Field guide: installing a subpanel, dry location considerations (edition 1)

Plan the load and pick the panel

Before pulling a wire, run the calc. Use Article 220 for feeder and service load calcs. For a typical residential subpanel feeding a workshop, finished basement, or addition, size the feeder for the calculated load, not the breaker you happen to have on the truck. Round up to a panel with at least 25% spare circuit capacity, the homeowner always adds a circuit later.

Match panel ampacity to the feeder OCPD, not the other way around. A 100A subpanel fed by a 60A breaker is fine, but a 100A panel maxed at its bus rating with undersized lugs is a callback waiting to happen. Check the panel label for max OCPD and bus rating.

• Calculate per NEC 220.40 through 220.61 for the feeder.
• Verify panel listing for the application (NEC 408.3).
• Confirm available fault current does not exceed panel SCCR (NEC 110.10).
• Leave at least two spare spaces, three is better.

Location, working space, and clearances

Dry location only means the panel is not subject to dampness or wetness under normal conditions (NEC Article 100 definitions). Garages attached to dwellings usually qualify, unfinished basements usually qualify, but a basement that floods every spring does not. If there is any question, treat it as damp and use a NEMA 3R enclosure or relocate.

Working space is non-negotiable. NEC 110.26(A) requires 36 inches of depth, 30 inches of width (or panel width if greater), and 6.5 feet of headroom. The space must be clear and dedicated. No shelving above the panel within the dedicated space defined in 110.26(E), no water heater six inches to the side, no storage stacked in front.

If a homeowner says "we'll just move the boxes when you need to work on it," put it in writing on the invoice that the space must remain clear. You will be back, and so will the next electrician.

Feeder conductors and the four-wire rule

For any subpanel in a separate structure or downstream of the service disconnect, you need four wires: two ungrounded, one grounded (neutral), one equipment grounding conductor. The neutral and ground bus must be isolated at the subpanel. This is NEC 250.32 for separate buildings and 408.40 in general. The pre-2008 three-wire feeder to a detached structure is gone, do not replicate it on a remodel.

Size the EGC per NEC Table 250.122 based on the feeder OCPD. If you upsize the ungrounded conductors for voltage drop, upsize the EGC proportionally per 250.122(B). This catches people on long runs to detached garages.

• Two hots, one neutral, one EGC. Always.
• Remove the main bonding jumper or green screw at the subpanel.
• Neutral bar floats, ground bar bonds to the enclosure.
• Size EGC by Table 250.122, upsize if conductors are upsized.

Mounting, supports, and conduit entries

Mount the panel so the highest breaker handle is no more than 6 feet 7 inches above the floor or working platform (NEC 240.24(A)). Plumb and square, lagged into studs or blocking, not just drywall anchors. If you are surface mounting on masonry, use proper masonry anchors rated for the panel weight plus the conductor pull.

Support feeders within 12 inches of the panel for NM cable (NEC 334.30) or per the appropriate article for whatever wiring method you used. Conduit knockouts get listed connectors, not duct tape and good intentions. Use reducing washers if the KO is oversized, and bond any concentric or eccentric KOs per NEC 250.97 if the circuit is over 250V to ground (rare in residential, common in light commercial).

Bonding, grounding, and the GEC question

Same building as the service: no separate grounding electrode required for the subpanel. The EGC in the feeder does the work. Separate building or structure: NEC 250.32(A) requires a grounding electrode system at the second structure. Drive a ground rod, or two if you cannot prove 25 ohms or less with one (250.53(A)(2) Exception). Two rods 6 feet apart is the standard practice, easier than testing.

Bond the GEC to the ground bar in the subpanel of the separate structure. Do not bond neutral to ground at the second structure subpanel, that is a 2008-and-later requirement under 250.32(B)(1). The neutral stays isolated, the ground bar bonds to the enclosure and to the local electrode.

On a detached garage feed, label the disconnect at the garage clearly. If the inspector cannot find the means to kill power to the structure in under 30 seconds, expect a correction notice.

Final checks before you energize

Torque every lug to the manufacturer spec. NEC 110.14(D) makes this explicit, use a calibrated torque tool or torque screwdriver. Eyeballing a feeder lug is how you end up with a thermal event two months later. Document torque values if the AHJ asks, some now do.

Megger the feeder before connecting if the run is long or pulled through wet conduit. Verify phase rotation only matters on three-phase, but on single-phase residential confirm both legs are hot at the panel and 240V across them. Label every breaker per NEC 408.4(A), generic labels like "lights" do not pass anymore in most jurisdictions.

1. Torque all terminations to spec.
2. Verify neutral isolation and ground bonding.
3. Confirm AFCI and GFCI requirements per 210.8 and 210.12 for the circuits served.
4. Label the panel directory with specific room and load descriptions.
5. Update the service panel directory to reference the subpanel feed.