Field guide: installing a subpanel, residential version (edition 3)

Plan the load and the location before you touch a wire

A residential subpanel lives or dies on two decisions made before the truck gets loaded: calculated load and physical placement. Run the load calc per NEC Article 220, Part III. Detached structures follow 225.39 for the disconnect rating. If the subpanel feeds a workshop with a welder, a mini-split, and an EVSE, that 60A feeder you had in mind is probably undersized.

Location matters as much as ampacity. Working clearance is 30 inches wide, 36 inches deep, and 6.5 feet high per 110.26(A). No panels in bathrooms (240.24(E)), no panels in clothes closets (240.24(D)), and keep it out of the path of stored junk the homeowner will absolutely pile in front of it within six months.

• Confirm main panel has breaker space and bus ampacity headroom.
• Measure the feeder run. Voltage drop over 3% on a branch or 5% total per 210.19 informational note means upsizing conductors.
• Verify the grounding electrode situation at the destination, especially for detached structures.

Size the feeder and the OCPD correctly

Feeder conductors are sized per 215.2 and Table 310.16, adjusted for ambient temp and conduit fill. A 100A subpanel in a typical residential install lands on 3 AWG copper THHN or 1 AWG aluminum at the 75°C column. Don't mix and match termination temperature ratings. If the breaker is listed 60/75°C, you're using the 75°C column, not 90°C, even with THHN.

The feeder breaker in the main panel is the overcurrent device for the subpanel. Size it to protect the feeder conductors, not the subpanel bus rating. A 125A subpanel fed with a 100A breaker on 3 AWG copper is perfectly legal and common. The panel rating is a maximum, not a requirement.

If the AHJ is picky about aluminum terminations, bring the antioxidant compound and torque the lugs with a calibrated wrench. "Tight enough" has failed more inspections than wrong wire size.

Four wires, always, for separate structures and indoor subs alike

Since the 2008 NEC, detached buildings require a 4-wire feeder with the grounded and grounding conductors kept separate at the subpanel. 250.32(B) killed the old 3-wire exception for existing installations with no other metallic paths. Run two hots, a neutral, and an equipment grounding conductor. Period.

At the subpanel: remove the main bonding jumper. The neutral bar floats. The ground bar bonds to the enclosure. If the panel shipped with the neutral bonded to the can via a green screw or strap, pull it. This is the single most common subpanel violation in the field, and it creates parallel neutral paths that energize metal parts during a fault.

1. Land both hots on the main lugs or backfed breaker.
2. Land the neutral on the isolated neutral bar.
3. Land the EGC on the ground bar, which is bonded to the enclosure.
4. Verify the bonding screw or strap is removed and set aside in the panel (some inspectors want to see it).

Grounding electrodes at detached structures

A detached building with a subpanel needs its own grounding electrode system per 250.32(A). One ground rod is rarely enough. 250.53(A)(2) requires supplementing a single rod unless you can prove 25 ohms or less resistance to earth, which almost nobody tests. Drive two rods, 6 feet apart minimum, bond them with 6 AWG copper, and move on.

If the detached structure has a metal underground water line, a concrete-encased electrode (Ufer), or a building steel frame in contact with earth, those count and are often superior. The EGC from the main panel does not replace the local grounding electrode system. It supplements it.

Breaker selection, AFCI, and GFCI coverage

Branch circuits in the subpanel follow the same 210.8 and 210.12 rules as anywhere else. A subpanel in a finished basement feeding bedroom circuits still needs AFCI protection per 210.12(A). Kitchen, bath, laundry, garage, outdoor, and unfinished basement receptacles need GFCI per 210.8(A). The subpanel's location does not exempt any of this.

Match breaker brand to panel. Classified breakers exist, but most inspectors and manufacturers' listings prefer native breakers. A Square D QO panel gets QO breakers. Don't fight this on a residential job.

Label every breaker before you close the dead front. Coming back in three months to troubleshoot a "miscellaneous" circuit is its own punishment.

Inspection-ready finish work

Torque every lug and termination to the manufacturer's spec. 110.14(D) made this mandatory, not optional, and inspectors are checking. Use a torque screwdriver, not your wrist's memory. Document it if the AHJ wants a record.

Fill unused openings per 408.7. Install a directory per 408.4(A) with circuit descriptions specific enough to be useful ("kitchen SABC #1" beats "kitchen"). Verify working clearance is clear, the dead front closes flush, and the panel is labeled with the feeder source and available fault current per 110.24 where applicable.

• Torque spec documented and met.
• Neutral and ground separated, bonding screw removed.
• Grounding electrode system installed and bonded for detached structures.
• Directory complete and legible.
• AFCI and GFCI coverage verified per 210.8 and 210.12.