Field guide: installing a subpanel, new construction version (edition 2)

Plan the feeder before you touch a wire

New construction gives you the luxury of doing this right. Start with the load calc per NEC Article 220. Size the feeder conductors and OCPD to the calculated load, not to whatever breaker happens to be sitting in the truck. If this subpanel feeds a detached structure, you are in 225.30 and 225.31 territory, which changes your disconnect and grounding story entirely.

Confirm the panel location meets 110.26 working space before the framers close up the wall. You need 36 inches of depth, 30 inches of width or the width of the equipment (whichever is greater), and 6.5 feet of headroom. Nothing else gets to live in that envelope, not the water heater, not the shelving the GC wants to sneak in.

Map out your circuit count with spares. A 30 percent spare rule keeps you honest and keeps the HO from calling you back in two years when they finish the basement.

Feeder: conductors, conduit, and the 4-wire rule

For a subpanel in the same structure, you are pulling 4 wires: two ungrounded, one grounded (neutral), one equipment grounding conductor. Separate neutrals and grounds at the subpanel per 250.24(A)(5) and 408.40. The bonding screw or strap comes out. The ground bar gets added if the panel did not ship with one.

Size conductors per 310.16 and apply 310.15(B) adjustment if you are bundling in conduit. For a 100A feeder in a dwelling, 310.12 lets you use the reduced dwelling feeder table, but only for the service or main power feeder to the dwelling, not every subpanel downstream. Read that section carefully before you downsize.

• 100A subpanel: #3 Cu or #1 Al THHN/THWN-2 typical, #8 Cu EGC per 250.122
• 125A subpanel: #1 Cu or 2/0 Al, #6 Cu EGC
• 200A subpanel: 2/0 Cu or 4/0 Al, #6 Cu EGC
• Always verify with the actual load calc and termination temp rating (75C column at the lugs)

Mounting and rough-in

Set the panel so the highest breaker handle lands at or below 6 feet 7 inches per 240.24(A). On a finished wall, that usually means the top of the cabinet around 72 to 74 inches. Center it on a stud bay, shim it flush with the future drywall using the depth gauges on the can, and screw through the back into solid framing.

Drive your KO hits where they make sense for the home runs. Cluster NM entries on the top and sides, keep the feeder on its own KO with a proper connector. Every NM cable needs a listed connector and 6 inches of free conductor inside the box per 300.14.

Drill your bore holes for home runs before the panel goes on the wall. Reaching over a mounted can with a hole-hawg is how you end up with a sprained wrist and a crooked hole through a top plate.

Grounding and bonding the right way

This is where inspectors fail jobs. The subpanel neutral bar must be isolated from the enclosure. The EGC from the feeder lands on the ground bar, which is bonded to the can. No green screw, no bonding strap, no neutral-to-case jumper.

If the subpanel is in a detached structure, 250.32 applies. You run an EGC with the feeder and drive a grounding electrode at the detached building (ground rods, Ufer, or whatever the structure has). The GEC from those electrodes lands on the ground bar, not the neutral. The old "re-bond the neutral at the detached building" trick has been dead since 2008, so stop doing it.

Torque every lug to the value printed inside the can door. 250.8 and 110.14 are not suggestions. A calibrated torque screwdriver is cheap insurance against a callback for a loose neutral cooking the bar.

Circuit population and AFCI/GFCI requirements

Populate breakers by load, not by whim. Balance the phases so neither leg is carrying the lion's share. New construction dwelling circuits hit you with a stack of protection requirements under 210.8 and 210.12.

1. 210.12(A): AFCI on nearly every 15A and 20A 120V circuit in dwellings (kitchens, laundry, bedrooms, living areas, hallways, closets, and more)
2. 210.8(A): GFCI for bathrooms, garages, outdoors, crawl spaces, unfinished basements, kitchens, laundry, and within 6 feet of sinks and tubs
3. 210.8(F): GFCI for outdoor outlets serving dwelling units, including HVAC receptacles
4. 210.11(C): dedicated 20A laundry, 20A bathroom, and two 20A small-appliance circuits minimum

Use dual-function breakers where both are required. Label every circuit legibly at the directory per 408.4(A). "Bedroom" is not a circuit description when there are three bedrooms.

Final checks before you call for inspection

Walk the panel with your own inspector's eye. Conductor fill in the gutter cannot exceed 75 percent at any cross section per 312.8. Breakers are listed for the panel brand (no Sylvania guts in a Square D can). Neutrals and grounds are one-per-hole unless the bar is listed for multiple, which almost none are on the neutral side.

Snap a photo of the open panel with all wiring dressed and labels on before you cover anything. The inspector will love it, and your future self will thank you during the service call five years from now.

Verify the feeder breaker at the main matches or is below the subpanel bus rating and the conductor ampacity. Double-check the panel schedule matches what is actually installed. Clean the can out, vacuum the drywall dust, and put the dead front on straight.