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I have no noticeable problems with my house's electricity, but when my Air Conditioner kicks in (and here in Central Texas, it kicks in year round) my lights dim for a second or so and then run normally. It doesn't seem to have any bad effects, except that it's annoying and makes me worry that something might be wearing out from the load. How can I change things so that my lights stay even when the A/C turns on?

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Do all the lights in the entire house dim when the AC kicks on, or just certain ones? –  Scott Mitchell Feb 6 '12 at 20:34
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3 Answers

The reason the lights dim is because the fans on the AC units have a very large inductive startup load. Basically the power lines coming into the house just aren't able to handle the burst of energy that the AC unit is trying to pull.

There isn't much that you can do about it, but it also isn't something you should worry about. This happens everywhere I have ever lived. If it really bothers you and you have a deep pocket you could have your local electric company install larger service lines to your house, but they will most likely charge you a pretty penny to do so.

I have heard of some people who have replaced the startup capacitors in their AC unit with larger ones. This allows more of the load that is needed to start the motor to be pulled from the capacitors instead of from the power lines, however you will never be able to supply all of the power from the capacitors and you will need to be careful if your unit is still under warranty.

If you used to not have this problem, but now you do or you have seen it getting worse, then this could point to the capacitors you have being bad and just needing to be replaced.

But as a note, the thing that would be most likely to wear out from the load would be the transform that takes the high voltage lines down to the voltage that is used in your house. However, this isn't anything for you to be concerned about because the power company is in charge of sizing them correctly and will replace them in the event of any failure.

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First I would check the electrical connections to the compressor and the breaker in the panel, see that they are tight and not corroded. Next I would measure the starting current of the compressor with a clamp on ammeter. I had a problem like this on a friends AC, only his problem was tripping the breaker at times. His compressor is about 10 years old, and is harder to start now. When I measured the starting current it measured 175 Amps! Which on this compressor is the locked rotor current! We replaced the starting capacitor and added a "Hard start kit". Now the starting current is a respectable 50 Amps.

Some tech info: The induction type motors used in these compressors have an extra motor winding. This is called a "start" winding and is used with the "start capacitor" it is there only to get the motor to initially turn and a mechanical switch disengages it at a low speed. What happens over time is that the motor load is increased due to wear, and the start capacitance becomes weaker. So why not just build a compressor with a large start up capacitance to begin with? Some higher end units do just that, but most don't to save $$$. Contrary to popular belief, the start capacitor size does not "Give a bigger jolt" to the motor, but delays the 60 cycle feed to the second winding fooling the motor to think it is driven by a poly phase feed. The larger the capacitor value, the bigger the delay - hence the lower the starting current. Without a start winding, the motor will sit there humming until it trips the breaker. Three phase motors (poly phase) do not have start windings because they have three windings.

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The reason for lights dimming is that when the AC starts it consumes very high startup current and it also happens that the lights share some of the wiring with the AC and that wiring has resistance high enough to allow for noticeable voltage drop.

I saw a similar issue once many years ago - a fridge motor starting would make the lights dim. In that case it happened that all the load was connected to a line that was initially made of 4 mm2 copper wire but then the panel was moved and the line was extended with a piece of 1,5 mm2 copper wire which was undersized but wouldn't exhibit any problems like overheating - the only noticeable problem was that voltage drop when the fridge was starting. That extension section was replaced with a thicker wire and the problem went away.

So the number one thing you can do is to find which wires go all the way from the AC to the main panel and to the line entering your house. It might happen that some of that wiring is rather thin (it might even be up to code) and is shared with the lights. You can test it by temporarily connecting the AC to the main panel breaker directly so that it bypasses all possible shared wires. If that's the case - you can redesign the wiring so that no thin wires are shared between the AC and the lights. If you happen to have rather thin wires entering the house - that might also be upgradable but will require the electricity company to participate and will likely be rather pricey.

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