Field guide: installing a subpanel, troubleshooting (edition 6)

Before you pull a single wire

Subpanel troubleshooting starts before the cover comes off. Ask the homeowner when the problem started, what was added recently, and whether anything was worked on at the main. Nine times out of ten, someone moved a neutral, forgot a bonding screw, or double-lugged a breaker in a panel that was never listed for it.

Kill the feeder at the main, verify dead with a known-good tester, then retest on a live circuit to confirm your meter. NEC 110.26 working space rules still apply to a subpanel in a closet or a finished basement ceiling. If you can't stand square in front of it with 30 inches of width, document it before you start pulling conductors.

Check the label. A lot of older subs are fed from a panel that was converted to a main and never relabeled. If the feeder overcurrent device and the subpanel bus rating don't agree, you've already found one problem.

The neutral and ground separation problem

This is the number one call back on a remodel subpanel. Per NEC 250.24(A)(5) and 408.40, the grounded (neutral) and equipment grounding conductors must be kept separate at any panel downstream of the service disconnect. The bonding screw or strap in the subpanel has to come out, and the neutral bar must float.

Symptoms of a bonded sub: current on the EGC, tingles on metal appliances, nuisance GFCI tripping on shared circuits, and a reading between neutral and ground that won't sit at zero under load. Clamp the EGC of the feeder. If you see more than a few tens of milliamps of steady current, the neutral is finding a parallel path through the grounds.

• Remove the green bonding screw or strap from the neutral bar.
• Install a separate grounding bar, bonded to the enclosure.
• Land every EGC on the ground bar, every grounded conductor on the neutral bar, one conductor per terminal unless the bar is listed otherwise.
• Verify the feeder has four conductors: two hots, a neutral, and an EGC. Three-wire feeders to separate structures are no longer permitted under 250.32(B).

If you find a bonded sub in a detached garage fed with three wires and no EGC, that's a 250.32(B) violation from the 2008 cycle forward. The fix is a fourth conductor back to the service, not adding a local ground rod and calling it good.

Tripping breakers and nuisance trips

Start with load, not the breaker. Clamp each leg at the feeder with everything the customer normally runs turned on. If a 100 amp sub is sitting at 85 on one leg and 40 on the other, you have a balance problem before you have a breaker problem. Move circuits between phases to even it out, and remember multiwire branch circuits have to stay on opposite legs per 210.4(B).

AFCI and GFCI nuisance trips in a subpanel almost always trace back to shared neutrals, a bonded neutral bar, or a neutral landed on the wrong breaker's pigtail. Check that every AFCI or GFCI breaker has its own dedicated neutral, landed on the breaker, not on the bus.

If a standard breaker trips cold with no load, don't just swap it. Meg the circuit, or at minimum ring it out with the breaker off and the neutral lifted. Insulation failure in a buried staple is a real thing and a new breaker won't fix it.

Heat, discoloration, and loose connections

Pop the dead front and look before you touch. Browning on a bus stab, a breaker that rocks when you press it, or a neutral lug with a halo of discoloration are all telling you the same story: a loose or failing connection has been cooking.

Torque matters. NEC 110.14(D) requires terminations to be torqued to the manufacturer's listed value. Get a calibrated screwdriver, look at the label inside the panel door, and hit every lug on the feeder and every breaker you're not 100 percent sure about. Aluminum feeders especially need a retorque and anti-oxidant compound per the listing.

1. De-energize and verify.
2. Thermal scan the panel under load before shutdown if you have the tool.
3. Retorque feeder lugs, neutral lug, and ground lug to spec.
4. Replace any breaker with a scorched stab or a loose grip on the bus.
5. If the bus itself is pitted, the panel is done. Quote a replacement.

Documenting and closing out

Update the directory. Half the troubleshooting time on the next call is going to be someone tracing circuits that weren't labeled, or were labeled wrong because somebody swapped breakers around during a kitchen remodel. Per 408.4(A), every circuit has to be legibly identified as to its clear, evident, and specific purpose.

Note the feeder size, the OCPD at the main, the bus rating, and whether the sub is in a separate structure. If you corrected a bonding issue, write it on the panel schedule and on your invoice. That paper trail protects you when the next electrician, or the inspector, shows up two years later.

Leave the panel better than you found it. Torqued, labeled, neutrals and grounds separated, dead front screws all present. The next guy in there might be you at 2am on a service call.

Subpanels are simple boxes with a short list of ways to go wrong. Separation, torque, balance, and documentation. Work that list in order and most of the mystery calls stop being mysteries.