Field guide: installing a subpanel, wet location considerations (edition 5)

Pick the right enclosure before you cut a single wire

Wet location subpanels live or die by the enclosure rating. NEC 312.2 requires raintight enclosures for wet locations, and 110.28 spells out the Type ratings. A NEMA 3R is the floor for outdoor exposed installs, but if the panel sits where pressure-washing or coastal salt spray hits it, jump to Type 4 or 4X. Stainless 4X is not overkill near pools, marinas, or dairy washdowns.

Check the listing label on the can itself, not the catalog page. Some manufacturers ship a 3R tub with a flat cover that voids the rating once you swap a deadfront. If the label only lists Type 1 with the cover you have, it is a Type 1 panel until you order the correct kit.

• NEMA 3R: rain, sleet, external ice formation. Outdoor general use.
• NEMA 4: hose-directed water. Washdown areas, equipment rooms.
• NEMA 4X: same as 4 plus corrosion. Coastal, chemical, food processing.
• NEMA 6P: prolonged submersion. Rare for subpanels, common for pull boxes below grade.

Feeder sizing and the 6-disconnect rule

Size the feeder to the calculated load per Article 220, not to the main panel ampacity. A 100A subpanel fed from a 200A service still needs a feeder rated for the actual load plus the OCPD ahead of it. Run the neutral the same size as the ungrounded conductors unless you have done a 220.61 calculation that justifies reduction.

If the subpanel is in a separate structure, 225.30 limits you to one feeder, and 225.33 caps the disconnects at six. The disconnect has to be at the structure where the feeder enters, readily accessible, and marked. Do not assume the main lug panel inside satisfies this. If the panel is 30 feet down a wall from the entry point, you need a disconnect at the entry.

Mark the disconnect location on the as-built before you leave. The next electrician, the AHJ, and the fire department all need to find that handle in the dark.

Grounding and bonding in the second structure

This is where good electricians get sloppy. At a separate structure, 250.32(B) requires an equipment grounding conductor run with the feeder, and the neutral must be isolated from ground at the subpanel. Pull the green bonding screw. Add a separate ground bar. The grounded and grounding conductors only meet again back at the service.

The structure still needs its own grounding electrode system per 250.32(A). That usually means two ground rods 6 feet apart, or a Ufer if the slab is new. Bond them to the ground bar in the subpanel with a conductor sized from 250.66. Do not tie the electrode conductor to the neutral bus.

1. Run 4-wire feeder: two hots, neutral, equipment ground.
2. Remove the main bonding jumper or green screw at the subpanel.
3. Install a separate ground bar bonded to the enclosure.
4. Drive electrodes and bond per 250.50 and 250.66.
5. Verify with a clamp meter, no current on the EGC under load.

Conduit, fittings, and keeping water out

Wet location conduit work is mostly about gravity and breathing. Water gets in through condensation, capillary action at fittings, and direct ingress at the top. Use listed wet location fittings, and use them in the right orientation. A compression connector rated for wet location is not the same as a setscrew connector with a rubber gasket.

For PVC, glue every joint. For EMT outdoors, use raintight compression fittings (NEC 358.42), not setscrew. LFMC and LFNC are fine for short whips but check the listing for the exact wet location language. Hubs on top of the panel beat KO connectors every time. If you must enter the top, use a Myers hub or a listed raintight connector, never a plastic bushing as the only seal.

Drill a 1/8 inch weep hole in the bottom of any outdoor enclosure that does not already have one. Water will get in. Give it a way out.

Conductor selection and derating you cannot skip

Wet locations require conductors marked W. THWN-2, XHHW-2, USE-2, RHW-2 are all common choices. THHN alone is not rated for wet locations, even though the dual-rated THHN/THWN-2 you pull off the spool usually is. Read the print on the jacket, not the label on the box.

For underground feeders in conduit, treat the raceway as a wet location per 300.5(B). That means W-rated insulation top to bottom, even the 6 inches of conductor sticking up into a dry basement. Apply 310.15(B) ambient and conduit fill adjustments. A 4-conductor feeder with a current-carrying neutral (nonlinear loads) counts all four for fill derating per 310.15(E).

Final walkthrough before you energize

Before you close the cover, do a physical and electrical check. Torque every lug to the manufacturer specification, not by feel. NEC 110.14(D) made this a code requirement, and inspectors are checking. A calibrated torque screwdriver pays for itself the first time you avoid a callback for a loose neutral.

Verify polarity, verify the EGC is continuous from subpanel back to service, and verify nothing reads voltage to ground that should not. Photograph the inside of the panel with the directory filled out and the torque marks visible. That photo has saved more than one electrician during a warranty dispute.

• Torque all lugs and breakers to spec, mark with paint pen.
• Megger the feeder before energizing if the run is long or buried.
• Confirm neutral isolation with an ohmmeter, neutral to ground should read open.
• Fill out the panel directory in pen, not pencil.
• Photograph the finished work, inside and out, before closing.