Field guide: installing a subpanel, common mistakes (edition 5)

Separate the neutrals and grounds, every time

The single most common subpanel failure on inspection is a bonded neutral bar in a downstream panel. NEC 250.24(A)(5) is clear: the grounded conductor (neutral) is bonded to ground only at the service disconnect. Downstream of that, neutrals and equipment grounding conductors must be kept separate.

That means pulling the bonding screw or strap from the subpanel before energizing. Add an isolated equipment grounding bar if the panel did not ship with one. Land neutrals on the floating neutral bar, grounds on the EGC bar, and never double up a neutral and a ground under the same lug.

If the subpanel's main breaker is back-fed and the neutral is still bonded, you have created a parallel path on every metal raceway between the two panels. You will read current on the EGC with a clamp meter. That is your tell.

Pull four wires, not three

Three-wire feeders to a separate structure were allowed under older code, but 250.32(B) has required four conductors (two hots, a neutral, and an equipment grounding conductor) for any new feeder since the 2008 cycle. There is no grandfathering on new work. If you are extending an existing three-wire feeder, you are creating new work, and the four-wire rule applies.

Size the EGC per 250.122 based on the upstream overcurrent device, not the feeder ampacity. A 100 A feeder on #3 copper still only needs a #8 copper EGC. Size the neutral for the actual unbalanced load per 220.61, but never smaller than the required EGC.

• Two ungrounded conductors, sized for the load and voltage drop
• One grounded conductor (neutral), sized per 220.61
• One equipment grounding conductor, sized per 250.122
• Conduit or cable assembly listed for the install method, not the EGC

Grounding electrodes at a separate structure

If the subpanel feeds a detached structure (garage, barn, shop), 250.32(A) requires a grounding electrode system at that structure. A single ground rod is not enough on its own unless you can prove 25 ohms or less to earth, which almost no one measures. Drive two rods, six feet apart minimum, and you are done with the resistance test requirement per 250.53(A)(2).

Bond those rods to the EGC bar in the subpanel with a #6 copper grounding electrode conductor (250.66(A)). Do not bond to the neutral bar. The neutral stays isolated at the detached structure exactly the same as a subpanel inside the same building.

Breaker sizing, panel rating, and the 6-throw rule

The feeder breaker at the main panel protects the feeder conductors and the subpanel busbar. The subpanel's bus rating must equal or exceed the feeder breaker rating. Dropping a 100 A feeder into a panel with an 80 A bus is a code violation and a fire waiting on a fault.

If the subpanel does not have a main breaker, you are relying on the upstream feeder breaker as the disconnect. That is fine for a panel inside the same structure, but at a separate building, 225.31 and 225.33 require a disconnect at the structure, with no more than six switches or breakers to kill all power. A main breaker subpanel solves this in one move.

If your AHJ is on the 2020 or newer NEC, check 225.36 and 230.85 for emergency disconnect requirements at dwellings. The detached garage subpanel may need to be service-rated, not just a main breaker subpanel.

Working space, conduit fill, and the boring stuff that fails inspection

110.26(A) gives you 36 inches of clear depth in front of the panel, 30 inches of width or the panel width (whichever is greater), and 6.5 feet of headroom. Inspectors measure this. Do not mount a subpanel behind a water heater, above a workbench you cannot move, or in a closet (240.24(D) and (E) bar panels in clothes closets and bathrooms in dwellings).

Conduit fill kills more rough-ins than wire size. Three current-carrying conductors plus an EGC in 3/4 inch EMT is fine for #6 THHN. Add a fourth current-carrying conductor (a multiwire branch circuit feeding back) and you are derating per 310.15(C)(1). Check Chapter 9 Table 1 for fill and Annex C for quick lookups.

1. Verify panel location meets 110.26 working clearance
2. Confirm bus rating equals or exceeds feeder OCPD
3. Pull bonding screw, install isolated ground bar
4. Land neutrals and grounds on separate bars
5. Drive and bond electrodes if at a separate structure
6. Torque every lug to the label spec, then mark it

Torque marks and the final walk

110.14(D) requires terminations to be torqued to the manufacturer's spec, and most AHJs now want to see a calibrated torque tool on site. Mark every lug with a paint pen after torquing. The inspector sees the mark, you have a record, and the next electrician knows what was checked.

Before you energize, megger the feeder if the run is long or the conduit was wet during the pull. Verify phase rotation if you are feeding three-phase equipment downstream. Then close the main, close the subpanel main, and clamp the EGC. Any reading above noise floor means a neutral-ground bond you missed.