Lights Dimming When AC Kicks On? (5 Common Causes)
⚡ Voltage drop when your AC starts is common, but excessive dimming indicates a problem. Learn why it happens and what to do.
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⚡ Voltage drop when your AC starts is common, but excessive dimming indicates a problem. Learn why it happens and what to do.
A very brief, slight dim when your AC starts is usually normal. Air conditioners draw a huge amount of power when they start—often 3-5 times their running current. This sudden inrush of current can cause a momentary voltage drop throughout your home, resulting in a brief flicker or dim of your lights.
However, if the dimming is significant, lasts more than a second, or happens frequently, this indicates an electrical problem that should be addressed. Excessive dimming can damage sensitive electronics and is a sign your electrical system is struggling.
If your home has an older electrical service (100 amps or less), it may not have enough capacity to handle modern electrical loads including central air conditioning. When the AC starts, it draws so much power that voltage drops throughout the home.
Solution: Consider upgrading your electrical service to 200 amps. This provides significantly more capacity and reduces voltage drop when high-draw appliances start.
If your AC unit shares a circuit with your lighting, the massive inrush current when the AC starts will directly affect the lights on that circuit. This is more common in older homes where circuits weren't planned with modern appliance loads in mind.
Solution: The AC should be on its own dedicated circuit. An electrician can run a new circuit specifically for the air conditioner, separating it from lighting and other circuits.
Loose connections in your electrical panel, at the AC disconnect, or in the AC unit itself can cause excessive voltage drop. When connections are loose, resistance increases, which causes voltage to drop when high current flows through.
Solution: Have a licensed electrician inspect and tighten all electrical connections. This includes the panel, the AC disconnect, and the connections inside the AC unit.
If the wiring to your AC unit is undersized for the distance it runs, voltage drop will be more pronounced. The longer the wire run, the larger the wire gauge needs to be to prevent voltage drop. Undersized wiring is a code violation and a fire hazard.
Solution: An electrician can assess the wire gauge and run properly sized wiring if needed. This is particularly important for long wire runs from the panel to the AC unit.
Older air conditioners or units with failing components may draw more starting current than they should. Worn capacitors, failing compressors, or other issues can cause the AC to draw excessive power when starting, leading to more pronounced dimming.
Solution: Have your AC unit inspected by an HVAC technician. They can check the components and determine if repairs or replacement are needed.
Dimming is concerning when:
These signs indicate your electrical system is struggling and needs professional attention.
A hard start kit is a capacitor that helps your AC compressor start more efficiently. It reduces the inrush current by up to 50%, which can significantly reduce dimming. This is often the most cost-effective solution for older AC units.
Note: This should be installed by an HVAC technician, not an electrician, as it involves working inside the AC unit.
Ensure your AC is on its own dedicated circuit with no other loads. This prevents the AC's inrush current from affecting other circuits in your home. If your AC already has a dedicated circuit, the issue may be elsewhere in your electrical system.
If your home has an older 100-amp service, upgrading to 200 amps provides significantly more capacity. This is particularly important if you've added appliances, EV charging, or other electrical loads over the years.
A soft starter gradually ramps up the voltage to the AC compressor instead of applying full power instantly. This dramatically reduces inrush current and prevents voltage drop. Soft starters are more expensive than hard start kits but provide better protection for your electrical system.
Have a licensed electrician inspect all electrical connections in your panel, at the AC disconnect, and in the AC unit. Loose connections are a common cause of voltage drop and should be corrected for safety.
While AC units are the most common cause, other high-draw appliances can also cause lights to dim:
If multiple appliances cause dimming, this strongly suggests your electrical service needs upgrading.
Call a licensed electrician if:
An electrician can assess your electrical system, identify the cause of dimming, and recommend appropriate solutions.
If you're experiencing excessive dimming when your AC starts or other electrical problems in Newcastle or the surrounding areas, Clarity Electric can help. We can diagnose the issue and provide safe, effective solutions.
Call us at (425) 210-4791 or request a free quote for electrical inspection and repair.