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

Pick the location before you pick the panel

Dry location subpanels live or die by where you set them. Working clearance per NEC 110.26 is non-negotiable: 36 inches deep, 30 inches wide or panel width (whichever is greater), and 6.5 feet of headroom. That clearance starts at the live parts and projects out, not from the wall finish.

Confirm the space stays dry under realistic conditions. A finished basement corner reads dry today, but if it sits below a laundry stack or a slab that sweats in August, you are setting up a corrosion call in two years. Check humidity, check the slab, check above the ceiling for plumbing.

Verify dedicated equipment space per NEC 110.26(E): 6 feet above the panel or to the structural ceiling, whichever is lower, with nothing foreign passing through. Sprinkler piping is the common exception, and only if properly protected.

Size the feeder, not just the panel

The panel rating sets a ceiling, not the actual feeder. Run a load calculation per NEC Article 220 before you pull a single conductor. For a typical 100A subpanel feeding a detached workshop or accessory unit, do not assume 100A is the answer. Calculate the demand, then size up to the next standard breaker.

Conductor ampacity comes from NEC 310.16 with adjustments for ambient temp and conduit fill. The 75 degrees C column is your friend on terminations rated for it, which most modern breakers and lugs are. Verify the listing on the panel, do not assume.

• 100A feeder: typically 1 AWG copper THWN-2 or 1/0 aluminum, 75 degrees C column
• Equipment grounding conductor sized per NEC 250.122, based on the overcurrent device protecting the feeder
• Neutral sized per the calculated unbalanced load, NEC 220.61
• Run a separate EGC, do not bond neutral and ground at the subpanel

Four-wire feeder, isolated neutral, no exceptions

This is where guys still get tripped up on remodels. NEC 250.142(B) requires the neutral and the equipment ground to be separated at every panel downstream of the service. The neutral bar floats. The ground bar bonds to the enclosure. The bonding screw or strap that came with the panel stays in the bag.

If you are replacing an old three-wire feeder to a detached structure, you are pulling a fourth conductor. The 2008 code cycle closed that loophole and there is no grandfathering when you touch the panel. Pull the EGC, install a separate ground bar, and remove the bonding jumper.

Tip from the field: tape the bonding screw to the inside of the deadfront cover before you energize. Next guy in there will know at a glance the panel was set up correctly as a subpanel.

Grounding electrode at separate buildings

Subpanel in the same structure as the service: no grounding electrode required at the subpanel, the service handles it. Subpanel in a detached building: you need a grounding electrode system per NEC 250.32, and it ties to the ground bar in the subpanel, not the neutral.

Two ground rods 6 feet apart, or one rod with a documented resistance of 25 ohms or less per NEC 250.53(A)(2). Most guys just drive two rods and move on, it is faster than testing. Bond to existing electrodes if they exist (rebar in footer, metal water line within 5 feet of entry).

1. Drive rods at least 6 feet apart, fully below grade
2. Use listed acorn clamps or exothermic welds, no split bolts
3. Run #6 copper minimum to the panel ground bar, NEC 250.66
4. Protect the GEC from physical damage where exposed

Breaker selection and panel layout

Use breakers listed for the panel. Classified breakers exist for a reason, but the panel manufacturer's listing controls. AHJs vary on this and you do not want to argue it on a Friday afternoon inspection.

Lay out circuits with future loads in mind. Leave space at the top for a future generator interlock or a 240V circuit. Balance the legs as you go, do not stack all the heavy loads on one phase and tell yourself you will fix it later. Label as you land each conductor, not at the end.

AFCI and GFCI requirements follow the load, not the panel location. NEC 210.8 and 210.12 still apply downstream. A subpanel feeding bedroom circuits needs AFCI protection on those branch circuits. A subpanel in a garage feeding receptacles needs GFCI per NEC 210.8(A).

Final checks before energizing

Torque every termination to the manufacturer spec, NEC 110.14(D). This is enforceable now, not a suggestion. Use a calibrated torque screwdriver or wrench, not your wrist. Mark each lug with a paint pen as you go so you know what you have done.

Tip from the field: take a phone photo of the panel interior with all conductors landed and labeled before you set the deadfront. Saves the callback when somebody asks what is on circuit 14.

Megger the feeder if it is a long run or pulled through wet conduit during rough-in. Verify phase rotation if you are feeding any three-phase equipment. Energize the main first, verify voltage at the lugs, then bring up branch circuits one at a time.