Field guide: installing a subpanel, common mistakes (edition 3)

Neutral and ground separation

The single most common subpanel failure on rough inspection: bonded neutral at the sub. The main bonding jumper belongs at the service disconnect, one time, one place. NEC 250.24(A)(5) prohibits a neutral-to-ground connection on the load side of the service.

At the subpanel, pull the green bonding screw. Remove any bonding strap between the neutral bar and the can. Install a separate equipment ground bar, bolted to the enclosure, and land all EGCs there. Neutrals land on the isolated neutral bar, floated from the can per 408.40.

If the sub is in a detached structure, same rule applies under the 2008 and later cycles. 250.32(B) eliminated the old re-grounded neutral allowance. Run four wires, keep neutrals and grounds separate, drive a ground rod (or two) at the detached building and bond the EGC to it.

Before you energize, ohm from the neutral bar to the can with the main off and the feeder neutral lifted. Any continuity means you missed a bond somewhere. Find it.

Feeder sizing and the 83% rule

Dwelling service and feeder conductors get the 310.12 treatment, but only for the main power feeder supplying the entire dwelling. A subpanel feeder inside the dwelling does not automatically qualify. Read 310.12(B) carefully: it applies to a feeder that carries the total load of the dwelling.

If the sub carries the whole load (common in a meter-main to interior-panel setup), 83% applies. If it's a sub off a main that still has branch circuits, size the feeder at 100% of calculated load per Article 220, then pick conductors from 310.16 at the correct termination temperature.

• 100A sub carrying whole dwelling load: #4 Cu or #2 Al per 310.12(B).
• 100A sub as a secondary panel off a loaded main: #3 Cu or #1 Al per 310.16 at 75C.
• Always verify breaker and lug termination ratings, 110.14(C). Most residential gear is 75C.

Grounding electrode at detached structures

Anyone running a sub to a detached garage, shop, or barn: 250.32 requires a grounding electrode system at the second building. One ground rod is not enough unless you can prove 25 ohms or less to earth, which nobody tests. Drive two rods, six feet apart, and call it done per 250.53(A)(2).

Bond the grounding electrode conductor to the equipment ground bar in the subpanel, sized from 250.66. For a 100A feeder with #4 Cu EGC, a #6 Cu GEC to the rods is code minimum. Irreversible crimp or listed acorn clamp, no wire nuts to a rod.

If there's metal water piping, a concrete-encased electrode, or structural steel at the detached building, those get bonded too per 250.50. The rule is every qualifying electrode present, not pick one.

Conductor fill, wire management, and the 6-throw rule

Subpanels tend to get crowded fast. Watch box fill at the feeder splice point if there is one, 314.16. Inside the panel itself, 312.8 governs through-wiring and splices. You can splice and feed-through in a panel enclosure, but total conductor cross-section cannot exceed 75% of the wiring space at any cross-section, and splices plus taps cannot exceed 40%.

Keep neutrals and hots from the same circuit together to avoid induced heating in metal enclosures, 300.3(B) and 300.20. Multi-wire branch circuits need a handle tie or 2-pole breaker at the sub, 210.4(B), and the neutrals must be pigtailed at the device, 300.13(B).

If you can't close the dead-front without forcing the wires, you don't have a finished panel. Pull conductors out, dress them to the sides, and land them again. Ten extra minutes now beats a callback for a tripped AFCI that's really just a pinched neutral.

Breaker compatibility and series ratings

Not every breaker fits every panel. UL listing and 110.3(B) are not suggestions. A classified breaker that "fits" the bus is not the same as a breaker the panel manufacturer lists on the label. Inspectors are catching this more often, especially in resale inspections.

When a sub is fed from a main with higher available fault current than the sub is rated for, check the series combination rating on the main panel label. If the sub is 10kAIC and the available fault current exceeds that, you need a series-rated combination documented on the equipment, 240.86 and 110.24.

• AFCI and GFCI requirements follow the circuit, not the panel, 210.8 and 210.12.
• Two-pole common trip for MWBCs, not handle ties, where 210.4(B) requires simultaneous disconnection of ungrounded conductors supplying a single yoke.
• Confirm the panel label lists the exact breaker catalog number before you pop it on the bus.

Disconnect, working space, and labeling

A subpanel inside the same structure as the service does not need its own disconnect ahead of it, but a sub at a detached structure does, 225.31 and 225.32. That disconnect must be at a readily accessible location nearest the point of entry of the feeder.

Working space is not negotiable. 110.26(A) wants 36 inches of depth, 30 inches of width or the width of the equipment (whichever is greater), and 6.5 feet of headroom. No storage, no shelves, no water heater sitting in front of the panel. Dedicated equipment space above the panel per 110.26(E) runs to the structural ceiling or six feet, whichever is lower.

Label the sub with its source per 408.4(B). Label every circuit with a description specific enough to identify the load, 408.4(A). "Spare," "lights," and "outlets" do not cut it on a modern inspection.