There are many reasons that your lights may dim when the A/C comes on. One of the most common reasons has to do with the capacitor.
The capacitor is going to be low tolerance, and this means the compressor is taking more energy from the capacitor, and this is causing it to pull more energy from the house.
If you find the capacitor isn’t the issue, the compressor on the system may also be going out. If this is the case, it will be pulling more amperage than it should, therefore causing the lights to dim.
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