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g/electrical • Posted by u/vfd_victor •

Neutral-to-ground voltage climbing through the day on one branch — 0.4V at 7am, 6.1V by 3pm Troubleshooting

Plant office wing, 120/208 wye, one 20A branch circuit feeding a row of workstations. IT is complaining about intermittent equipment resets, always in the afternoon.

Measured N-G at the receptacle:

07:00 — 0.4V
11:00 — 3.2V
15:00 — 6.1V
19:00 — 1.1V

Circuit is not overloaded. Measured 11A at peak on a 20A circuit. Insulation resistance is fine. No shared neutral on this one, it is a dedicated 12AWG homerun about 140 feet back to panel LP-2.

The daily curve says thermal to me but 11A on 12AWG should not be producing that kind of drop. What am I missing?

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u/admin_jake ADMIN •
✓ Accepted Solution

N-G voltage is the voltage drop on the neutral conductor between your measurement point and the bond at the service. 6.1V across 140 feet at 11A means about 0.55 ohms in that neutral path. 140 feet of 12AWG should be roughly 0.22 ohms. You have twice the resistance you should have and it gets worse as it warms up.

That is a loose or corroded neutral termination, not a conductor problem. Positive temperature coefficient — the connection heats, resistance rises, it heats more. The afternoon peak is the connection being hot, not the building being busy.

Pull the panel cover and thermal-image the neutral bar while the circuit is loaded. You will find it.

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✓ Solution
u/vfd_victor OP •

Ran the Ti480 on the neutral bar this morning with the circuit loaded. 61°C on one lug, everything else in the bar sitting at 28°C. It was backed off maybe a full turn. Retorqued to 20 in-lb and N-G is holding at 0.5V all afternoon.

Ambient in that panel room is 22°C. That lug was cooking.

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u/breaker_bob •

Classic. And note what it was doing to you diagnostically — every symptom pointed at load because the load and the heat peak at the same time of day. The tell was that the numbers did not match Ohm's law for the conductor you actually installed. Always do that arithmetic first, it is thirty seconds and it tells you whether you are chasing a conductor or a connection.

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u/nec_nerd •

110.14(D) has required torque to the manufacturer's spec with a calibrated tool since the 2017 cycle. Not a suggestion, not a best practice — it is code. Anyone still setting lugs by hand feel is installing a defect with a delay fuse on it.

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u/gen_set_gina •

Worth checking the rest of that bar while you are in there. A lug that loose usually means the whole panel got torqued by feel rather than by wrench, and the others are on the same schedule. Torque them all, log it, and you have a document that gets you out of an argument later.

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