Field guide: installing a subpanel, wet location considerations (edition 3)

Plan the feed before you cut a single knockout

Wet location subpanels punish bad planning. Calculate your load per NEC 220, size the feeder for the actual demand plus future growth, and confirm the upstream OCPD can handle it. A 100A subpanel fed with #2 copper THWN-2 is the common spec, but verify ampacity at the terminal temperature rating per 110.14(C), not the wire's insulation rating.

Voltage drop matters more than rookies think. For runs over 100 feet, bump the conductor size. Three percent on the feeder, five percent total to the farthest load, per the informational note in 210.19(A). Document the calc on the panel schedule so the inspector and the next electrician know why you upsized.

Decide the grounding scheme before you order parts. A subpanel in a separate structure follows 250.32; a subpanel in the same structure follows 408.40 and keeps neutrals and grounds isolated. Mixing these up is the single most common subpanel failure on inspection.

Pick an enclosure rated for the actual environment

Wet location is not a vibe, it's a NEMA rating. Outdoor, washdown areas, unconditioned crawlspaces, and damp basements all qualify under NEC 100 definitions. Use NEMA 3R minimum for outdoor exposed locations, NEMA 4 or 4X where direct spray, hose-down, or coastal salt is in play. 4X stainless or non-metallic for pool equipment rooms, car washes, and food processing.

Confirm the panel is listed for the application. A NEMA 1 indoor panel inside a "weatherproof" cover is not a wet location panel. The listing follows the enclosure, not the cover. Check the UL label and the manufacturer's installation instructions, which 110.3(B) makes part of the Code.

Field tip: if you can see daylight through the knockouts after installation, it's not wet-location compliant. Plug every unused opening with a listed closure plate, not tape, not a coin, not a wire nut.

Mounting, clearances, and working space

Working space per 110.26 does not relax for outdoor or wet installations. 36 inches deep, 30 inches wide or the width of the equipment, 6.5 feet high, clear and dedicated. Snow load, vegetation, and seasonal flooding all count as obstructions. If the meter is on a post and the subpanel is six inches behind a rhododendron in July, that's a violation.

Mount the enclosure so the bottom of the panel is at least 4 feet above grade in flood-prone areas, and keep the top under 6 feet 7 inches to the highest breaker handle per 240.24(A). On masonry or stucco, use stand-off straps or spacers to keep a 1/4 inch air gap behind the box. Trapped moisture between the enclosure and the wall rusts panels from the back, where you can't see it until it's too late.

• Stainless or hot-dip galvanized fasteners only in wet locations
• Drip loops on every conductor entering from above
• Hub fittings (Myers hubs or equivalent) on top entries, not just locknuts and sealing washers
• Bottom or side entries preferred for wet locations whenever the layout allows

Conduit, fittings, and the breathing problem

PVC, RMC, and listed liquidtight flexible conduit are all fair game per Chapter 3, but the transitions are where water gets in. Use expansion fittings on long PVC runs per 352.44, and pitch every horizontal run so condensate drains away from the panel, not toward it.

Conduit bodies and panels breathe. Warm conduit pulls humid air in during the day, cools at night, and condenses water inside the enclosure. Drain holes in the bottom of outdoor panels (typically a listed 1/4 inch weep, not a drilled hole) are required, and a listed conduit drain like a Crouse-Hinds ECD at the low point of long underground runs is cheap insurance.

Field tip: pull a wet rag through any underground PVC run before pulling conductors. If the rag comes out muddy, you've got a cracked joint or a missing bell end. Fix it now, not after the panel fills up next spring.

Bonding, grounding, and GFCI requirements

For a subpanel in the same building as the service, remove the main bonding jumper, install a separate equipment grounding bus, and isolate the neutral bar from the enclosure per 408.40. Four-wire feeder: two hots, neutral, and equipment grounding conductor sized per 250.122 against the feeder OCPD.

For a subpanel in a separate structure (detached garage, pool house, well house), 250.32(B)(1) requires the four-wire feeder plus a grounding electrode system at the second structure. Drive two ground rods 6 feet apart unless you can prove 25 ohms or less with a single rod, which you can't without a clamp meter, so just drive two.

GFCI protection follows the loads, not the panel, but a wet-location subpanel often serves outlets that fall under 210.8(A) or 210.8(B), plus the equipment-specific rules in 210.8(F) for outdoor outlets and 680 for pools and spas. Use 2-pole GFCI breakers at the subpanel for branch circuits where downstream device-level protection isn't practical.

Final inspection checklist

Before you call for inspection, walk the install with the AHJ's eyes. Torque every lug to the manufacturer's spec and mark each one (110.14(D) requires calibrated torque tools as of the 2017 cycle). Label the panel directory with circuit descriptions, not just "lights" or "outlets," per 408.4(A).

1. Neutral and ground separated, MBJ removed
2. EGC sized to 250.122, bonded to enclosure
3. Grounding electrodes installed and bonded if separate structure
4. All knockouts closed, hubs on top entries, drip loops in place
5. Working clearances clear and permanent
6. Panel directory complete, torque marks visible
7. GFCI/AFCI per the load locations, tested with a tester, not just the button

The panels that pass first time are the ones where the electrician thought about water, heat cycling, and the next person to open the cover. Build for the worst day of the year, not the day you're installing.