Field guide: installing a subpanel, troubleshooting (edition 4)

Subpanel troubleshooting starts before you pull a single cover. Most callbacks on subpanels trace to four root causes: bonded neutrals on the load side, undersized feeders, missed GEC requirements on detached structures, and breakers that don't match the bus. Run these checks in order and you'll solve 90% of what comes in.

Verify the neutral and ground separation first

Pop the cover and look at the neutral bar. If the green bonding screw is in, or a bonding strap ties neutral to the enclosure, you've found your problem on a subpanel fed from another structure's service. NEC 250.24(A)(5) prohibits a neutral-to-case bond on the load side of the service disconnect. The only exception is an existing installation to a separate building under the old 250.32(B)(2), which was removed in the 2008 NEC and no longer applies to new work.

Pull the bonding screw. Separate the equipment grounding conductors onto a dedicated ground bar that's bonded to the can. Neutrals land on an isolated bar. If you're seeing nuisance GFCI trips, humming on metal parts, or stray voltage on gas lines, a bonded subpanel neutral is the first suspect.

If the neutral bar has a green screw AND a bare copper bonding strap, pull both. I've seen panels where someone removed the screw but left the strap riveted to the can. Still bonded.

Check the feeder sizing against actual load

A 100A subpanel fed with #4 copper or #2 aluminum is common on paper but gets sketchy when the run is long or the load is real. Per NEC 215.2(A)(1), feeder conductors must have ampacity not less than the non-continuous load plus 125% of the continuous load. Voltage drop isn't a code requirement in 215.2(A)(1) Informational Note No. 2, but 3% on feeders is the working target.

Measure the run. For a 100A subpanel 80 feet out in a detached shop, #4 CU is on the edge. At 150 feet you're into #2 CU or #1/0 AL territory. Check the terminal temperature rating on both ends, most residential breakers and lugs are 75C, so use the 75C column in Table 310.16.

• 100A feeder, under 50 ft: #4 CU or #2 AL typical
• 100A feeder, 50-100 ft: verify VD, consider #3 CU or #1 AL
• 100A feeder, 100+ ft: #2 CU or #1/0 AL
• Always confirm terminal temp rating before final sizing

Detached structures need a grounding electrode

This is the most missed requirement on rural jobs. NEC 250.32(A) requires a grounding electrode system at any separate building or structure supplied by a feeder, with limited exceptions for single-branch-circuit feeds. That means ground rods, Ufer, or both at the detached shop, garage, or barn, even if the main service 40 feet away already has a full GES.

The equipment grounding conductor runs with the feeder per 250.32(B)(1). The new local electrode bonds to the subpanel ground bar, not the neutral. This is where the old bonded-neutral method used to live, and it's where most legacy installations go wrong.

Two 8-foot rods, 6 feet apart, #6 copper to the subpanel ground bar. Drive them before you set the panel, not after. Saves trenching twice.

Breaker and bus compatibility

Mixing brands is a liability and often a code violation under NEC 110.3(B), which requires equipment to be installed per listing. A Square D QO breaker in a Homeline panel, or a Siemens in a Cutler-Hammer, may physically snap in but isn't classified for that bus. UL classified replacement breakers are the exception, and they're marked specifically for cross-use.

Check the panel label. It lists acceptable breaker catalog numbers. If you're troubleshooting a panel that trips under load or shows heat discoloration at the bus stab, pull the breaker and inspect the stab. Arcing damage means breaker and possibly bus replacement. Don't polish and reuse a pitted stab.

1. Read the panel label for listed breakers
2. Torque lugs to spec, typically printed on the can
3. Inspect stabs for discoloration or arc marks
4. Replace, don't repair, damaged bus sections

Troubleshooting live symptoms

Warm panel cover, tingling on metal conduit, or intermittent breaker trips usually point to one of three things: a loose neutral at the feeder lug, a shared neutral serving an MWBC that lost its handle tie, or the bonding issue from section one. Pull the dead-front, thermal-scan if you have it, and retorque every feeder and branch termination to the label spec. Infrared won't lie about a loose connection.

For MWBCs in a subpanel per NEC 210.4(B), all ungrounded conductors must be disconnected simultaneously. If someone swapped a 2-pole for two singles, you've got a shock hazard on the neutral when one leg is off and the other is energized. Handle ties are the minimum, common-trip 2-pole breakers are better practice.

Final walk before you close it up

Before the dead-front goes back on, verify: neutral bar isolated, ground bar bonded, EGC landed on ground bar, GEC to local electrode at detached structures, feeder conductors torqued, breakers match the panel listing, and every branch circuit labeled. Take a photo of the interior for your records, it pays off on the next service call.

Test with the meter, not the nose. Neutral to ground should read near zero volts under load. Anything over 2V on a 120V circuit points back to a bonding or feeder issue, and you start over at section one.