Field guide: installing a subpanel, code citations (edition 1)

Before you pull permits

A subpanel is a feeder-fed distribution point. That framing drives every decision: conductor sizing under 215, OCPD selection under 240, grounding and bonding under 250, and working space under 110.26. If the feeder origin is on a separate building or structure, 225.30 through 225.40 also apply.

Load calc first. Run Article 220 Part III or IV for the new panel and verify the existing service still has headroom after the feeder is added. Document the existing service amp rating, the main OCPD, and the calculated load before and after. If the upstream service is marginal, a subpanel will not fix it, and tapping a 200A service that is already running 175A calculated is how callbacks start.

If the homeowner says "just add a subpanel for the shop," pull the meter and check the service conductors before you quote. A 100A service cannot feed a 60A subpanel pulling real load, no matter what the sparky before you told them.

Sizing the feeder

Feeder conductors are sized per 215.2 using the ampacity tables in 310.16, with terminal temperature ratings per 110.14(C). Most residential and light commercial breakers land on 75C terminals, so size off the 75C column unless the equipment label says otherwise. Do not size off the 90C column for the final ampacity, even if the conductor insulation is THHN/THWN-2.

Common residential feeders:

• 60A subpanel: #6 Cu THWN-2 (65A at 75C) or #4 Al
• 100A subpanel: #3 Cu or #1 Al (per 310.16, 75C)
• 125A subpanel: #1 Cu or 1/0 Al
• 200A subpanel: 3/0 Cu or 250 kcmil Al

For dwelling services and feeders that carry the full load of the dwelling, 310.12 allows reduced conductor sizes. That rule does not apply to a subpanel fed from a main panel unless the subpanel carries the entire dwelling load. Read the exception carefully before you downsize.

Grounding, bonding, and the four-wire rule

This is where most inspector corrections happen. A subpanel in the same structure as the service gets four conductors: two hots, a neutral, and an equipment grounding conductor. The neutral bus must be isolated from the enclosure. The EGC lands on a grounding bus bonded to the can. 250.24(A)(5) prohibits a neutral-to-ground connection on the load side of the service disconnect.

Pull the factory bonding screw or strap out of the subpanel. Every brand labels it differently, Square D uses a green screw, Siemens uses a bonding strap, Eaton BR uses a green screw with a tag. If it is still installed, the neutral is bonded to the case and you have parallel neutral current on the EGC and any metal raceway. That is a shock hazard and a code violation under 250.6.

EGC sizing is per 250.122 and Table 250.122, based on the upstream OCPD, not the feeder ampacity. A 100A feeder needs a #8 Cu EGC. Upsized feeders for voltage drop require a proportionally upsized EGC per 250.122(B).

Separate structure feeders

Feeding a detached garage, barn, or shop is its own animal. 225.30 limits you to one feeder per structure with limited exceptions. A disconnect is required at the separate structure per 225.31, readily accessible and grouped per 225.32 and 225.33.

Grounding electrodes are required at the separate structure per 250.32(A). Drive two ground rods 6 feet apart, or use a Ufer if available, and bond to the EGC bus in the subpanel. Do not re-bond the neutral at the separate structure unless you are running a three-wire feeder under the limited conditions of 250.32(B) Exception, which applies only to existing installations with no parallel metallic paths. New work is always four-wire with isolated neutral.

If there is a metal water line, gas line, or any communication cable running between the buildings, you cannot use the three-wire exception. Run the EGC. Every time.

Working space and physical install

110.26(A) governs working space: 36 inches of depth, 30 inches of width or the width of the equipment if larger, and 6.5 feet of headroom. The space must be clear and dedicated per 110.26(E). Do not mount a subpanel behind a water heater, in a clothes closet (240.24(D)), or in a bathroom (240.24(E) for dwellings).

Mounting height: the operating handle of the highest breaker cannot exceed 6 feet 7 inches per 240.24(A). Secure the can to structural framing, not just drywall. In garages and unfinished basements, protect feeder cable from physical damage per 300.4 if run across the face of framing members below 8 feet.

Torque every lug. Use a calibrated torque screwdriver and follow the label inside the panel, usually printed on the deadfront. 110.14(D) requires torque values be applied per manufacturer instructions, and inspectors are increasingly asking to see the torque tool on site.

Before you close it up

Verify the install against this checklist before the deadfront goes back on:

1. Bonding screw removed, neutral bus isolated from can
2. EGC sized per Table 250.122 based on upstream OCPD
3. Neutrals and grounds on separate buses, one conductor per terminal unless listed otherwise
4. Feeder OCPD at the source matches conductor ampacity after all adjustments
5. All AFCI and GFCI requirements per 210.8 and 210.12 carried through to the subpanel branch circuits
6. Panel directory filled out per 408.4(A), not "lights" and "plugs"
7. Torque values applied and documented

Label the feeder disconnect at the source panel identifying what it feeds per 408.4(B). If the subpanel is in a separate structure, label the disconnect there as the service disconnect for that building. Close up, megger if the job calls for it, energize, and verify voltage phase to phase and phase to neutral before loading any circuits.