Why do house lights dim when Air Conditioner comes on?

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  #1  
Old 11-22-13, 01:03 AM
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Why do house lights dim when Air Conditioner comes on?

Why do house lights dim when my Air Conditioner comes on? Is it from too small of a fuse outside near the compressor?
Rick
 
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Old 11-22-13, 02:12 AM
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Welcome to the forums.

It could be one of several reasons. Unit getting older and drawing more power. Service not big enough coming into the house. Utility company transformer lacking enough reserve power.

It's not caused by the fuse at the A/C compressor.
 
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Old 11-22-13, 05:39 AM
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With a residential unit there probably isn't even a fuse at the unit and a fuse is just there to open the circuit in case of a short or overload. There is no logical reason it could cause it.
 
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Old 11-22-13, 05:49 AM
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What do you have for a panel, fuses, circuit breakers?
What's the main breaker or fuses say for amperage?
What's the make of the panel?
Checked for loose connections?
 
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Old 11-22-13, 07:14 AM
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The lights dim from voltage sag. As the unit starts it draws a large amount of power and the voltage dips.
 
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Old 11-22-13, 08:15 AM
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All wires have some resistance. The greater the resistance, all other things being equal, the greater the voltage sag (voltage drop). The more amperes (voltage does not matter), all other things being equal, the greater the voltage sag.

In this case only the voltage sag in those wires shared by the air conditioner and the lights in question count. Those would be the wires between the breaker panel and the pole transformer out in the street. For example there may be a 10 volt sag in the hot line coming into the panel and a 10 volt sag in the hot line going back out (the 2 hot legs of the 120/240 volt circuit) so the air conditioner gets 220 volts instead of 240. The lights draw from one side of the line only and there isn't enough current on the neutral to cause a measurable sag there so the lights get 110 volts instead of 120.

The number of amperes might not be large enough and drawn long enough to trip a breaker but the number might still be in excess of the nominal rating for the wires. For such a short draw, like under 2 seconds, and not much too much amperage, it's still safe.

There might be additional voltage sag in the wires between the panel and the air conditioner itself but since the lights don't share those wires, the lights are not affected by that portion of the sag.

After the 2 seconds the air conditioner hopefully has gotten going to the point it doesn't need that many amperes to keep running and the voltage sag becomes much less.

Loose connections introduce additional resistance and can result in additional voltage sag. For large numbers of amperes even for a few seconds, considerable heat is generated at that spot and over time, damage or a fire can result. For voltage sag in a length of wire, heat is also generated but the heat is spread out over the entire length of the wire and is usually not lasting long enough to cause damage.
 
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Old 11-22-13, 05:14 PM
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Hmm - Lots of food for thought. I know a good bit about electronics as far as basic theory and radio frequency communications but not much about house wiring conventions. The dim lights have been evident ever since the house was built. That leads me to believe that the problem lies either with the power company's selection of wire gauge or with the electrician's choice of wire gauge from the side of the house to the circuit breaker box.

I'm inclined to suspect the power company for this reason: My house is a good 150ft from the road and they had to put up a pole about 75ft in with a separate transformer and then run underground wires to the house. I'm betting that the stretch from the transformer to the house is the culprit. I'm gonna have to get hold of them and see what's up - while still reading answers to this thread on other things I might have missed. Though, also, the circuit breaker box is all the way on the other side of the house from where the power comes in. Another good 50 to 60 feet.
Thanks!
Rick
 
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Old 11-22-13, 05:21 PM
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With a 200' run between the street and your house..... it's very likely the power company feed is the cause of the sag.
The power company normally runs 100A service cable and that's it. Take it or leave it. It doesn't matter if you even have a 200A service.... they very RARELY will upgrade their wiring. Since their wiring is in "free air" they aren't bound by the same codes on wire sizing as found in a home.

You may be able to opt for upsized service cable for a nominal price.
 
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Old 11-27-13, 01:31 AM
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Thanks, y'all! I reckon I'll live with it, for now - have been for 14 years, anyways - unless the power company's charge to install heftier wire isn't exorbitant.
C-ya,
Rick
 
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Old 11-27-13, 06:40 AM
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A slight dimming when a large load, such as an air conditioning unit, comes on is normal in most house.
 
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