What is a subpanel?
Updated August 2026
A subpanel is a second breaker box fed by a single large circuit from the main panel, used to add breaker spaces or to put circuits closer to where the load lives. It looks like a small main panel, usually 6–12 spaces, and it is fed by a four-wire feeder: two hots, a neutral, and a separate ground. Feeder size follows the breaker protecting it, so a 60-amp subpanel typically runs on 6-gauge copper and a 100-amp on 3-gauge copper or 1-gauge aluminum.
Where it is
Typical homes for one: a detached garage or workshop, a finished basement, a room addition, an accessory dwelling unit, or a kitchen remodel that suddenly needs six new circuits when the main panel has two spaces left. Placement follows the working-space rule in NEC 110.26, which wants 30 inches of width, 36 inches of clear depth in front, and 6 feet 6 inches of headroom. Bathrooms and clothes closets are off limits. In a detached building the subpanel also needs its own grounding electrode, usually two 8-foot rods driven 6 feet apart.
What it does
Everything a subpanel does electrically is ordinary. The part that goes wrong is bonding. In the main panel, the neutral and ground are deliberately joined at exactly one point in the system. In a subpanel they must stay apart: neutrals land on an isolated bar floating on plastic standoffs, grounds land on a bar bonded to the enclosure, and the green bonding screw that ships with the box gets removed and left out. Tie them together and normal return current starts flowing on ground wires and on any metal it can reach, which can be conduit, a water line, or a gas pipe. The breakers still work, the lights still light, and nothing looks wrong, which is why this is the single most common defect inspectors write up on subpanel work. The other frequent finding is a feeder breaker sized larger than the feeder wire, a mistake that removes the wire protection entirely.
Maintenance
Treat it like the main box. Look it over annually for rust at the bottom, condensation at the top knockouts, insects, and a directory that still matches reality, and pay extra attention in a detached garage or shop where humidity swings. Feel the cover for warm spots and listen for buzzing. Have an electrician check feeder lug torque every 5–10 years and confirm the neutral and ground bars are still separated after any remodel. Both boxes age out around 25–40 years. The full routine is on the panel maintenance checklist.
What it costs to replace
A replacement subpanel runs $500–1,500 installed for a typical 60–100 amp subpanel, or $1,500–3,500 with a trench and feeder to a detached garage.
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Frequently asked questions
Does a subpanel add electrical capacity to my house?
No, and this is the misunderstanding that costs people money. A subpanel adds breaker spaces, not amps. If the house has 100-amp service and the load calculation is already tight, hanging a subpanel off it changes nothing about the ceiling; you have simply given yourself more places to exceed it. Adding real capacity means a service upgrade at $2,000–4,000. A subpanel at $500–1,500 solves a full-panel problem, not an undersized-service problem.
Why must neutrals and grounds be separated in a subpanel?
Because the system needs exactly one neutral-to-ground bond, at the service. If a subpanel adds a second bond, return current splits between the neutral conductor and every parallel metal path available, which puts current on grounding wires, conduit, and sometimes water piping. That defeats the low-impedance fault path a breaker relies on to trip fast, and it can energize metal surfaces. The fix is cheap during installation and annoying afterward: separate bars, and the bonding screw removed.
Do I need a permit for a subpanel?
Yes, in essentially every jurisdiction, and the inspection is worth having. Permit fees typically run $75–250 and the inspector checks the items that matter most: feeder size against breaker size, bar separation, grounding at a detached structure, and working clearance. Unpermitted panel work also tends to surface at resale, where a buyer's inspector flags it and the correction happens on your timeline instead of theirs. Have the electrician pull it as part of the quote.