Answered by a licensed electrician
Why do my lights dim when the AC turns on?
Quick answer
A brief, slight dim when the air conditioner starts is normal — compressors draw a large inrush current for a split second. Worry when the dimming is severe, lasts more than a moment, has been getting worse, or happens with small appliances too. Those patterns point to an overloaded or undersized service, a loose neutral, or failing panel connections — all worth a professional look.
Every house does this a little. The question is whether yours is doing it normally — or telling you something. Here's how we triage it.
The normal version
An air conditioner compressor needs four to six times its running current for the fraction of a second it takes to start spinning — a 20-amp-running compressor might gulp 80–100 amps momentarily. That inrush briefly pulls down voltage everywhere on the service, and incandescent and some LED lights show it as a quick, subtle dim. If your dimming is momentary, mild, happens only with big motor loads (AC, well pump, shop tools), and has been the same for years, it's physics, not a defect. An HVAC tech can add a hard-start kit to soften it further.
The patterns that mean something
Four patterns earn a professional look. Severe dimming — lights dropping dramatically, electronics rebooting — suggests the service or panel can't deliver the inrush, common on 100-amp services now carrying AC, an EV charger, and a modern kitchen. Worsening dimming — same AC, deeper dips than last summer — points at a connection loosening somewhere between the utility transformer and your breakers; connections fail progressively, and "it's getting worse" is exactly how that sounds. Dimming with small loads — microwave, kettle, hair dryer — shouldn't happen on a healthy service at all. And lights that brighten while others dim is a loose neutral — a genuine emergency that damages electronics and warrants an immediate call.
What we check and what fixes it
The diagnostic visit measures voltage drop at the panel while loads start, thermal-scans every connection, and checks the utility-side terminations. The fix matches the cause: re-torquing or repairing connections (cheap), moving the AC or other big loads to properly sized dedicated circuits, a 200-amp panel upgrade when the load calculation says the service is simply full, or a utility repair when the weak link is their splice at the pole — which we identify and they fix free.
If your dimming fits any of the four patterns — or it just bothers you — we'll diagnose it properly: 416-837-4038. For flicker unrelated to appliance starts, our flickering lights guide covers that separate family of causes.
Want a licensed electrician to take a look?
Free written quote, usually the same day. ESA permits handled.
People also ask
Can a hard-start kit fix AC-related light dimming?
Often, yes. A hard-start capacitor kit reduces the compressor's inrush current substantially, which shortens and softens the dip. It's an HVAC-side fix that helps when the electrical system is otherwise healthy — but it masks, rather than fixes, dimming caused by loose connections or an undersized service.
Why do my lights flicker even when the AC isn't running?
Random flicker unrelated to appliance starts has different causes: a loose neutral, failing switch or fixture connections, or utility-side supply problems. Whole-house flicker is the serious variant — see our flickering lights guide, and treat brightening-and-dimming swings as an emergency.
Does dimming damage LED bulbs or electronics?
Momentary dips don't harm LEDs or electronics. Sustained voltage swings — especially the brightening caused by a loose neutral — absolutely do, cooking power supplies and appliance boards. The pattern matters more than the dimming itself.