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Q.I am having an electrical problem on a second-story addition I am currently building. The clients report that the lights dim periodically during the early morning hours, but that it’s not related to the refrigerator starting nor any other piece of electrical equipment in the house. I have tested the voltage and have found no fluctuations greater than two or three volts during the course of a day.
The service is 200 amps, and the entire house has been completely rewired within the past six years. The wiring looks to have been properly installed.
Is there something I have overlooked in trying to determine the cause of these electrical surges? The adjacent houses have not reported any surges.

A.Rex Cauldwell, a master electrician in Copper Hill, Va., responds: Dimming of lights is not caused by a surge but by a lowering of voltage. When the full voltage and brilliance of the lights return, it just looks like a surge.

I would advise you to hire an electrician skilled in advanced troubleshooting. The problem with situations like these is that you have to be there when the dimming occurs to be able to isolate the cause. I once had to move into a house for a couple of days to troubleshoot an intermittent problem.

Most of the time, however, that’s not necessary. There’s a good chance you can simulate the conditions that are causing the problem. Despite the homeowners’ insistence that the dimming is not related to electrical equipment ( homeowners are often wrong in what they report), the early morning hours are usually a time of heavy electrical usage — hairdryers going, cooking in the kitchen, lots of lights on, the well pump if there is one, and so forth. I’ve also seen lights dim when a gas dryer was running — the belt was too tight and the motor was having a hard time starting.

First, try to verify whether the utility is responsible for incoming low voltage. To do this, I turn off the breaker for the electric stove — usually the heaviest power draw in the house. I then turn all the cooking elements and the oven on high. With my VO meter probes on the panel’s main lugs, I then throw on the stove breaker to see if the voltage drops below 240. If it goes down 10% or more, then there’s probably a problem at the transformer.

If the voltage remains steady, next test the two phases. Use a portable electric space heater to load first one side, then the other. With the heater running, measure the voltage between each phase and neutral. If one phase goes down several volts while the other phase goes up several volts, then most likely the SEC (service entrance cable) neutral is starting to deteriorate and the SE cable will have to be replaced.

Another possibility is that the SEC neutral splice at the panel or the meter has loosened and needs attention. This happens with SE cable because the soft aluminum compresses under the lug and loses contact.

The problem could also be caused by a bad neutral on the utility side as well. This is verified by measuring at the meter base, but you’ll have to have the utility there to cut the seal.