Field guide: installing a subpanel, code citations (edition 5)

Plan the feeder before you cut drywall

Size the subpanel by calculated load, not by panel slot count. Run Article 220 against the actual circuits going in, then pick a feeder that lands you under 80% continuous per 215.3. A 100A subpanel on a 60A feeder is a callback waiting to happen when the homeowner adds a mini-split.

Feeder conductor ampacity per 215.2(A)(1) must carry the calculated load. For a 100A subpanel in a detached structure, 1 AWG copper or 1/0 aluminum at 75C terminals gets you there. Check terminal temperature ratings on both ends per 110.14(C), the lug rating is what governs, not the insulation.

Voltage drop is not a code requirement under 215.2(A)(1) Informational Note, but it is a quality marker. On runs over 100 feet, bump the conductor one size. Your inspector will not fail you for 3% drop, your customer will call you back for dim lights.

Grounding and bonding, the part everyone gets wrong

In a subpanel, neutrals and grounds are separated. Period. The bonding screw or strap comes out, and you install a separate equipment ground bar per 408.40. The neutral bar floats on its insulated standoffs. This is the single most common failure on rough inspection.

For a subpanel in the same structure, 250.32(B)(1) requires a 4-wire feeder: two hots, insulated neutral, and equipment grounding conductor. No grounding electrode at the subpanel unless it feeds a separate structure under 250.32(A).

If you see a 3-wire feeder to a subpanel in a 1995 detached garage, that was compliant then under the old 250.32 exception. The 2008 NEC killed it. On any alteration or added circuit, you are pulling a 4-wire and installing a local grounding electrode system.

Feeder routing, protection, and terminations

Romex through studs needs 1.25 inches of setback per 300.4(A)(1), or a 16-gauge steel nail plate. Through-bored holes in joists for horizontal runs follow 300.4(A)(2). If you are running SER cable in an interior wall, it is still cable, and 334.80 ampacity rules apply in thermal insulation: derate to the 60C column if bundled or wrapped.

For detached structures, bury depth under 300.5 depends on wiring method. Direct-buried UF at 24 inches, PVC conduit at 18 inches, rigid metal at 6 inches. GFCI protection upstream does not change the burial depth. Sleeve the riser in Schedule 80 or RMC where exposed to damage per 300.5(D)(4).

• Torque every lug to the manufacturer spec stamped on the panel, 110.14(D) made this enforceable in the 2017 cycle.
• Use anti-ox on aluminum feeder terminations, wire-brush the strands first.
• Re-identify the white conductor of a SER cable used as an ungrounded conductor per 200.7(C)(1) with black, red, or blue tape at every accessible point.

Overcurrent protection and panel selection

The feeder OCPD sits at the supply end, in the main panel. Size it per 240.4 to protect the feeder conductor ampacity. For a 100A subpanel fed with 1 AWG copper, a 100A breaker is correct, even though 1 AWG is good for 130A at 75C, because 240.4(B) lets you round up only up to 800A and only when there are no receptacles on the circuit, which does not apply to a panel feeder.

The subpanel itself needs a main breaker or a feed-through lug kit depending on application. Detached structure: main breaker required as the disconnect per 225.31 and 225.36, unless the existing disconnect meets those rules. Same structure: main lug is allowed, but a main breaker gives you a local shutoff that will save you on service calls.

Watch the bus rating. A 125A bus panel with a 100A main is fine. A 100A bus with 42 circuits of tandem breakers may exceed the listing, panels are listed for a specific number of poles per 408.54.

AFCI, GFCI, and the branch circuits landing in the subpanel

Branch circuit protection follows the occupancy, not the panel location. A subpanel in a detached garage feeding a finished living space still needs AFCI on the bedroom circuits per 210.12, and GFCI on receptacles per 210.8 based on location. Kitchen countertop in an in-law suite: GFCI. Garage receptacles: GFCI. Outdoor: GFCI.

If the subpanel is the first disconnect for a detached structure and feeds outdoor outlets, 210.8(F) requires GFCI protection for the outdoor branch circuits, and since the 2020 cycle, 210.8(B) covers all 125V through 250V receptacles up to 50A in commercial locations you may be touching.

Install dual-function AFCI/GFCI breakers at the subpanel for any circuit that previously had a GFCI receptacle at the point of use. Fewer parts in the wall, easier troubleshooting, and you stop the nuisance trips from long home-run capacitance.

Labeling, documentation, and the final walk

408.4(A) requires every circuit to be legibly identified as to purpose or use. Handwritten in pencil on a torn label sheet does not meet "legibly." Print the directory, laminate it, tape it to the inside of the door. 408.4(B) requires the source marking: "Fed from main panel, basement east wall."

Available fault current marking per 110.24 applies to service equipment, but if your subpanel is the service disconnect for a detached structure, it qualifies. Note the calculated fault current and the date. Inspectors are catching this one now.

• Verify neutral-ground separation with a meter before energizing, should read open.
• Confirm torque on every termination, including the neutral and ground bars.
• Test every GFCI and AFCI with the test button and a plug-in tester.
• Photograph the open panel for your records, label legibility counts on callbacks.