Field guide: installing a subpanel, for master electricians (edition 5)

Plan the load and the location first

Before pulling a single conductor, calculate the subpanel load per NEC 220, Part III. Add up lighting, receptacles, appliances, and any dedicated circuits feeding from the new panel. Apply demand factors where allowed, especially for dryers (220.54) and ranges (220.55). Undersize the feeder and you will be back inside a wall in two years.

Location matters as much as math. Working space per 110.26 is non-negotiable: 30 inches wide, 36 inches deep, 6.5 feet of headroom. No panel above a sink, inside a clothes closet (240.24(D)), or in a bathroom of a dwelling (240.24(E)). Garages, basements, utility rooms, and dedicated electrical closets are the usual wins.

Confirm the main panel has a breaker slot available and that the existing service can carry the added load. If the service is already maxed, you are having a service upgrade conversation with the homeowner before you touch the subpanel.

Feeder sizing, conduit, and grounding

Size the feeder conductors from 310.16 based on the calculated load and the breaker protecting them at the main. A 100A subpanel typically runs on 3 AWG copper or 1 AWG aluminum at 75C terminations, but always verify against the actual calculation and termination ratings per 110.14(C). Do not default to the old "60A needs 6 AWG" rule without checking.

For the equipment grounding conductor, size per Table 250.122 based on the overcurrent device ahead of the feeder, not the conductor ampacity. A 100A feeder needs an 8 AWG copper EGC minimum. If you upsize the ungrounded conductors for voltage drop, upsize the EGC proportionally per 250.122(B).

• 4-wire feeder required for remote subpanels: two hots, neutral, and separate EGC (250.32(B)).
• Neutral and ground bars must be isolated at the subpanel. Remove the bonding screw or strap.
• Ground bar bonded to the enclosure. Neutral bar floats.
• No ground rods at a subpanel in the same structure as the service. Rods only apply to separate structures (250.32(A)).

Breaker compatibility and panel fill

Use breakers listed for the panel. Classified breakers are a legal alternative under 110.3(B) only when the panel's listing and the breaker's classification agree. UL white book is your friend here. Mixing random brands because the supply house was out of stock is how panels burn.

Count circuits against 408.54 if you are installing a panelboard with a circuit limit, though most modern residential load centers no longer carry that restriction. Still, leave at least 20 percent spare capacity for future loads. Homeowners add EV chargers, heat pumps, and hot tubs faster than you think.

"I carry a cheat sheet of which breakers are classified for which panels. Saves me twenty minutes of arguing with an inspector who thinks every Eaton breaker fits every Square D panel. They do not."

GFCI, AFCI, and the 2023 code realities

The subpanel itself does not change where GFCI and AFCI are required, but every new branch circuit you land on it must comply with 210.8 and 210.12 as adopted in your jurisdiction. Dwelling unit basements, garages, kitchens, laundry areas, and outdoor receptacles all need GFCI protection per 210.8(A). AFCI protection follows 210.12(A) for most habitable rooms.

If the subpanel is feeding a detached garage, outbuilding, or pool equipment, the feeder itself may need GFCI protection per 210.8(F) for outdoor outlets or 680.25 for pool-related feeders. Read the section carefully. The 2020 and 2023 cycles expanded GFCI requirements significantly, and not every AHJ has caught up.

Dual-function breakers solve both problems on a single device but cost more and take more space if the panel uses plug-on neutral. Price it into the bid. Do not eat the cost because you forgot.

Terminations, torque, and labeling

Torque every lug to the manufacturer's spec, verified with a calibrated torque screwdriver or wrench. 110.14(D) now explicitly requires it. "Gut tight" is not a specification and will fail an inspection in most jurisdictions.

Label the subpanel per 408.4(A): every circuit identified by its purpose, legibly, with permanent marking. "Misc" and "lights" are not legal descriptions. Include the feeder source on the panel directory: which main panel breaker and where that panel is located, per 408.4(B). This is the single most skipped requirement on the inspector's checklist.

1. Verify torque values on main lugs, ground bar, and neutral bar.
2. Confirm neutral-to-ground bond is removed.
3. Label every breaker with specific room and load.
4. Add source panel reference at the top of the directory.
5. Install the dead front, close the door, and photograph the finished panel for your records.

"I photograph every panel I close up. When a homeowner calls six months later saying something is wrong, I have the original terminations, labels, and torque marks on file. Has saved me three callbacks this year already."

Final inspection checklist

Walk the job like the inspector will. Working clearances, neutral isolation, EGC sizing, breaker compatibility, GFCI and AFCI coverage, torque, and labeling. Miss one and you are scheduling a re-inspection and burning a half day.

The jobs that pass first time are the ones where the electrician did the code lookup before the install, not after. Keep the NEC, the panel instructions, and the AHJ amendments in reach. A subpanel is not complicated work, but it is work the code treats seriously, and so should you.