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? Feb 6, 2012 at 20:34
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    You may want to put any sensitive equipment (TVs ect.) on a UPS. Your AC doesn't really care but electronics don't like low voltage.
    – Mazura
    Aug 8, 2015 at 3:09
  • 1
    Would a surge protector for the whole house work Feb 17, 2019 at 13:28

6 Answers 6


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.


The reason the lights dim is because the COMPRESSOR on the AC unit requires a lot of energy to start which reduces the amount of energy going to the rest of the house. Your electric service can only handle so much electric load at a time and sometimes machines (especially ones with large motors or compressors) require most of that load to start, leaving less than enough to supply the rest of the house.

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.


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.


This is not uncommon. Reasonably cheap to fix, and you should fix it to extend the life of your A/C compressor. If the compressor is being starved for voltage, it could wear out sooner than it should. This guy explains it in great detail.

You need what's called a "Hard Start Kit" - it's essentially a second capacitor to help out. Think of a capacitor like a little battery - it's there to provide the additional voltage that your compressor is trying to draw as it starts.

Any reputable A/C guy should be able to talk to you about it. And if you're handy, you can tackle it yourself in about 30 minutes - lots of wiring diagrams to be found online.


The Compressor has 2 winding's, a Run winding and a Start winding, the Common terminal is the point at which both of the compressor winding's connect, thus the "Common" designation. For example,say you read 3 ohms resistance for your Run winding (Common to Run), and you read 5 ohms resistance for your Start winding (Common to Start). This means between the Run and Start terminals you SHOULD read 8 ohms, as Run to Start is reading both winding's resistance in total, if they do not jive then one winding is shorted to itself or ground or the other winding. This is a good indicator of whether a compressor might have a shot at running if you send power to the compressors Common and Run terminals.

The Run capacitor is wired to the Run and Start terminals of the compressor, the Run capacitor stays in the circuit full time, however the Start capacitor only stays in the circuit for at least 50% yet no no more than 80% of the Start winding voltage or BEMF, back electromotive force.

BEMF is above the line voltage feeding the compressor, say it's 230 volts, the Start winding may read 300 volts, this is due to a motors similarity to a generator and the motor actually is creating voltage above the line voltage applied. To read BEMF you "bump start" the compressor by touching the Start capacitor's terminals to the Run capacitors terminals briefly!! too long and the Start capacitor will explode, too little and the compressor stalls. So after the compressor starts up rerad the Start winding voltage, we said it was 300 volts, so to size the potential relay that controls the Start capacitor we take the 300 volts and x .50 = 150 volts minimum. Then take the 300 volts x .80 = 240 volts maximum. You will want a potential relay that drops out no lower than 150v yet stays in no longer than 240v. And for the Continuous rating we take the 300v x 1.20 = 360 volts Continuous.

So 150v min & 240V max & 360v continuous.

A capacitor adds a pulse of power in the East and West poles while the line voltage is feeding the North and South poles of the motors armature, in effect its adding torque by having 4 guys spin the merry go round rather than just 2 guys if that makes sense.enter image description here


Check that the main power line lugs in your breaker box are tight. Some homes have aluminum wires from the power pole which expand when they get hot therefore they need to be tightened/checked every year or so. Had this issue on my first home. Freebee!!!


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