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

Load calc and panel sizing first

Before you pull a single wire, run the load calc on the feeder. NEC 220 Part III is your friend here. Most residential subpanels I install end up at 60A, 100A, or 125A depending on what the homeowner is adding: a detached garage with EV charging, a finished basement, a shop with a welder. Size the feeder to the expected load plus reasonable future growth, not to whatever breaker happens to be in the truck.

Remember that the subpanel itself has to be rated for the full calculated load, and the feeder ampacity has to match. NEC 408.36 requires the panel to have a main rating at least equal to the feeder overcurrent device when fed through. Count your circuits too: a 20-space panel fills up faster than people expect once you add AFCI and GFCI breakers that eat real estate.

If the customer mentions "maybe a hot tub later," size the feeder for it now. Going back to upsize a run of 4/0 SER through finished drywall is a conversation no one wants to have.

Feeder conductors and the neutral

Pick your wire method based on the run. SER cable is common for interior installs, but once you hit a garage or exterior run, you are usually in conduit with THHN/THWN-2 or direct-burial USE/XHHW. Check 310.16 for ampacity and 310.15(B) for adjustment factors if you are bundling. For a 100A subpanel, 1 AWG copper or 1/0 aluminum is standard at 75 deg C terminations.

The neutral is where rookies get burned. A subpanel feeder needs four wires: two hots, a neutral, and a separate equipment grounding conductor. The neutral carries unbalanced current back; the EGC carries fault current. They do not get bonded together at the subpanel. Ever.

• Hot 1 and Hot 2, sized per 310.16
• Insulated neutral, sized per 220.61 (can sometimes be smaller than the hots)
• Equipment grounding conductor per 250.122
• All four land on separate terminals at the sub

Bonding and grounding at the subpanel

This is the part that trips up even experienced guys when they have not done a sub in a while. At the subpanel, the neutral bar must be isolated from the enclosure. Remove the main bonding jumper, the green screw, the bonding strap, whatever the manufacturer ships to tie neutral to case. NEC 250.24(A)(5) and 408.40 make this explicit: the neutral-to-ground bond happens only at the service disconnect.

Install a separate equipment ground bar, bonded to the enclosure, and land all EGCs and the feeder EGC there. Neutrals go on the isolated neutral bar. If the panel ships with only one bar, order the ground bar kit for that panel; do not improvise with self-tappers into sheet metal.

For a detached structure, 250.32 governs. Modern code (2008 and later) requires the four-wire feeder plus a grounding electrode system at the separate building. That means ground rods, or a Ufer if available, bonded to the EGC bus. No more three-wire feeders to outbuildings.

Overcurrent protection and panel placement

The feeder breaker at the main panel is your overcurrent protection for the subpanel and the feeder conductors. Size it to the conductor ampacity, not above. A 100A sub on 1 AWG copper gets a 100A breaker at the main, not a 125A just because you had one on the truck.

Working clearance under 110.26 is non-negotiable: 36 inches deep, 30 inches wide, 6.5 feet high, clear in front of the panel. I have seen inspectors fail installs because the homeowner's water softener lived 24 inches in front of the sub. Document the clearance before you close up.

If the sub is in a clothes closet, stop. 240.24(D) prohibits overcurrent devices in clothes closets. Find another wall.

Circuit loading and AFCI/GFCI at the sub

All the dwelling-unit protection rules still apply at a subpanel just like at a main. 210.8 for GFCI (bathrooms, garages, outdoors, kitchens, laundry, basements, and more in the latest cycles). 210.12 for AFCI on most 15A and 20A 120V circuits in dwelling living areas. The breaker location does not exempt you.

Balance your 120V loads across the two hot legs. An unbalanced sub runs hot on one phase and underloaded on the other, and the neutral carries the difference back. Spread AFCIs and the heavy 120V stuff (microwave, fridge, bathroom) across A and B.

1. List every circuit you plan to install
2. Assign each to leg A or leg B based on estimated load
3. Target within 20 percent balance between legs
4. Fill the directory legibly, per 408.4

Inspection-ready finish

Torque every lug to the manufacturer's spec. 110.14(D) made this a code requirement, not a best practice. Use a calibrated torque screwdriver, not feel. Mark the lugs with a paint pen after torquing so the inspector can see you actually did it.

Label the feeder disconnect at the main with the subpanel location per 408.4(B). Fill out the panel directory completely. Take a photo of the inside of the panel with the deadfront off, in case something gets questioned later. Clean up the wire routing, use the gutter space, and keep neutrals and grounds on their correct bars. A tidy sub passes inspection and makes the next guy's life easier when he opens it up in ten years.