Field guide: installing a subpanel, new construction version (edition 5)

Plan the feeder before you touch the walls

Subpanel work on new construction lives or dies by the feeder sizing. Pull the load calc per NEC 220 before you rough anything in. A 100A subpanel fed with #3 Cu THHN at 75°C terminations works for most detached garages and ADUs, but if the run exceeds 100 feet you are voltage-drop territory and need to upsize.

Check the ampacity table in 310.16 against the terminal rating in 110.14(C). Most residential breakers and lugs are 75°C, so do not chase the 90°C column unless every termination in the path is rated for it. Size the grounding electrode conductor from 250.66 and the equipment grounding conductor from 250.122.

• 60A feeder: #6 Cu (75°C) with #10 EGC
• 100A feeder: #3 Cu or #1 Al with #8 EGC
• 125A feeder: #1 Cu or 2/0 Al with #6 EGC
• 200A feeder: 2/0 Cu or 4/0 Al with #6 EGC

Locate the panel for code and for the homeowner

NEC 110.26 governs working space. 30 inches wide, 36 inches deep, 6.5 feet high, and the space in front of the panel cannot be used for storage. On new construction you have the luxury of framing around it, so push back if the GC wants to tuck it behind a water heater or stuff it in a clothes closet. Panels in clothes closets are prohibited per 240.24(D).

Height matters too. 240.24(A) caps the highest breaker handle at 6 feet 7 inches. Mount the panel so the top breaker lands around 6 feet, which puts the bottom knockouts at a comfortable working height and leaves room for future circuits.

Frame a full stud bay around the panel with blocking top and bottom. It gives you clean mounting, keeps the drywallers honest, and saves you from chasing sheetrock dust out of the gutter six months later.

Run the feeder clean

Four-wire feeders only. Two hots, a neutral, and an equipment grounding conductor. The three-wire feeder to a detached structure went away in the 2008 NEC and is gone for good under 250.32(B). Keep neutrals and grounds separate at the subpanel, which is the single biggest mistake rookies make on this job.

For interior subpanels, SER cable is the fast option if your AHJ allows it and you follow 338.10(B)(4)(a) for ampacity (60°C column in thermally insulated assemblies). For runs through unfinished basements or mechanical rooms, EMT with THHN gives you a cleaner install and room to pull extra conductors later. Outdoors or buried, go PVC Schedule 40 or 80 per 352 and bury per 300.5.

Land the conductors, terminate once

Bring the feeder in through the top or bottom, never the side where the breakers live. Use the right connector for the wiring method: a proper SER connector with the strain relief seated, or a grounding bushing on EMT runs into concentric knockouts per 250.97.

At the subpanel, the neutral bar must be isolated from the can. Pull the bonding screw or bonding strap, check the panel manufacturer's instructions, and verify with a meter before you energize. The ground bar bonds to the can. Neutrals on one bar, grounds on the other, one conductor per hole unless the bar is listed for two.

1. Strip conductors to the gauge marked on the lug
2. Torque to the value printed inside the panel door, NEC 110.14(D) is not optional anymore
3. Verify each breaker is fully seated on the bus stab
4. Label the feeder breaker at the main panel with the subpanel location

Grounding at a detached structure

If the subpanel feeds a separate building, you need a grounding electrode system at that building per 250.32(A). Two ground rods 6 feet apart driven full depth, bonded to the ground bar with a #6 Cu GEC. The EGC in the feeder still comes back to the main panel. Do not bond the neutral at the detached structure unless it qualifies under the exception for a single branch circuit, which almost never applies on new construction.

Test the ground rods if your AHJ requires it. 25 ohms or less from a single rod, or drive the second rod and call it done under 250.53(A)(2). Most inspectors accept the two-rod rule without a megger reading, but bring one if you want to sleep well.

Inspection-ready checklist

Before the rough inspection, walk the panel with fresh eyes. Open the can, look at your work, and fix anything that looks rushed. Inspectors see hundreds of panels a month and they can spot a sloppy install from the driveway.

Take a photo of the torque wrench on each lug with the value visible. If an inspector questions your terminations, you have receipts. If a breaker fails in five years, you have receipts then too.

• Neutral bar isolated, bonding screw removed and bagged inside the can
• EGC landed on ground bar, sized per 250.122
• Feeder breaker at main panel labeled and sized to the feeder ampacity
• Panel directory filled out, not left blank for the homeowner
• AFCI and GFCI protection mapped per 210.12 and 210.8 for the circuits fed
• Working space clear, no studs, pipes, or ducts in the 110.26 envelope