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

Plan the load and the location before you cut a hole

A subpanel install lives or dies on the prep. Before you pull a knockout, sit down with the load calc per NEC 220 Part III and confirm the feeder ampacity, the panel rating, and the available space at the proposed location. Dry location means dry, not "mostly dry" or "dry except when the washer overflows." Garages, finished basements, mechanical rooms, interior closets that meet 240.24(D) clearances are fair game. Bathrooms, clothes closets, and over stairs are not.

Working clearance is the first thing inspectors check and the first thing electricians forget. NEC 110.26(A) wants 36 inches of depth, 30 inches of width or panel width (whichever is greater), and 6.5 feet of headroom. That space stays clear, dedicated, and unobstructed. No shelving above, no stored paint cans, no water heater elbow swinging into the zone.

Confirm the feeder path before you commit. Measure twice, count your bends, and check that the conduit or cable assembly you plan to use will actually fit through the studs and joists you have to cross.

Sizing the feeder and the panel

Match the feeder to the calculated load, not to whatever breaker happened to be on the truck. A 100A subpanel fed with #4 copper THHN at 75C terminations gets you there for most residential additions. Going bigger? Run the numbers. NEC Table 310.16 and the termination temperature limits in 110.14(C) decide the conductor size, not your gut.

The panel itself should have headroom. A 20-space panel with 18 circuits already planned is a panel that will be torn out in five years when the homeowner adds a heat pump. Spec a panel with at least 30 percent spare capacity in both spaces and bus ampacity.

• Confirm panel bus rating equals or exceeds feeder OCPD.
• Verify the panel is listed for use as a subpanel (most are, but check the label).
• Order the correct main lug or main breaker kit. They are not interchangeable.
• Get a separate equipment ground bar if the panel did not ship with one.

Grounding and bonding, the part everyone messes up

This is the single biggest failure point on subpanel inspections. In a subpanel, the neutral and ground are separated. Period. Remove the main bonding jumper, remove the bonding screw, remove whatever green screw the manufacturer shipped tightened down. NEC 250.24(A)(5) and 408.40 do not leave room for interpretation here.

The feeder is a 4-wire run: two hots, an insulated neutral, and a separate equipment grounding conductor sized per NEC Table 250.122 to the feeder OCPD. The neutral lands on the isolated neutral bar. The EGC lands on the ground bar that is bonded to the can. Do not share lugs, do not double up neutrals and grounds.

If you can read continuity between the neutral bar and the panel can with the main bonding jumper removed, stop. You have a hidden bond somewhere, usually a neutral pigtail touching a screw or a factory strap you missed on the back of the bus.

If the subpanel is in a separate structure, NEC 250.32 changes the rules. For a feeder to a detached building, you still keep neutral and ground separated at the second panel and you drive a grounding electrode at that structure. Single branch circuits to outbuildings get different treatment, read the article before assuming.

Mounting, conduit entries, and physical protection

Mount the panel plumb, with the top breaker no higher than 6 feet 7 inches above the finished floor per 240.24(A). Use the right fasteners for the wall type. Drywall anchors are not fasteners. Hit studs or use a plywood backer.

For dry location interior installs, NM cable through the top with listed connectors is common and cheap. EMT or MC gives you a cleaner job and better mechanical protection where the cable runs through unfinished space. Use listed connectors rated for the cable type, and torque the clamps. Loose NM connectors are a code violation under 110.3(B) and a fire waiting to happen.

• De-burr every conduit cut. Plastic bushings on EMT 1 inch and larger, per 300.4(G).
• Support conduit within 3 feet of the panel and every 10 feet thereafter for EMT.
• Seal unused knockouts with listed KO seals, not tape.
• Maintain cable bend radius at the panel entry. Sharp bends crack THHN insulation over time.

Circuit work and torque

Land your branch circuits with the panel deenergized and verified dead. Strip to the gauge mark on the breaker, no nicks, no exposed copper past the lug. Tighten to the manufacturer torque spec, which is printed on the panel label and called out by NEC 110.14(D). A calibrated torque screwdriver is not optional anymore on inspections in jurisdictions that have adopted the 2017 NEC or later.

Label every circuit before you close the cover. NEC 408.4(A) requires a legible, specific directory. "Lights" is not specific. "Kitchen counter receptacles, south wall" is.

Snap a phone photo of the finished panel with the directory visible before you put the cover on. Saves you a callback when the homeowner peels the sticker off in two years.

Final checks before you energize

Before the cover goes back on the main, walk the install. Megger the feeder if the run is long or you have any doubt about insulation integrity. Verify neutral-to-ground isolation at the subpanel with an ohmmeter, with all branch loads disconnected. Check torque on the feeder lugs at both ends.

Energize the feeder first with all subpanel branch breakers off. Confirm voltage at the main lugs, confirm 240V phase to phase and 120V phase to neutral. Then bring branches up one at a time. Document the load readings under typical operation. That documentation is what saves you when a nuisance trip shows up six months later and the homeowner swears nothing has changed.