Field guide: installing a subpanel, troubleshooting (edition 1)

Before you pull the cover

Most subpanel troubleshooting calls come back to three things: a neutral-ground bond that shouldn't be there, a feeder that was sized for the breaker instead of the load, or EGC terminations that nobody checked after the drywall crew finished. Before you start chasing phantom voltage or a tripping GFCI, open the panel and verify the basics. Ninety percent of the time, the answer is visible once the dead front is off.

Confirm the feeder breaker at the main is off, lock it out, and test dead on both legs and neutral. Then test neutral-to-ground. If you see more than a few millivolts on that reading with the main breaker still on and the subpanel feeder off, you've got a parallel neutral path somewhere downstream. Flag it before you go further.

The neutral-ground bond mistake

This is the single most common subpanel failure an electrician gets called back for. Per NEC 250.24(A)(5) and 408.40, the grounded (neutral) conductor and equipment grounding conductor must be bonded only at the service disconnect, not at the subpanel. If the green bonding screw or strap is still installed in the subpanel, pull it. If there's a separate ground bar, move every EGC to that bar and leave neutrals on the insulated neutral bar.

Symptoms of a bonded subpanel include nuisance GFCI trips, current on the EGC (NEC 250.6 objectionable current), hot ground terminals on receptacles, and ground rods warm to the touch. A clamp meter on the EGC between panels tells you everything: any measurable current there means neutral is finding a path through the ground.

If the panel shipped with a factory green bonding screw, it's usually in the neutral bar threaded into the can. Back it out, bag it, and tape it to the inside of the dead front so the next tech knows it was intentionally removed.

Feeder sizing and the 83% rule

Undersized feeders show up as warm lugs, voltage drop complaints at the far end of the subpanel, and occasionally breakers that trip under load but test fine. For a dwelling served by a single feeder, NEC 310.12 lets you use the 83% rule when the feeder carries the entire load of the dwelling. For a detached structure or a subpanel that isn't carrying the full dwelling load, size per NEC 215.2 and Table 310.16 at the standard ampacity.

A 100A subpanel in an attached garage fed from the main is not a dwelling service and does not get the 83% discount. Use 3 AWG copper or 1 AWG aluminum at 75°C terminations. For voltage drop on runs over 100 feet, bump one size. NEC 215.2(A)(1) Informational Note 2 recommends 3% on feeders, 5% total including branch circuits.

• 60A subpanel: 6 AWG CU or 4 AWG AL, 4-wire
• 100A subpanel: 3 AWG CU or 1 AWG AL, 4-wire
• 125A subpanel: 1 AWG CU or 2/0 AL, 4-wire
• Always 4-wire for a separate structure or any subpanel installed after 2008

The four-wire feeder and separate structures

NEC 250.32(B) requires a 4-wire feeder (two hots, insulated neutral, EGC) to any separately derived or remotely located subpanel. The 2008 code cycle eliminated the old 3-wire feeder exception for detached structures. If you're working on a property where the garage or barn subpanel was fed with a 3-wire before 2008, that existing install is legal but any replacement or modification triggers the current rule.

Grounding electrodes at the separate structure are still required per NEC 250.32(A), but they bond to the EGC at the subpanel, not the neutral. Two ground rods 6 feet apart, or a single rod with a verified 25 ohm resistance reading. UFER or a metal water pipe within 5 feet of entry counts if present.

Troubleshooting the live call

When the homeowner calls saying half the basement is dead and breakers look fine, work the problem in order. Check the main breaker at the service for a tripped or half-tripped position. Then the feeder breaker. Then voltages at the subpanel lugs: L1-N, L2-N, L1-L2. A reading of 120/0/120 or 0/120/120 points to an open leg, usually at a loose feeder lug or a failed breaker.

If both legs read 120V to neutral but L1-L2 reads 208V instead of 240V, you have a lost neutral at the service or the subpanel, and the loads are back-feeding through the neutral bar. Shut it down immediately. Per NEC 110.14(D), retorque all feeder terminations to manufacturer spec with a calibrated torque screwdriver.

1. Verify main and feeder breakers, visually and with a meter
2. Measure L1-N, L2-N, L1-L2 at the subpanel lugs
3. Check neutral and EGC terminations for tightness and separation
4. Clamp the EGC between panels for objectionable current
5. Retorque every lug to spec, not by feel

What to document before you leave

Write torque values, feeder size, breaker ratings, and the date on a label inside the dead front. Per NEC 408.4(A), every circuit in the subpanel needs a legible directory. Handwritten in pencil on masking tape does not meet the intent of the code, and it won't pass inspection in most jurisdictions.

Photograph the panel interior before you close it up. Every call-back starts with "what did it look like when you left it." A timestamped photo settles that question in thirty seconds.

If you removed a bonding screw, corrected a feeder, or replaced a breaker, note it on the directory and on the service ticket. The next electrician on that panel will either thank you or curse you based on how clearly you documented the change.