Field guide: installing a subpanel, residential version (edition 1)

Plan the load and the feeder before you open a box

Subpanel work starts at the kitchen table, not at the truck. Sum the continuous loads at 125%, non-continuous at 100%, and apply demand factors from NEC 220 Part III. If the sub feeds a detached structure, you are in 225 territory, not just 408. Size the feeder so voltage drop stays under 3% to the panel per the 210.19(A) informational note, especially on long runs to a garage or ADU.

Pull permits. Confirm AHJ amendments. Some jurisdictions still require a disconnect at the structure even when the sub is the only panel, and some quietly enforce the 2023 GFCI expansions for dwelling units under 210.8(F). Know which code cycle you are working under before you price the job.

• Main panel busbar rating and breaker space available
• Feeder length, conductor material, and conduit fill per Chapter 9 Table 1
• Grounding electrode system at the supplied structure (250.32)
• Available fault current and interrupting rating of the sub (110.9, 110.24)

Feeder conductors, OCPD, and the four-wire rule

For a residential subpanel, run four wires: two ungrounded, one grounded (neutral), one equipment grounding conductor. The 2008 NEC killed the old three-wire feeder to detached buildings, so 250.32(B) now requires an EGC in every case unless you are grandfathered and nothing has changed. If anything has changed, it is not grandfathered.

Size the OCPD at the main for the feeder ampacity per 240.4, then size conductors from 310.16 at the 75C column for terminations rated 75C, which is most modern gear. A 100A sub on copper typically wants #3 THHN with a #8 EGC; a 60A sub is usually #6 with a #10 EGC. Check the breaker and lug ratings on the actual panel you are installing, not the one you installed last year.

If the homeowner asks for "room to grow," sell them a 100A sub on #2 aluminum SER in 1.25 inch PVC. Cheaper than copper, code compliant under 338.10(B), and you will not be back in three years adding a mini split circuit they forgot to mention.

Bonding, grounding, and the neutral you must float

This is where most failed inspections happen. At a subpanel, the grounded (neutral) bus must be isolated from the enclosure. Remove the factory main bonding jumper, green screw, or bonding strap. The EGC lands on a separate equipment ground bar bonded to the can. Neutrals and grounds never touch downstream of the service disconnect per 250.24(A)(5) and 408.40.

For a detached structure, drive ground rods or use an existing electrode system per 250.50 and 250.52, and bond the EGC to that electrode system at the sub. You are not creating a parallel neutral path; you are giving the structure its own reference. If there is metal water piping or a CSST gas line at the structure, bond those too per 250.104.

1. Pull main bonding jumper from the subpanel
2. Install a separate ground bar, screw it to the enclosure
3. Land all EGCs and electrode conductors on the ground bar
4. Land all grounded conductors on the isolated neutral bar
5. Verify continuity from sub ground bar back to main panel ground bar

Mounting, working space, and conduit entries

Working space is not negotiable. 110.26(A) still wants 36 inches of depth, 30 inches of width or the width of the equipment (whichever is greater), and 6.5 feet of headroom. In a garage or basement with low framing, measure before you mount. An inspector will red tag a panel that is two inches too close to a furnace flue.

Use listed connectors for every entry. NM cable needs a listed NM connector, not a KO bushing with a bunch of Romex shoved through it. For SER going into a panel, use a proper SE connector sized to the cable. Strap within 12 inches of the panel and every 4.5 feet after that per 334.30 for NM, 338.10 referencing 334 for SE in the same conditions.

Circuit layout and the arc-fault/GFCI matrix

Load balancing matters more on a sub than on a service panel because the feeder is a single bottleneck. Alternate circuits between the two legs and keep heavy loads (dryer, range, EVSE) split across phases. Label every circuit at install. A sharpie on masking tape is not a label per 408.4(A); use a real directory.

For dwelling units, AFCI is required on almost every 120V 15 and 20A circuit in habitable rooms per 210.12(A). GFCI per 210.8 has expanded every cycle: kitchens, baths, laundry, garages, outdoors, basements, within 6 feet of a sink, and since 2020, dishwashers and ranges where the AHJ enforces it. When in doubt, install a dual-function breaker and move on.

Keep a mix of CAFCI and DFCI breakers on the truck for the big three: SquareD HOM/QO, Eaton BR/CH, Siemens QP. Driving back to the supply house costs you more than the 12 dollar premium on a dual-function.

Energize, test, and document

Before you flip the feeder breaker, megger the feeder conductors if the run is long or was pulled through wet conduit. Verify the neutral is isolated with an ohmmeter between neutral bar and ground bar, you want infinity. Confirm torque on every lug to the manufacturer spec per 110.14(D); a calibrated torque screwdriver is cheap insurance.

Energize, check voltage leg to leg and leg to neutral, then leg to ground. All three should match expected values within a volt or two. Test every GFCI and AFCI with the test button and a plug-in tester. Hand the homeowner the panel directory, the permit card, and your invoice. Next job.