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Old 01-18-2009, 06:39 PM   #1
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Bedroom lights dim w/microwave


I discovered this weekend that our bedroom lights dim when the microwave in the kitchen runs. Aside from the side benefit of alerting me that DH is cheating on his diet, I assume this represent some sort of problem. All my electrical is getting upgraded (permits and to code ) slowly, but surey. Do I need to worry about this?

Facts: kitchen and bedroom are on unrelated circuits (and 65 feet away from each other), we have 200A service to the house, bedroom circuit runs off K&T (circa 1890s!).

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Old 01-18-2009, 07:08 PM   #2
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Bedroom lights dim w/microwave


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I discovered this weekend that our bedroom lights dim when the microwave in the kitchen runs. Aside from the side benefit of alerting me that DH is cheating on his diet, I assume this represent some sort of problem. All my electrical is getting upgraded (permits and to code ) slowly, but surey. Do I need to worry about this?

Facts: kitchen and bedroom are on unrelated circuits (and 65 feet away from each other), we have 200A service to the house, bedroom circuit runs off K&T (circa 1890s!).

How much?

How do you know the microwave is not tapped off of the bedroom?

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Old 01-18-2009, 08:19 PM   #3
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Bedroom lights dim w/microwave


Are the bedroom lights on a dimmer? Dimmers are terribly sensitive to even the slightest voltage fluctuation. (sp.)

Do the lights dim (or brighten) with any other big load applied? Like a water heater, or all the burners on a stove. How about a basic 1500 watt heater plugged in at various locations around the house? If the lights dim under all these tests, it's likely not the house wiring.

How far from the POCO transformer is the panel? Overhead with the typical ultra-small triplex wire?

If you're comfortable with it, take a voltage reading at the panel, then start the microwave. 1 or 2 volts difference would be about normal.

With a bit more info, we can determine if it's in the house or outside.

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Old 01-19-2009, 04:09 PM   #4
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Bedroom lights dim w/microwave


ho-ho! The bedroom lights are, in fact, on a dimmer (ceiling fan).

POCO supplies power underground to the house. The transformer is 30-50 feet away in my backyard.

I'll try taking some measurements.
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Old 01-19-2009, 04:34 PM   #5
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Bedroom lights dim w/microwave


I had this problem,with my kitchen lights after some one decided to put the microwave on the same circuit as the kitchen lights. They were also on a dimmer. But the problem was solved when I opened the wall enough to run a wire from the stove plug up to the microwave plug. So now my stove and microwave are shared and not on the light circuit. My problem was solved. Altho the problem could be related to the dimmer, I would think it would happen. Do you have any other lights in the house on a dimmer? Do those lights dim also? I would think the circuit would have to be shared, but could be wrong. Whats on the microwave circuit?
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Old 01-19-2009, 05:06 PM   #6
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Bedroom lights dim w/microwave


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I had this problem,with my kitchen lights after some one decided to put the microwave on the same circuit as the kitchen lights. They were also on a dimmer. But the problem was solved when I opened the wall enough to run a wire from the stove plug up to the microwave plug. So now my stove and microwave are shared and not on the light circuit. My problem was solved. Altho the problem could be related to the dimmer, I would think it would happen. Do you have any other lights in the house on a dimmer? Do those lights dim also? I would think the circuit would have to be shared, but could be wrong. Whats on the microwave circuit?
You cant tap off your stove to feed your microwave, it puts an uneven load on the circut. check art. 210.11 (b) in the code book
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Old 01-19-2009, 05:13 PM   #7
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Bedroom lights dim w/microwave


Plus the feeders and circut breakers are under sized under full load conditions.
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Old 01-19-2009, 05:15 PM   #8
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You cant tap off your stove to feed your microwave, it puts an uneven load on the circut. check art. 210.11 (b) in the code book

Assuming this was not a gas stove and they did come off an electric range receptacle, then the violation would be 210.21(B)(3)
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Old 01-19-2009, 05:19 PM   #9
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Bedroom lights dim w/microwave


And that it wasn't hard wired, not that they normaly are, but just in case. But you are correct I assume to much sometimes.

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Old 01-19-2009, 05:47 PM   #10
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I thought there was something that said the microwave had to have its own circut now because the newer ones draw close to 15 amps?
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Old 01-19-2009, 06:25 PM   #11
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Bedroom lights dim w/microwave


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kitchen and bedroom are on unrelated circuits
If they're on the same side of the neutral, I'd say bad connection on the leg or the neutral, at the panel or upstream.
If they're on different sides of the neutral, I'd say the symptoms don't make sense or there are multiple bad connections.
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Old 01-19-2009, 07:38 PM   #12
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Bedroom lights dim w/microwave


Given the electrical in this house I would count on multiple points of failure. I will check on what sides the neutrals are on and report back.

Also, we have run a fairly high draw space heater on the same circuit as the lights and it does not cause the lights to dim.
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Old 01-19-2009, 08:12 PM   #13
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I thought there was something that said the microwave had to have its own circut now because the newer ones draw close to 15 amps?
Nope................................ Check out section 210.23
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Old 01-19-2009, 08:25 PM   #14
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Nope................................ Check out section 210.23
do you mean 210. 23 (C)?
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Old 01-19-2009, 08:31 PM   #15
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do you mean 210. 23 (C)?
No, not (C), you asked if a Microwave needed a dedicated circuit, so I pointed you to 210.23 check out (A)(1) & (2)...... This should answer your question.

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