in a detached garage, #4 copper THHN in conduit gets you there at 75°C terminations, but check your terminal ratings before you commit. Aluminum at #2 SER or #1/0 XHHW is common for residential, just confirm the panel lugs are listed for AL.

Voltage drop kills more subpanels than overloading does. Past 100 feet on a 100A feed, you're upsizing whether the calc demands it or not. Target 3% on the feeder, 5% combined per 210.19(A) Informational Note No. 4.

Grounding and bonding at the second panel

This is where most failed inspections happen. A subpanel is not a service. The neutral and the equipment grounding conductor must be kept separate. Remove the bonding screw or strap that ties the neutral bar to the enclosure. NEC 250.24(A)(5) and 408.40 are unambiguous on this.

You need four conductors on the feeder for a standard subpanel: two ungrounded, one grounded (neutral), one equipment grounding conductor. Three-wire feeders to detached structures were allowed under older code but 250.32(B) closed that door in 2008. If you're working on an existing three-wire feed, this is the moment to upgrade it.

• Neutral bar: floating, isolated from enclosure
• Ground bar: bonded to enclosure, separate from neutral
• EGC sized per 250.122 based on the feeder OCPD
• Detached structure? Drive a ground rod or rods per 250.32(A), connected to the EGC, not the neutral

For detached buildings, the GEC at the subpanel connects to the local grounding electrode system. The neutral stays isolated. Old habits from service work will get you written up here.

Wet location and damp location, know which you have

Article 100 defines them and the difference matters for your enclosure rating, conductor selection, and device choice. A wet location means saturation with water or other liquids, or unprotected exposure to weather. A damp location is protected from direct weather but still subject to moisture, like under an open canopy or in an unheated basement.

For an exterior subpanel or one in a wash bay, pump house, or boat dock, you need a NEMA 3R minimum, often 4 or 4X for direct spray or marine environments. The panel listing has to match. NEC 312.2 requires raintight construction where exposed to weather, with arrangements that prevent moisture from entering or accumulating inside the enclosure.

If the panel sits below grade level even by a few inches, treat it like a wet location regardless of the cover rating. Water finds its way in, and a 3R panel mounted in a window well will fill up in the first heavy rain.

Conductors and conduit in wet locations

Conductor insulation has to be listed for wet locations when installed in conduit underground or in any wet location raceway. NEC 310.10(C) is the reference. THWN-2, XHHW-2, and USE-2 are the common players. Plain THHN is not wet rated, despite what the spool says, unless it's dual-marked THHN/THWN-2.

Any conduit run underground or exposed to weather is considered a wet location per 300.5(B) and 300.9. That includes the inside of PVC buried at 18 inches, the inside of RMC stubbed up out of a slab, and the interior of any raceway in an exterior wall cavity that breaches the building envelope. Use wet-rated conductors throughout.

• Seal conduit entries with listed duct seal or expanding foam where temperature differentials cause condensation, per 300.7(A)
• Slope horizontal runs to drain points, install drain fittings at low spots
• Use weatherproof bushings and gasketed hubs, not just teflon tape on threads
• LB and LR fittings need gasketed covers in wet locations

GFCI, AFCI, and the field reality

Subpanels in dwelling units fall under 210.8 and 210.12 just like service panels. Garages, basements, outdoor receptacles, kitchens, bathrooms, laundry, all of it. If you're feeding a detached garage subpanel, every 125V receptacle in that garage needs GFCI protection per 210.8(A)(2). You can do it at the subpanel breaker or downstream at the device, but doing it at the breaker simplifies troubleshooting later.

AFCI requirements per 210.12(A) cover most dwelling unit branch circuits. The 2020 and later code cycles have expanded coverage. Verify the panel manufacturer's breaker availability before you spec a brand. Some smaller load centers don't have dual-function breakers in every amperage you need.

On wet location subpanels feeding outdoor equipment, set GFCI at the breaker, not at a downstream receptacle. When that breaker trips during a storm, you want to reset it from a dry location, not a flooded box.

Working space at the subpanel is non-negotiable. NEC 110.26 wants 36 inches of depth, 30 inches of width, and 6.5 feet of headroom. In a wet location install, that working space cannot be a puddle. Build a pad, raise the panel, or relocate. Inspectors fail this regularly, and they're right to.