Did You Know: Lights Dimming When the AC Starts Is a Clue
A brief flicker can be normal. A deep, lingering dim is telling you something.
Nobody calls about a flicker. They call about the thing the flicker was warning them about.
Summary
- Large motors draw a surge of current at startup.
- A brief, slight dip is ordinary; a deep or long one is not.
- The cause ranges from a failing capacitor to a loose service connection.
Key Takeaways
- Note how deep the dim is and how long it lasts.
- If it has got worse over time, that change is the useful information.
- Dimming across the whole house points upstream of any one circuit.
In a North Texas summer your air conditioner starts and stops many times a day, so this is one of the most-noticed electrical quirks in the region. It is also one of the most useful, because the character of the dim tells you a lot.
Why it happens at all
An air conditioning compressor is a large motor, and motors draw substantially more current in the instant they start than they do while running. That surge briefly pulls down the voltage across the system, and the lights show it. A momentary, slight dip when a big motor kicks on is ordinary and not a cause for concern.
When it stops being ordinary
Four things turn this from normal into worth investigating.
Depth. A barely perceptible flicker is one thing. Lights visibly dropping to half brightness is another.
Duration. The dip should be essentially instantaneous. If lights stay dim for a second or more, the motor is struggling to get going.
Change over time. This is the most valuable signal. If it never used to do this and now it does, or it has steadily got worse over a season, something is degrading.
Spread. If the dimming happens throughout the entire house rather than on one circuit, the cause is upstream, at the panel or the service itself.
What it can point to
On the equipment side, a failing start capacitor is a very common cause of a compressor that draws hard and long at startup. That is an HVAC repair, not an electrical one, and it is worth ruling in or out.
On the electrical side, it can indicate a loose or corroded connection in the panel or at the service entrance, an undersized or aging service for a house that now runs far more than it was built for, or a shared neutral problem. These are not DIY-diagnosable, and the service entrance in particular is not an area for homeowner investigation under any circumstances.
What is worth telling an electrician
How deep, how long, whole-house or one area, and whether it has changed. Those four details narrow the search enormously and make the visit shorter.
Need this done right?
Jarrett Page has 30 years in the trade and works Dallas-Fort Worth personally.
