Field guide: installing a subpanel, damp location considerations (edition 2)

Sizing the feeder before you cut a single hole

A subpanel is a feeder calculation problem first, a mounting problem second. Pull the load list before you spec the conductors. Apply NEC 220.40 through 220.61 for the feeder demand, then size the OCPD at the source panel and the conductors per NEC 215.2(A)(1). Do not default to a 100A feeder because that is what you have on the truck.

The grounded conductor in a feeder to a separate structure or a remote subpanel is no longer your equipment ground. Per NEC 250.32(B), you run four wires: two ungrounded, one grounded (neutral), one equipment grounding conductor. The neutral floats at the subpanel. The bonding screw or strap comes out. If you forget this, every metal part downstream becomes a parallel neutral path, and you will hear about it the first time someone meters a hot box.

Pull a fresh tap on a known good neutral at the source before you energize. If the source panel neutral is loose or shared with a sub already, your new sub will inherit every gremlin upstream.

Picking the enclosure for the location

Damp, wet, and dry are not opinions. NEC Article 100 defines them, and the inspector reads the same book. A covered porch, an unheated basement against a foundation wall, the underside of a canopy at a car wash bay: damp. Outdoors exposed, below grade, or subject to saturation: wet. A finished interior with conditioned air: dry.

For damp locations, a Type 3R enclosure is the floor, not the ceiling. NEC 312.2 requires a 1/4 inch air space between the cabinet and the wall surface in damp or wet locations to prevent moisture wicking into the back of the can. Slotted mounting feet on a 3R panel exist for this reason. Do not crush them flat against block.

• Type 1: indoor dry only. Do not use in a garage that floods in spring.
• Type 3R: rainproof, sleet resistant. Acceptable for damp and most outdoor wet.
• Type 4 / 4X: watertight, washdown rated. Required for hose down areas, marinas, food processing.
• Type 12: indoor, dust and dripping non corrosive liquids. Common in plant rooms.

Conductor and raceway choices in damp areas

Conductor insulation matters more than most apprentices think. NEC 310.10(C) requires conductors in wet locations to be listed for wet use. THWN-2, XHHW-2, and RHW-2 are your standard picks. Plain THHN is not rated wet, even though most building wire is dual marked THHN/THWN-2. Read the print on the jacket before you pull.

Raceway in damp or wet exterior runs needs to drain. PVC conduit fill of water is a when, not an if. Slope runs back to a fitting with a weep, or use a Myers hub at the top of the can. LB bodies installed cover-down trap water against the gasket and rust the screws into place within two seasons.

If you are running EMT outdoors in a damp location, use raintight compression fittings, not setscrew. Setscrews on EMT outdoors are a callback waiting to happen.

Grounding and bonding at the subpanel

This is where most field corrections happen. The four wire feeder lands with the EGC on the ground bar bonded to the can, and the neutral on an isolated, insulated neutral bar. If the panel shipped with a green bonding screw threaded into the neutral bar, back it out and bag it to the panel schedule pocket.

For a subpanel in a separate structure, NEC 250.32(A) requires a grounding electrode at that structure. A single ground rod needs the 25 ohm test or a second rod per NEC 250.53(A)(2). Two rods, six feet apart, bonded with a #6 copper, is the standard answer because nobody on a service truck carries a fall-of-potential tester.

1. Land the feeder EGC on the bonded ground bar.
2. Land the feeder neutral on the isolated neutral bar.
3. Remove the main bonding jumper or screw.
4. If separate structure, run a GEC to local electrodes per 250.32.
5. Bond any metal water and gas piping per 250.104.

Working space, clearances, and the inspector

NEC 110.26 is not negotiable. 30 inches wide, 36 inches deep, 6.5 feet high, clear and dedicated. A subpanel behind the water heater, under the laundry sink, or above a workbench with a vise welded to it will fail. Photograph the working space before you close the wall.

Dedicated equipment space per 110.26(E) extends from the floor to the structural ceiling, or 6 feet above the panel, whichever is lower. No plumbing, no ductwork, no foreign systems in that envelope. In a damp basement, that means routing the new condensate line around the panel column, not over it.

Final checks before you leave the site

Torque every lug to the manufacturer label. NEC 110.14(D) makes torque a code requirement, not a suggestion. Carry a calibrated screwdriver, not a guess. Re-torque the feeder lugs at the source panel too, because a loose breaker lug at the main is what cooks the new sub six months later.

Label the subpanel per NEC 408.4: every circuit, legibly, with the load it actually serves. "Spare" and "lights" do not count. Note the source panel and the breaker number on the inside of the dead front. The next electrician on this job is you in three years, and you will not remember.

• Verify neutral isolation with a meter, panel energized, between the neutral bar and the can. Should read zero volts and infinite resistance to ground only with the main off.
• Confirm GFCI and AFCI requirements per NEC 210.8 and 210.12 for any new branch circuits.
• Photograph the inside of the dead front, the directory, and the working space for the close-out package.