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Old 03-23-2011, 07:45 AM   #1
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Lights dim when Microwave runs.


Recently I have noticed when I run my microwave oven, the ceiling lights in my kitchen dim. I don’t get it because they are on different circuits. I took a voltage reading from the plug the microwave is plugged into and it read 114. When I fired up the microwave it dropped to 95. Could there be something wrong with the microwave or do I have other issues?

This is part of an addition which is feed from a 100 amp sub panel. I cut the power to the sub and went in and tightened all the breakers and the feed. But nothing seemed too loose.


Last edited by jhon; 03-23-2011 at 07:56 AM.
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Old 03-23-2011, 08:25 AM   #2
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Lights dim when Microwave runs.


Only in the addition or throughout the house?

Plug in some incandescent lamped lights here and there; they give more visible evidence of undervoltage or overvoltage.

Someone with a lot of electrical experience should measure the voltage across the big lugs in the main panel (hot to neutral) as you turn the microwave oven on. WHat do you get?

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Old 03-23-2011, 08:27 AM   #3
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Lights dim when Microwave runs.


Do like Allen said.
If you still have the same readings at the sub, then you need to make the same check at the main panel.
It could be that you have a problem from the power company side.
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Old 03-23-2011, 08:33 AM   #4
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Lights dim when Microwave runs.


Since this is a sub panel being fed from a main, I'd start by measuring the voltage on the output side of the breaker in the main that feeds the sub panel. Measure across the two connections (240V) and them from each connection to the neutral (120V) while the microwave is making popcorn.
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Old 03-23-2011, 01:47 PM   #5
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Lights dim when Microwave runs.


OK, Allen, jbfan and a7ecosair,

I think the problem is only in the addition. Have not noticed it any where else. But I will double check before I do anything.

I called the power company and they said they would check out the outside lines... But if they did not find anything wrong, it would cost me 85 bucks...

So before I have them do that, I will do the following tonight.

1. Turn on various incandescent lamps in the old and new part of the house and then run the microwave to make sure that it is isolated to the new addition. (Might even try pluging in two hair dryers in the old section to see I get a dimming there.

If it is isolated to the new section, I will do the following:

2. In the sub panel, test the voltage between the black and red wire coming into my sub panel and make sure it is 240. Then check the red to white and the black to white to make sure it is 120 Run the microwave and check for changes.


a7ecosair, not sure how to do what you want me to do in the main panel. Do you want me to test from the two screws where the wire is screwed in on the breaker and then from each of those spots to the neutral? The first part does not seem right.



Also, I am going to assume that if my problem is only in the new addition, it is not an power company problem.. or can it still be?


And is 114 voltage to low?


One other thing that may mean nothing. When I was testing voltage output on various outlets, one of them was registering 124. 10 volts higher than the rest...Any ideas on that.... Also, the voltage in the new and old part of the house seems to be the same except for that one outlet.


Thanks for your help. Also, I know my limits. If you tell me to do something I don't feel I am qualified to do, or have any doubts whatsoever that I might not understand what I am doing... I won't do it. So don't worry about me killing myself.
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Old 03-23-2011, 02:15 PM   #6
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Lights dim when Microwave runs.


You may have a loose neutral in your main panel. AE was trying to get you to check the connections there.

A loose neutral will show up as increased voltage on one leg while the other leg dips by the same amount, ie 140 and 100.
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Old 03-23-2011, 02:29 PM   #7
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Lights dim when Microwave runs.


Quote:
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You may have a loose neutral in your main panel. AE was trying to get you to check the connections there.

A loose neutral will show up as increased voltage on one leg while the other leg dips by the same amount, ie 140 and 100.
Yes, but I am just not sure how to measure that.
Yesterday, I shut down the sub panel and tightened every screw in the box. But I did not fine anything that was overly loose. I did find that the guy that disconnected my electric dryer when I got gas, left the neutral connected to the box in my sub panel. He only disconnected the two hots from the breaker.
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Old 03-23-2011, 02:53 PM   #8
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Lights dim when Microwave runs.


