Field guide: installing a subpanel, common mistakes (edition 2)

Neutrals and grounds: separate them or fail inspection

The most common subpanel failure on a rough-in is a bonded neutral bar. In a subpanel, the neutral bus and the equipment grounding bus must be isolated. Only the service disconnect gets the main bonding jumper. NEC 250.24(A)(5) prohibits any connection between the grounded conductor and equipment ground on the load side of the service disconnect.

Pull the green bonding screw out of a new load center before you set it. Most manufacturers ship that screw installed or bagged to the deadfront. If it ends up threaded into the neutral bar at a subpanel, every parallel path through the EGC becomes a current carrying conductor.

If you feel warmth on a ground wire at the subpanel under load, stop. You have a neutral to ground bond somewhere downstream. Meg it out before you energize anything else.

Four wires, always, for a separate structure or interior feed

Since the 2008 NEC cycle, the three wire feeder to a detached structure is gone. You need two ungrounded conductors, an insulated neutral, and an equipment grounding conductor. NEC 250.32(B)(1) is explicit: run an EGC with the feeder and keep the neutral floating at the remote panel.

For interior subpanels fed from the service, the same rule applies under 215.6 and 250.134. The EGC size follows Table 250.122 based on the overcurrent device protecting the feeder, not the ampacity of the ungrounded conductors. A 100 amp feeder gets a #8 copper EGC, not a #6 because the phase conductors happen to be #3.

• Two hots, sized to the load and the breaker
• Insulated neutral, full size unless calculated per 220.61
• EGC per 250.122, separate from the neutral
• No reliance on conduit alone if you are running PVC or flex without a listed grounding path

Feeder sizing and the 83 percent rule

Residential feeders to a dwelling subpanel that carries the entire load of a dwelling unit can use 310.12, the 83 percent rule. A 100 amp subpanel feeding a whole house off a 200 amp service can be fed with #4 copper or #2 aluminum. That allowance does not apply to a subpanel in a detached garage or a workshop that is not the entire dwelling load.

For everything else, use 310.16 at the 75 degree column for terminations on panels rated 100 amps or less, and verify the terminal rating stamped inside the can. Do not size off the 90 degree column because the THHN jacket says 90. Terminals are the limiting factor.

Voltage drop bites on long runs to outbuildings. Three percent on a feeder, five percent total including branch circuits, is the informational note in 210.19 and 215.2. It is not enforceable, but a tripping well pump at 240 feet is your problem to diagnose at 11pm.

Overcurrent protection at both ends, or just one?

A feeder to a subpanel needs overcurrent protection at its supply end, sized to the conductor ampacity per 240.4 and 240.21. The subpanel itself does not need a main breaker unless it is a service disconnect, a disconnect for a separate structure under 225.31, or the panel has more than six disconnects.

For a detached structure, 225.31 and 225.33 require a disconnect at the structure, grouped, with no more than six throws. A main breaker panel at the outbuilding satisfies this. A main lug panel at a detached garage fed from the house does not, unless you add a fused disconnect or breaker ahead of it.

If the subpanel is in the same building as the service and has six or fewer branch breakers, a main lug panel is legal. But a $40 main breaker saves the next electrician an hour of head scratching. Spec the main.

Grounding electrodes at a separate structure

A detached building supplied by a feeder needs its own grounding electrode system per 250.32(A). That means a ground rod, or two rods eight feet apart if a single rod cannot meet 25 ohms and you do not want to test. Bond the EGC of the feeder to that electrode system at the subpanel, and keep the neutral isolated.

Do not drive a rod and run a #6 back to the house panel thinking you are helping. Parallel paths on the grounded conductor are exactly what 250.6 tells you to eliminate. The rod at the outbuilding bonds to the EGC bar, full stop.

• Minimum #6 copper to the ground rod per 250.66(A)
• Acorn clamp listed for direct burial
• If a metal water line or structural steel is present at the remote building, bond it too per 250.104

Working space and the stuff inspectors actually check

30 inches wide, 36 inches deep, 6.5 feet high. NEC 110.26. Inspectors will measure. A subpanel behind a water heater, tucked in a closet next to a clothes rod, or crammed above a workbench will red tag faster than any wiring error.

Label the feeder breaker at the source panel with the subpanel location. Label the subpanel with the source. 408.4 and 110.22 both require it. Use a legible permanent label, not a Sharpie scrawl that will smear in a year.

Finally, torque every lug. A calibrated driver, to the value printed inside the panel door. Loose neutrals at a subpanel are the number one cause of intermittent problems the homeowner will call you back for six months later, and 110.14(D) has made it a code requirement since 2017.