Tracking down a defective neutral


Old 02-11-11, 11:11 AM
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Tracking down a defective neutral

When certain loads are on (microwave, gas furnace blower), certain lights in my house brighten noticeably. I believe this indicates that we have a fault in the neutral wiring somewhere, most likely a loose connection, which I need to find before it overheats and we get into real trouble.

So, what's the best way to isolate the fault, especially since it could well be in the power company's wiring?

I can pull the panel cover and re-torque the neutrals easily enough. I don't think the lights and the loads are on two sides of the same multi-wire branch circuit, but I need to double check that. If they aren't, does that confirm that the problem is either inside of or upstream of the breaker box?

The set-up is pretty standard: transformer on a pole with a 3-wire drop to a meter (time-of-use) integrated with a 200-amp main breaker, with all the house breakers in that same box. Main panel professionally installed about 3 years ago. Rest of the house a hodgepodge of old (1950 house) and newish (kitchen and bath remodels by previous owner in 1990s). Draws are almost always under 100 amps. Photovoltaic system installed 3 years ago as well, but no difference in this problem day or night, so it seems unlikely to be involved.

FWIW, oven and cooktop cause no change in lights, only the 110volt items.

-- Rich
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Old 02-11-11, 12:37 PM
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Based on the description I would be certain that the neutral problem is in the main panel, meter box or power company wiring. A good place to start is usually the power company because most will come out for free to check their side. Many utility workers will also check and retorque the meter can, even though it's technically your responsibility.

I would not recommend that you retorque your main panel neutral lug unless you have PPE for live work or have the power company pull the meter before you do the work.
Old 02-11-11, 06:57 PM
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I called PG&E and, after navigating the labyrinth of a voice mail system they have, got a service appointment. It must have been a slow day because the technician showed up in less than an hour. He popped out the meter and put in a testing device that loads the legs independently and displays the voltage (to neutral) on each leg.

Before loading, both legs showed 122 volts. During load, they changed to about 118 on the loaded side and 124 on the unloaded side. Both sides were the same. He said it wasn't enough difference to account for the lights brightening when the microwave was on. However, he wanted to change the connectors between the service wires and the house wiring anyway when he noticed that the "temporary" connection my electrician had made 3 years ago when he replaced the panel was still in use. Note that the electrician told me that about 1/2 the time PG&E never replaces the connections he makes.

So, the PG&E tech cut out the bolt-style connectors and put in crimp-style connectors. He also went to the top of the pole in his bucket and inspected the connections there, replacing one he said was a little corroded.

After all this, the tester showed about 120 on the loaded side and 123 on the unloaded side. The tech said he didn't think we'd be able to see a difference.

So far, testing things that used to pretty reliably cause lights to brighten are no longer having that effect. So, pending further observation, I'm thinking there was in fact a problem in the PG&E connections, even though his testing device did not seem to show it.

Thanks for the help!

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