Quote:
Originally Posted by jhon View Post
a7ecosair, not sure how to do what you want me to do in the main panel. Do you want me to test from the two screws where the wire is screwed in on the breaker and then from each of those spots to the neutral? The first part does not seem right.
Yes. The first check will tell us what voltage is feeding the sub panel. The second part will tell us if the two 120 volt legs are balanced. When you are in the main panel be careful. Also while you have the main panel cover off, locate the white neutral that goes to the sub panel with the other two wires. At some point you may need to make sure this white wire is securely fastened. If you do any work other than voltage checks, kill the main breaker.
Quote:
Also, I am going to assume that if my problem is only in the new addition, it is not an power company problem.. or can it still be? This is correct.
Yes
Quote:
And is 114 voltage to low?

One other thing that may mean nothing. When I was testing voltage output on various outlets, one of them was registering 124. 10 volts higher than the rest...Any ideas on that.... Also, the voltage in the new and old part of the house seems to be the same except for that one outlet.
All your 120 volt outlets should read the same. If some are high and some low that is pointing to neutral problem as others have mentioned.
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Old 03-23-2011, 04:51 PM   #9
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Lights dim when Microwave runs.


OK, The lights in the other section of the house dimmed a bit as well when I tested it with the microwave. So I measured the sub panel, I get 240 from the black and red.... the black wire (To the neutral) gives me 127 volts, the red to the neutral gives me 114 volts...
Then I turn on the microwave, and I get 104 from the black and 135 from the red.

I am going to test the main panel now....

PS yesterday I tightened up everything in the sub box. It all looked ok.
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Old 03-23-2011, 04:59 PM   #10
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Lights dim when Microwave runs.


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Originally Posted by jhon View Post
OK, The lights in the other section of the house dimmed a bit as well when I tested it with the microwave. So I measured the sub panel, I get 240 from the black and red.... the black wire (To the neutral) gives me 127 volts, the red to the neutral gives me 114 volts...
Then I turn on the microwave, and I get 104 from the black and 135 from the red.

I am going to test the main panel now....

PS yesterday I tightened up everything in the sub box. It all looked ok.
Same issue in the main panel.

Do I call the power company or the electric company.

I have doubts that it would be safe to try to tighten the leads in the main box. When I did it in the sub panel, I was able to cut the power.

I am going to now shut off the breaker to the sub panel and take readings.
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Old 03-23-2011, 05:04 PM   #11
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Lights dim when Microwave runs.


With the sub panel off, I still get 115/125.

HELP!
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Old 03-23-2011, 05:16 PM   #12
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Lights dim when Microwave runs.


Where did you connecting your meter when you measured voltage in the main? Do not try and tighten the incoming power leads or the neutral.
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Old 03-23-2011, 05:26 PM   #13
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Lights dim when Microwave runs.


Quote:
Originally Posted by a7ecorsair View Post
Where did you connecting your meter when you measured voltage in the main? Do not try and tighten the incoming power leads or the neutral.
I connected on the hex holes where you would tighten the wires that come into the box. The two on the top and in the same spot on the neutral on the bar on the right.

I did push on the wires about 6 inches away where it is insulated just to see if any of them were visibly loose. They seemed fine. The terminal moved a little but not the wire alone.

I kind of figured not to touch them babies.... Here is where I know as a handyman, my job is probably finished......
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Old 03-24-2011, 05:10 AM   #14
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Lights dim when Microwave runs.


I am just a little confused. It would seem to me that I have different voltage coming into the house from the power company as one side of the box is higher than the other. But the sub panned feeds off of only one side. So why would I have different voltage in the sub panel?
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Old 03-24-2011, 07:18 AM   #15
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Lights dim when Microwave runs.


Disconnect all electronic equipment (TV sets, microwave ovens, computers, etc) from every circuit where the voltage is seen to or you think will seesaw above 127 volts. Find and fix the problem (it's in the neutral somewhere) boefore plugging those items back in.

One thing you can do yourself is to tighten up all the small screws and set scrws in the panel. Suggest undoing each a quarter turn before tightening, which cleans off and improves the connection somewhat.

If the red/white and black/white voltages seesaw before entering your house then the problem is in the power company lines or equipment.

If your experienced person can measure at the fat wire ends themselves as opposed to the lugs they are attached to, you can get a slightly surer measurement that you should call the power company. But you can never be 100.0 percent sure whether to call the power company first or call an electrician to go over your panel first.

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Last edited by AllanJ; 03-24-2011 at 07:27 AM.
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