Field guide: installing a subpanel, dry location considerations (edition 6)

Plan the load and the location first

Before you pull a permit or open a knockout, size the subpanel to the actual load and pick a spot that meets clearance and environment rules. Dry location means dry per NEC Article 100, not "mostly dry most of the year." A garage wall that sees blowing rain through a louvered vent is not a dry location, and neither is the corner of a basement that floods every spring thaw.

Confirm the feeder ampacity against the calculated load per NEC 220, and verify the main panel has capacity and a breaker space for the feeder. If the existing service is already loaded heavy, a load calc is not optional. Document it.

• Working space: 30 in. wide, 36 in. deep, 6 ft 6 in. high per NEC 110.26(A).
• Dedicated space: 6 ft above the panel kept clear of piping and ducts per NEC 110.26(E).
• Mounting height: top breaker no higher than 6 ft 7 in. per NEC 240.24(A).
• Never install in a clothes closet or bathroom per NEC 240.24(D) and (E).

Feeder sizing, conductors, and conduit

Size the feeder conductors to the calculated load and the overcurrent device, applying 75 deg C terminations on most modern gear unless the listing says otherwise, per NEC 110.14(C). For a 100 A subpanel, that is typically 3 AWG copper or 1 AWG aluminum THHN/THWN-2, with an appropriately sized equipment grounding conductor from Table 250.122.

Run four wires: two ungrounded, one grounded (neutral), and one equipment grounding conductor. The neutral and ground must remain isolated at the subpanel. This is the single biggest mistake on residential remodels, and it will fail inspection every time.

Pulled a 100 A feeder in 1-1/4 in. EMT once and the apprentice swore it would "go easy." It did not. Use 1-1/2 in. for 1 AWG aluminum if you value your shoulders, and check Chapter 9 Table 4 and Table 5 fill before you commit.

Bonding, grounding, and the neutral

At a separately derived system you bond. At a subpanel fed from the same service you do not. Remove the main bonding jumper or green bonding screw at the subpanel per NEC 250.24(A)(5) and 408.40. The neutral bar floats, the ground bar bonds to the enclosure.

Add a separate equipment grounding bar if the panel did not ship with one, and land all EGCs there. If the feeder runs in metallic raceway that qualifies as an EGC under NEC 250.118, you still want a continuous bonding path verified end to end. Do not rely on a loose locknut to carry fault current.

1. Confirm the main panel is the service disconnect and is bonded there.
2. At the subpanel, remove or do not install the main bonding jumper.
3. Land the grounded conductor on an isolated neutral bar.
4. Land all EGCs on a bonded ground bar.
5. Verify continuity from subpanel enclosure back to the grounding electrode system.

Dry location specifics that bite people

"Dry location" lives in Article 100. It means an area not normally subject to dampness or wetness, but that may be temporarily damp during construction. A finished basement with conditioned air is dry. An unfinished basement with a sump and visible efflorescence on the block is damp at best, and often wet by code. Use NEMA 3R or better and adjust wiring methods accordingly per NEC 312.2.

Even in a dry location, watch for condensation paths. Conduit running from a cold attic or crawl down into a warm panel will pump moisture into the enclosure. Seal the raceway per NEC 300.7(A) with duct seal or a listed sealing fitting. This is not a Class I location requirement, it is a corrosion and nuisance trip prevention move.

If you can see dust, you can see moisture six months later. Mount panels off masonry walls with 1/4 in. spacers in basements, and keep the bottom knockouts unused when you can route from the top or sides.

Breakers, AFCI, GFCI, and labeling

Match breakers to the panel listing. Mixing brands voids the listing and the inspector's patience. Branch circuits served by the subpanel still need AFCI and GFCI protection per NEC 210.8 and 210.12 based on the room and circuit type, not based on which panel they originate in.

If the subpanel feeds a detached structure, NEC 225.31 through 225.36 require a disconnect at the second building, and the feeder needs its own grounding electrode system per 250.32. That is a different animal than a same-building subpanel and is worth confirming before you run wire.

• Label every breaker with the specific load, not "lights" or "kitchen."
• Apply the panel directory legibly per NEC 408.4(A).
• Mark the feeder source location at the subpanel per NEC 408.4(B).
• Torque all lugs to spec per NEC 110.14(D) and document it.

Final checks before you energize

Megger the feeder before you land it if there is any chance of damage during the pull. Verify phase rotation if you are on three phase. Check that the neutral is not bonded at the subpanel by lifting it and reading resistance to ground, it should read open.

Energize one leg at a time if the panel allows it, and verify voltage leg to leg and leg to neutral before adding load. Walk the job with the homeowner or GC, point out the working clearance, and tell them not to store paint cans in front of it. They will anyway, but you said it.