Field guide: installing a subpanel, dry location considerations (edition 4)

Sizing the feeder before you uncoil anything

Subpanel feeders fail inspection most often because the conductor, OCPD, and grounding electrode conductor were sized off three different load assumptions. Pick one calculated load and ride it through. Use NEC 220 Part III for the feeder calc, then back-check against the panel busbar rating in 408.36.

For a typical 100A subpanel fed from a 200A service in a detached shop, 1 AWG copper THHN at 75C terminations gets you there per Table 310.16. Aluminum 1/0 AWG works if both ends are listed for AL/CU. Don't mix temperature columns; if any termination is 60C rated, you're using the 60C column for the whole run.

Voltage drop is not a code rule under 215.2(A)(1) Informational Note, but it is a callback waiting to happen. Run the numbers:

• Under 100 feet: standard ampacity table sizing is usually fine.
• 100 to 200 feet: bump up one size, especially for motor loads or welders.
• Over 200 feet: calculate at 3% drop for the feeder, 5% total to the furthest outlet.

Dry location: what actually qualifies

NEC Article 100 defines a dry location as one not normally subject to dampness or wetness, but may be temporarily subject to dampness or wetness during construction. That last clause trips people up. A finished basement with a dehumidifier running is dry. An unfinished basement with a sweating slab in July is damp at minimum, and your panel selection has to reflect that.

Garages attached to dwellings are generally treated as dry for panel purposes if the slab stays dry and there's no direct weather exposure, but check the AHJ. Detached garages, pole barns, and shops with overhead doors that get rained through count as damp or wet depending on conditions.

If you can see efflorescence on the block wall behind where the panel is going, that wall is not a dry location. Move the panel, fur out the wall, or switch to a NEMA 3R enclosure even if you're indoors.

Enclosure, mounting, and working space

NEMA 1 is your standard indoor dry location enclosure. It does nothing against drips from a copper line above or condensation from a poorly insulated rim joist. Look up before you mount. If there's plumbing, HVAC condensate, or any potential drip path overhead, either reroute the offending pipe or use a 3R can.

Working space comes from NEC 110.26. The numbers people forget:

1. 30 inches wide minimum, or panel width if wider, measured at the panel face.
2. 36 inches deep from the live parts, Condition 1 (dry wall behind you).
3. 78 inches headroom from the floor or platform per 110.26(A)(3).
4. Dedicated electrical space: the footprint of the panel up to 6 feet above it or to the structural ceiling, whichever is lower, per 110.26(E).

No plumbing, no ductwork, no shelving in that dedicated space. A water heater overflow pan three feet above the panel is a code violation and a guaranteed insurance fight when it lets go.

Grounding and bonding the subpanel

This is where the most field mistakes still happen, decades after the four-wire feeder rule went in. A subpanel in a separate structure or on a feeder from the main service has the neutral and ground isolated. Always. The bonding screw or strap that came in the panel comes out and goes in the parts bag.

Pull four conductors: two ungrounded, one grounded (neutral), one equipment grounding conductor sized per NEC Table 250.122 based on the upstream OCPD. A 100A feeder takes a minimum 8 AWG copper EGC. Do not rely on the metal raceway as your sole EGC on a long run; pull a wire.

For subpanels in a separate structure, NEC 250.32 requires a grounding electrode at that structure, bonded to the EGC bus in the subpanel. Ground rod, ufer, or whatever the building has. The neutral stays floating.

If you energize a subpanel and the EGC bus reads any voltage to a known ground, stop. You either missed the bonding screw removal or you have a neutral-to-ground crossover somewhere in the branch circuits. Fix it before anyone touches a metal box.

Circuit layout and breaker selection

Balance the legs as you fill the panel. Walk through the loads and assign them to A and B phase to keep the neutral current down. This matters more in the subpanel than the main because the feeder neutral is doing real work.

AFCI and GFCI requirements still apply downstream of the subpanel. A subpanel feeding bedrooms needs combination AFCI on those circuits per NEC 210.12. A subpanel in a garage needs GFCI on the 125V, 15 and 20 amp receptacle outlets per 210.8(A). The fact that it's a subpanel does not change the branch circuit rules.

Listed breakers only, matched to the panel. A square D breaker in a Siemens load center is not a code-compliant install, even if it physically clips in. Check the panel label for the manufacturers and series numbers it accepts.

Final walk before you energize

Torque every lug. Feeder lugs in particular, both ends. Use a calibrated screwdriver or wrench to the value printed on the breaker or panel label, not feel. NEC 110.14(D) makes this a code requirement, not a best practice.

Megger the feeder if the run is long or the conductors sat in a wet trench. Phase to phase, phase to neutral, phase to ground, neutral to ground. Anything under 1 megohm gets investigated before you close the main.

Label the subpanel with the source: "Fed from Panel A, Breaker 23, Building 1." Label the feeder breaker upstream with the destination. Six months from now when someone is troubleshooting, that label is the difference between a 10 minute job and a two hour scavenger hunt.