Field guide: installing a subpanel, new construction version (edition 1)

Plan the subpanel before you pull a single wire

New construction gives you the luxury of clean slate layout. Use it. Walk the prints, mark the main panel location, the subpanel location, and the run path before the framers close anything up. Subpanels feeding detached structures follow NEC 225.30 through 225.39; subpanels inside the same building follow 408 and the feeder rules in 215.

Size the feeder to the calculated load per Article 220, not to whatever breaker was sitting on the shelf. A 100A subpanel fed with #3 copper THHN at 75°C terminations is common, but verify the termination rating on both ends before you commit. If the run is long, voltage drop per 210.19(A) Informational Note 4 matters more than ampacity.

Measure the run twice. A 90 foot feeder on paper turns into 118 feet once you account for vertical drops into the panel and the trip through the top plate. Short by 10 feet on #3 and you are splicing in a junction box you did not plan for.

Feeder conductors and the neutral question

A subpanel in a separate building may or may not require a grounding electrode and a separated neutral, depending on whether it is the only feeder and whether metal paths exist between structures. Read 250.32 carefully. For subpanels in the same building, the neutral and ground must be isolated at the subpanel per 250.142(B). Remove the main bonding jumper. Remove the bonding screw. Verify.

Four wire feeders are the standard now: two hots, one neutral, one equipment grounding conductor. The EGC is sized per 250.122 based on the feeder overcurrent device, not the feeder conductor. A 100A feeder wants a #8 copper EGC minimum.

• Two ungrounded (hot) conductors sized per load calc
• One grounded (neutral) conductor, same size as ungrounded unless reduced per 220.61
• One equipment grounding conductor per 250.122
• Conduit or cable assembly rated for the install location

Mounting and working clearances

110.26 is not negotiable. 30 inches of width, 36 inches of depth, 6.5 feet of headroom. Do not let the HVAC guy or the plumber talk you into shaving it. If the panel sits in a finished space, dedicated equipment space per 110.26(E) still applies above the panel to the structural ceiling or 6 feet, whichever is lower.

Mount the panel so the highest breaker handle is no more than 6 feet 7 inches off the finished floor per 240.24(A). In new construction, account for finished floor height, not subfloor. A panel screwed to studs at rough-in often ends up an inch and a half too high once the tile goes down.

Snap a level line across the studs at 60 inches from the projected finished floor. That becomes your top of panel reference for every subpanel and main in the build. Consistency saves callbacks from the inspector.

Bonding, grounding, and the details inspectors check

For a subpanel in the same structure, the rules are simple if you follow them. Neutral bar isolated from the enclosure. Ground bar bonded to the enclosure. EGC lands on the ground bar. Neutrals land on the neutral bar. No neutral and ground sharing a single terminal, ever. 408.41 requires each grounded conductor to terminate in an individual terminal.

If the subpanel feeds a detached structure, 250.32(B) requires a grounding electrode system at that structure. Ground rods, Ufer, or whatever the soil and build permit. The EGC run with the feeder handles fault current; the electrode handles lightning and stray voltage reference.

1. Pull the factory bonding screw or strap from the neutral bar
2. Install a separate ground bar kit if not supplied
3. Land the feeder EGC on the ground bar
4. Verify neutral bar reads open to the can with a meter

Circuit layout and breaker selection

Balance the load across both legs as you populate the panel. A 42 circuit panel with everything stacked on A phase is a call back waiting to happen. Track the calculated load per circuit and alternate as you go.

AFCI and GFCI requirements under 210.12 and 210.8 have expanded in recent cycles. Kitchens, laundry, bathrooms, garages, outdoor outlets, dishwashers, and most 125V 15 and 20A outlets in dwelling living areas need AFCI, GFCI, or dual function. Stock the subpanel with the right breaker mix before you start landing circuits, not after.

• AFCI: bedrooms, living rooms, kitchens, laundry, most dwelling outlets per 210.12(A)
• GFCI: kitchens, baths, garages, outdoors, laundry, within 6 feet of sinks per 210.8(A)
• Dual function where both are required
• HACR rated breakers for HVAC feeds

Label, torque, and close it out

Every breaker gets a legible directory entry per 408.4(A). Room plus load, not just "lights." Torque every lug to the manufacturer spec listed inside the cover. 110.14(D) now requires a calibrated torque tool. Inspectors are checking for torque marks on more jurisdictions every year.

Before you energize, walk the panel one more time. Breaker handles off. Neutrals and grounds on their proper bars. Feeder torqued. Cover gasket seated if outdoors. Then megger the feeder if the run is long or the site was wet during rough-in. Catching a nicked conductor before you throw the main is the cheapest troubleshooting you will ever do.