Field guide: installing a subpanel, new construction version (edition 3)

Plan the load and the location first

Before you pull a single staple, know your calculated load and your feeder ampacity. A 100A subpanel fed off a 200A main is the common answer in new builds, but size it against the actual connected load and future circuits. NEC 220 Part III gives you the standard method, and the optional dwelling calc in 220.82 is usually kinder when the house has electric heat, an EV charger, or a heat pump water heater.

Location matters as much as the numbers. Keep the panel out of clothes closets (240.24(D)), out of bathrooms (240.24(E)), and give yourself the 30 inch wide, 36 inch deep, 6.5 foot tall working space from 110.26. If it lands in a garage, verify the wall finish and clearance to parked vehicles before the drywall crew shows up and you are stuck chasing a variance.

Coordinate with the framer early. A flush panel between studs needs a bay that is plumb, square, and not full of fire blocks where your conduits want to land.

Size the feeder and the conduit

For a 100A subpanel in a typical dwelling, 83% rule in 310.12 lets you run #4 copper THHN or #2 aluminum for the ungrounded and grounded conductors. The equipment grounding conductor is sized from Table 250.122, not upsized with the feeder unless you are upsizing for voltage drop (250.122(B)).

Separate the grounded conductor from the equipment ground at the subpanel. This is the rule that gets called out on nearly every rough inspection, and it is non negotiable: NEC 250.24(A)(5) and 408.40 require the neutral to float in the subpanel. Remove the bonding screw, strap, or jumper that shipped with the can, and land the EGC on a ground bar that is bonded to the enclosure.

• 100A feeder, dwelling: #4 Cu or #2 Al ungrounded and grounded (310.12)
• EGC: #8 Cu or #6 Al minimum (Table 250.122)
• Minimum conduit for four #2 Al THHN: 1-1/4 inch EMT or PVC (Chapter 9, Table 1 and Annex C)
• SER cable in interior walls, where allowed: follow 334.80 for ampacity reduction in thermal insulation

Grounding and bonding at the rough

If the subpanel is in the same building as the service, you run four wires: two hots, a neutral, and an equipment ground. No supplemental ground rods at the subpanel, no exceptions for "belt and suspenders." A second grounding electrode system at a subpanel in the same structure creates parallel paths for neutral current and will fail inspection under 250.32(B).

Detached structure is a different article. NEC 250.32 requires an electrode at the separate building, and you still run an EGC with the feeder. The 2008 exception that allowed a three wire feeder to a detached building with no metallic path is gone. Do not cite it, do not install it.

Mark the neutral bar with red tape at rough so the trim electrician does not accidentally bond it when he installs the ground bar kit. Saves a callback every time.

Rough-in: knockouts, supports, and conductor fill

Support the feeder within 12 inches of the panel and every 4.5 feet after that for NM, or per 358.30 for EMT. If you are stacking circuits through the top of the can, plan the knockouts before you start drilling. A panel with a dozen 1/2 inch KOs in a row looks tidy but the derate from 310.15(C)(1) will bite you if you bundle more than three current carrying conductors for over 24 inches.

Leave conductor length inside the can. NEC 312.6 gives you the minimum bending space, but beyond that, every trim electrician wants at least one full loop around the interior so they can land breakers without fighting the wire. Strip the cable jackets inside the panel, not outside, and keep the jacket intact at least 1/4 inch past the connector (312.5(C)).

• Bond all metal raceways with a bonding bushing or listed fitting where concentric KOs are present (250.97)
• Torque lugs to the value on the label, not "tight enough" (110.14(D))
• Identify the grounded conductor with white tape if you used a re-identified conductor in larger sizes (200.6(B))

Circuit layout and AFCI/GFCI planning

Lay out the branch circuits before you set breakers. Balance the phases, group kitchen and laundry small appliance circuits per 210.11(C), and plan the AFCI and GFCI coverage from 210.8 and 210.12 before you pick breakers. A subpanel full of single pole AFCI/GFCIs gets expensive fast, and some combinations (shared neutral multiwire branch circuits) require two pole devices.

For a 2023 code dwelling subpanel, expect AFCI on nearly every 15 and 20A 120V circuit in habitable spaces, and GFCI on kitchens, bathrooms, laundry, garage, outdoors, basements, and now dishwashers and ranges in many jurisdictions. Check 210.8(A) and the local amendments, because states are adopting and amending at different speeds.

Order your AFCI/GFCI breakers by the panel brand early. Square D, Eaton, and Siemens all have backlogs on certain part numbers, and swapping brands at trim means pulling the panel.

Trim, label, and inspection

Torque every termination, including the feeder lugs, the neutral and ground bars, and the breaker screws. Use a calibrated torque screwdriver, not feel. The 2020 code made 110.14(D) explicit, and inspectors are asking to see the tool.

Label the panel directory with the actual circuit and location, not "lights." NEC 408.4(A) requires a legible, specific description. If the subpanel is fed from a different structure or a different service, 408.4(B) wants the source identified on the panel too. A 3 dollar label maker prevents a 300 dollar return trip.