Lights Dimming When Using Microwave


Old 02-14-14, 08:59 PM
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Lights Dimming When Using Microwave

This is my first time back here in a while, thanks for all the advice over the years!

Recently I've had a problem with all of the incandescent lights in the house dimming whenever turning on the microwave or the blender in the kitchen. The kitchen receptacles are supplied by a 20A three wire circuit. The microwave and refrigerator do not have their own dedicated circuits, but they appear to be grandfathered onto this three wire circuit. The dishwasher and disposal have their own 20A three wire circuit.

I suspected the issue with the lights dimming was voltage drop. Normally each of the busbars measures exactly 120V per my voltmeter. But when the microwave is turned on, one busbar then reads 123V, and the other one reads 117V. I get those same measurements at the screw terminals to the main breaker, so I'm guessing the main breaker is still good.

The microwave is rated for 14A, and the blender is rated for 10A. So I'm surprised at that voltage fluctuation for a 20A circuit in the kitchen which is only about 40 feet from the panel.

I'm at a loss for what to do next. My current plan is to open up all the kitchen receptacles and look for loose connections, especially a loose neutral. Anything else I should look for?
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Old 02-15-14, 04:20 AM
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Loose neutral at the panel also.
Old 02-15-14, 05:38 AM
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Since you are measuring voltage irregularities in the panel and since you are observiing lights throughout the house dimming, you should start your troubleshooting at the panel. The problem is probably in the panel or even further upstream such as in the meter box or in the lines out to the utility pole.

Don't open up receptacles or boxes up in the house yet.

With the voltage irregularities seen at the big lugs holding the fat wires coming into the panel, chances are that the main breaker, too, is okay and the problem further upstream.

Most likely you cannot open up boxes and test voltage further upstream and even if you could, you need to be steady handed and experienced so as not to accidentally short together live terminals and parts which, up there, are not protected by small (like less than 30 amp) breakers and which short could result in a huge spark. Now is a good time to call the power company to do tests for you.
Old 02-15-14, 01:45 PM
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I had previously checked for loose screws on the breakers. I checked the neutrals in the panel and found a few which could have used a little tightening, but that didn't fix the problem.

Now that I think about it, I started having this problem sometime after my smart meter was put in. Perhaps there is an issue with one of the connections there. So I will call the power company and have them take a look.
Old 02-15-14, 02:25 PM
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If you call the power company do not call customer service. Call the 24 hour emergency service number and tell them you thin k there is a loose connection in the meter socket. They should send someone out within a few hours regardless of time or day.
Old 02-16-14, 01:19 PM
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Join Date: Mar 2007
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Thanks for the tip, I guess it's a similar situation to when you call the gas company to say you smell gas. It was a little painful describing the problem to the person on the phone, but once the tech came out and I explained it, he immediately knew to look at the neutral. He confirmed with his ammeter that it was completely dead. It was due to aluminum oxide in the underground connection going to my house, due to water getting in that box. He replaced my connection, as well as that of the two neighbors, since it was just a matter of time before my neighbors had the same problem. Thanks everyone!

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