Field guide: installing a subpanel, code-compliant approach (edition 1)

Plan the load and the location first

Subpanel work fails at the planning stage, not the wire-pulling stage. Before you touch a knockout, calculate the load per NEC 220, confirm the feeder ampacity, and verify the main panel has space and bus capacity for the feeder breaker. A 100A subpanel feeding a detached garage is a different animal than a 60A subpanel for a basement remodel, and the conductor sizing, grounding scheme, and disconnect requirements all shift with it.

Location matters as much as load. NEC 110.26 working space is not negotiable: 36 inches deep, 30 inches wide, 6.5 feet of headroom, with the panel never installed in clothes closets (240.24(D)) or bathrooms in dwelling units (240.24(E)). Walk the path the feeder will take before you commit. A subpanel that lands two feet from a finished wall but requires three 90s and a fire-rated penetration is not the win it looks like on paper.

Confirm whether the structure is served by one branch circuit or qualifies as a separate building under 225.30. That single question determines whether you need a disconnect at the second structure, which changes your material list before you leave the truck.

Size the feeder, breaker, and conductors correctly

Feeder sizing follows NEC 215.2 and the ampacity tables in 310.16. For a typical 100A subpanel run in conduit at 75°C terminations, you are looking at 3 AWG copper or 1 AWG aluminum for the ungrounded and grounded conductors. The equipment grounding conductor is sized from Table 250.122, not the same as the feeder, which trips up apprentices constantly. A 100A feeder gets an 8 AWG copper EGC, not a 3 AWG.

Voltage drop is a design consideration, not a code requirement under 215.2(A)(1) Informational Note No. 2, but the 3% recommendation matters on long runs to detached structures. At 100 feet plus, run the calculation. Upsizing from 3 AWG to 2 AWG on a long garage feeder is cheap insurance against nuisance issues with motor loads later.

Tip from the field: if you upsize the ungrounded conductors for voltage drop, NEC 250.122(B) requires you to upsize the EGC proportionally. Skip this and your inspector will catch it.

Grounding and bonding, the part everyone gets wrong

This is where most subpanel installations fail inspection. In a subpanel, the grounded (neutral) conductor and the equipment grounding conductor must be kept separate. The neutral bar is isolated from the enclosure, and the bonding screw or strap that ships installed in most panels gets removed. NEC 250.24(A)(5) and 408.40 are explicit: bonding the neutral to the case at the subpanel creates a parallel path on the EGC, which is a code violation and a real shock hazard.

The four-wire feeder rule applies anywhere you are feeding a separate panel: two ungrounded, one grounded, one equipment grounding. The 2008 NEC closed the old exception that allowed a three-wire feeder to a detached structure, so do not rely on what your foreman did in 2005. Pull a separate EGC every time.

For a detached structure, you also need a grounding electrode system at the second building per NEC 250.32. Ground rods, a concrete-encased electrode if you can get to the rebar, or a metal underground water pipe all qualify. The EGC from the feeder bonds to that electrode system at the subpanel, but the neutral stays floating.

Conduit, conductors, and the physical install

Pick the wiring method to match the environment. PVC for direct burial runs (NEC 300.5 gives you the 18-inch minimum cover for residential branch circuits and 24 inches for feeders without GFCI), EMT for interior exposed work, MC or NM where the cable assembly is permitted by 334.10. Box fill (314.16) and conduit fill (Chapter 9, Table 1) are not suggestions.

1. Mount the panel plumb and at a height where the highest breaker handle is no more than 6 feet 7 inches above the floor (NEC 240.24(A)).
2. Land the feeder with proper torque per the manufacturer's listing, NEC 110.14(D) now requires a calibrated torque tool.
3. Separate the neutral and ground bars, remove the main bonding jumper.
4. Install the feeder breaker in the main panel, verifying it does not exceed the panel's bus rating.
5. Label the disconnect at the source per 408.4(A) and the subpanel itself per 408.4(B).

Anti-oxidant on aluminum terminations is not required by code on listed AL9CU lugs, but it is still good practice and most inspectors expect to see it. Document everything with photos before you close up the wall.

Final checks before you energize

Before flipping the feeder breaker, verify the neutral-to-ground separation with a meter. You should read open between the neutral bar and the enclosure with the main off. If you read continuity, the bonding screw is still in or a neutral has been landed on the ground bar. Find it before you energize.

Check your AFCI and GFCI requirements per NEC 210.8 and 210.12 for any branch circuits originating in the new subpanel. A subpanel in a finished basement feeding bedroom circuits still needs combination AFCI protection, and the rules apply at the breaker, not at the main panel upstream.

Tip from the field: take a clamp meter reading on the EGC after you energize the loads. Any current flowing on the ground means a neutral is misconnected somewhere downstream. Fix it before you sign the job.

Walk the inspector through the four-wire feeder, the isolated neutral, the grounding electrode at the second structure, and your torque documentation. A clean install with the paperwork ready passes the first time, and that is the only metric that matters when you are billing the job.