Diagnostic guide
Why Do My Lights Flicker When the AC Turns On?
Last reviewed · Reviewed by the Pro Electrician technical team
Short answer
Lights flicker when the AC compressor turns on because the compressor draws a massive locked-rotor current (often 50 to 100+ amps) for a fraction of a second. This momentarily drops voltage across your entire electrical panel. Persistent or severe flickering indicates a weak capacitor or loose utility neutral.
Normal Brief Dimming Versus Severe Voltage Drop
A momentary dip in brightness (lasting under a quarter-second) is common when large motor loads engage. However, if lights dim by more than 10-15%, stay dim for several seconds, or flicker across the entire house, your electrical system is experiencing excessive voltage drop that can damage sensitive electronics.
- Brief quarter-second dimming is normal motor startup inrush current
- Dimming exceeding 10% indicates undersized service cables or weak capacitors
- Severe dimming paired with light brightening indicates a dangerous floating neutral
Failing HVAC Dual-Run Capacitors
Central air conditioning condensing units use a dual-run capacitor to assist the motor during startup and operation. In Florida relentless heat, capacitors degrade rapidly. When a capacitor weakens, the compressor draws significantly more startup current, causing deeper voltage drops and pronounced light flickering.
Loose Neutral Connections on Service Drop
If some lights dim while others get noticeably brighter when the AC turns on, you have a damaged or loose neutral wire. This dangerous condition allows voltage on one 120V phase to spike as high as 160V while the other phase drops to 80V, quickly destroying appliances.
