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

Plan the load before you cut a knockout

A subpanel install lives or dies at the load calc. Before you pull the truck up to the job, run Article 220 on paper. Total the continuous loads at 125%, add the noncontinuous, apply demand factors per 220.42 through 220.60, and compare against the feeder ampacity you intend to run. If the homeowner is "thinking about" a hot tub or an EVSE later, size for it now. Pulling a second feeder through the same chase next year is not a savings.

Confirm the supply side can carry the new load. The existing service main and bus rating cap everything downstream. A 200A service feeding a panel already at 160A calculated load has no business picking up a 100A subpanel without a load study or a service upgrade. Document the calc and leave a copy in the panel envelope.

Pick the panel itself based on circuit count plus 25% spare minimum, lug rating, and bus material. Aluminum bus is fine when terminations are torqued and treated, but verify the listing matches the breakers on the truck.

Feeder sizing, conductors, and the EGC

Feeder conductors follow 215.2 and Table 310.16. Run the ampacity at the 75 degree C column for terminations on standard breakers and lugs unless the equipment is specifically listed for 90 degree C. Voltage drop is not a code requirement under 215.2(A)(1) Informational Note, but 3% on the feeder is the working standard. On long runs to a detached structure or a far corner of a basement, upsize a gauge before you regret it.

The equipment grounding conductor is sized per 250.122 based on the overcurrent device ahead of the feeder, not the conductor ampacity. If you upsized the ungrounded conductors for voltage drop, 250.122(B) requires you to upsize the EGC proportionally. This is the line item inspectors love to flag.

• 100A feeder, copper: 3 AWG THHN ungrounded, 8 AWG copper EGC minimum
• 100A feeder, aluminum: 1 AWG XHHW-2 ungrounded, 6 AWG copper or 4 AWG aluminum EGC
• Neutral sized to the maximum unbalanced load per 220.61, never smaller than the EGC
• Upsized ungrounded conductors require proportional EGC per 250.122(B)

Grounding and bonding at the subpanel

This is where rookies blow the inspection. At a subpanel in the same structure as the service, the neutral and ground must be kept separate. Remove the main bonding jumper or the green bonding screw. The neutral bar floats on insulated standoffs, the ground bar bonds to the enclosure. Four-wire feeder, every time, per 250.32(B) when the subpanel is in a separate structure with its own grounding electrode system.

If the subpanel is in the same building, you do not drive a new ground rod for it. The EGC carried with the feeder is the bonding path back to the service. Driving a rod and bonding the neutral at the subpanel creates a parallel neutral path through the earth and the building steel. That is a fire and shock condition, not a belt-and-suspenders move.

Pull the bonding screw out of the can and tape it to the inside of the dead front before you energize. If a future tech opens the panel and sees no screw and no tape, they will assume one was never installed and chase a problem that does not exist.

Dry location: what actually qualifies

Article 100 defines a dry location as one not normally subject to dampness or wetness, but which may be temporarily subject to dampness, as in the case of a building under construction. A finished basement above grade with conditioned air qualifies. An unfinished basement with a sump pump cycling weekly does not. Garage walls that share a slab with the driveway frequently sweat in summer and shift to damp.

The panel listing matters. A standard NEMA 1 indoor enclosure goes in a dry location only. Move it to a damp or wet location and you need NEMA 3R or better, plus listed fittings on every conduit entry. Reading the label on the can takes ten seconds and saves a callback.

1. Check the wall for moisture with a pinless meter before mounting, especially exterior walls and below-grade walls
2. Mount with a 1/4 inch air gap behind the panel using listed standoffs if the wall is masonry or concrete
3. Seal conduit entries from unconditioned spaces per 300.7(A) to prevent condensation inside the can
4. Maintain working space per 110.26: 36 inches deep, 30 inches wide or panel width, 6 feet 6 inches high

Conduit, fill, and the working space rule

Feeder runs in EMT or PVC need to honor Chapter 9 Table 1 fill limits. Two conductors plus EGC in a single raceway lands at the 31% fill column. For a 100A copper feeder in EMT, 1 inch trade size handles three 3 AWG THHN and an 8 AWG EGC with room to spare. Do not stuff a 3/4 inch raceway because it was on the truck.

Working space under 110.26 is not negotiable. The 36 inch depth is measured from the live parts, not the wall. Water heaters, shelving, and the homeowner's deep freeze all violate this when the panel goes up first and the rest of the room fills in around it. Photograph the space at rough-in and at final.

If the homeowner asks to put a cabinet in front of the panel after final, hand them a copy of 110.26 and walk away. The inspector who fails the next permit on that house is not going to care who moved what.

Commissioning and the paper trail

Torque every lug to the manufacturer spec, not by feel. 110.14(D) made calibrated torque tools the rule, not the suggestion. Label every breaker per 408.4(A) with the specific load, not "lights." Update the directory on the service panel to reflect the new feeder and the subpanel location.

Megger the feeder before energizing if the run is long or pulled through a wet trench. Verify phase rotation if it feeds anything three-phase downstream. Check voltage line to line, line to neutral, and neutral to ground. Neutral to ground should read close to zero at the subpanel; anything above a couple of volts means the neutral is bonded somewhere it should not be.