A brief, slight dim when the AC starts is common. The compressor motor pulls a large burst of current as it starts, and voltage dips for a moment. Dimming that is deep, lasts, keeps getting worse or comes with flickering at other times can point to a loose connection, a failing AC part or a panel problem.
Why it happens
Electric motors draw several times their normal current for the first instant they start. Your AC compressor is the largest motor in most homes. That surge causes a short drop in voltage across the wiring, and lights, especially on circuits that share the same path, show it as a quick dip. Once the motor is running, the current settles and the lights recover.
Normal or a warning sign?
Probably normal: a quick, slight dip when the AC starts, and steady lights the rest of the time.
Worth a call: a deep dim, lights that stay dim, or dimming that has gotten worse over time.
Worth a call: lights that flicker or dim when nothing large is starting.
Call the utility first: some rooms unusually bright while others are dim, or half the house losing power.
Call right away: a burning smell, buzzing at the panel or warm switches and outlets.
Common causes of heavy dimming
A loose or corroded connection in the panel, at the meter or at the AC disconnect
A failing start capacitor or a struggling compressor, which makes the starting surge larger
A panel or service that is small for the home’s total load
A problem on the utility side, such as the service line to the house
What you can safely do
Notice when it happens and how strong it is, and write it down. Check whether neighbors see the same thing. Do not open the panel or the AC disconnect. Loose connections get worse under load and can overheat, which is why heavy dimming is worth checking. If you have an HVAC technician coming for service, mention the dimming to them too.
Talk to a real person
Call (602) 999-2770 and a person answers, any hour. Describe what you see, and we will tell you honestly whether it sounds like an electrical issue, an AC issue or a utility issue. If it is ours, you will hear the price before anyone drives out.
A burning smell, buzzing at the panel or a warm outlet means stop and call. If you see smoke or fire, call 911.
Common questions
Is it bad if my lights dim for a second when the AC starts?
A quick, slight dim is common. Deep or worsening dimming should be checked.
Can a bad AC capacitor make lights dim?
It can. A weak capacitor can make the compressor struggle to start. That part is for an HVAC technician.
Will LED bulbs stop the dimming?
They may show it differently, but they do not fix the cause. If the dip is strong, have the wiring checked.
