You are sitting in the living room and the air conditioner kicks on. For a split second, the lights flicker or dim slightly, then return to normal. It happens every time the AC cycles. Is this normal? Is it dangerous? Should you call an electrician? The answer depends on how severe the dimming is, how long it lasts, and whether the problem is getting worse. Here is what causes it and how to know when it crosses the line from normal to concerning.
What Causes Lights to Dim When the AC Starts
The short explanation is voltage drop caused by the high starting current (inrush current) of the air conditioner compressor motor.
How Inrush Current Works
When an electric motor starts, it draws significantly more current than when it is running at steady state. An air conditioner compressor might draw 15 to 20 amps while running but pull 60 to 80 amps for the first fraction of a second during startup. This is called inrush current or locked rotor amps (LRA).
This massive but brief current demand pulls voltage down momentarily across your entire electrical system. The voltage drop is visible in your lights as a quick dimming or flickering. As the compressor reaches running speed (within about half a second to a second), the current drops to normal running levels and the voltage recovers. The lights return to full brightness.
Why It Affects the Whole House
Even though the air conditioner is on its own dedicated circuit, the voltage drop from its inrush current affects the main bus bars in your electrical panel, which feed every circuit in your home. The lights in any room can dim momentarily because they all share the same voltage source at the panel. The closer a light circuit is to the AC circuit on the panel, the more noticeable the effect.
When Dimming Is Normal
Some degree of light dimming when a large motor starts is considered normal and is not a sign of an electrical problem. Here is what normal looks like:
- Duration: The dimming lasts less than one second, a quick blink or brief dip
- Severity: The lights dim slightly but do not go dark or flicker rapidly
- Recovery: Lights return to full brightness immediately and stay bright while the AC is running
- Consistency: The dimming has been the same since the AC system was installed and has not gotten worse over time
- Isolation: Only happens when the AC compressor starts, not at random times or when other appliances turn on
Most homes in Aiken experience some minor light dimming when the AC starts, especially during the summer when the compressor cycles frequently. This is a normal characteristic of how electrical systems respond to large motor loads and does not indicate a problem.
When Dimming Is a Warning Sign
Certain patterns of dimming indicate electrical problems that need professional attention. Call an electrician if you notice any of the following:
Severe Dimming
If the lights dim dramatically, going nearly dark for a second or more, the voltage drop is excessive. Normal voltage at a residential outlet is 120 volts. A brief dip to 110-115 volts causes barely noticeable dimming. A dip to 100 volts or below causes obvious, dramatic dimming and can damage electronics and appliance motors. Severe dimming indicates a problem with your service entrance wiring, panel connections, or utility supply.
Prolonged Dimming
If the lights dim and stay dim for several seconds or remain noticeably dimmer while the AC is running, this goes beyond normal inrush current effects. It may indicate the AC is drawing excessive current due to a failing compressor, the circuit wiring is undersized, or there is a poor connection somewhere in the circuit causing sustained voltage drop.
Dimming Getting Worse
If the dimming has been gradually worsening over months or years, something is deteriorating. Connections loosen over time, corrosion builds up at contact points, and these progressive issues cause increasing voltage drop. A problem that was barely noticeable last year but is obvious this year will be worse next year and may eventually cause overheating or failure.
Dimming with Other Appliances
If lights dim noticeably when other appliances start, such as the washing machine, dishwasher, microwave, or electric dryer, your panel or service entrance may be undersized for your home's demand. A 100-amp service feeding a home with central AC, an electric water heater, and modern appliances may be at or near capacity.
Flickering or Strobing
Rapid flickering or strobing (as opposed to a single smooth dim and recovery) can indicate a loose connection that is intermittently breaking and re-making contact. Loose connections are a fire hazard and should be investigated promptly.
Buzzing or Humming from the Panel
If the dimming is accompanied by buzzing, humming, or clicking sounds from the electrical panel, a breaker or connection inside the panel may be arcing or failing. This is a safety concern that warrants immediate inspection.
Common Causes of Excessive Dimming
Undersized Service
Many older Aiken homes were built with 100-amp or even 60-amp electrical service. Modern homes with central air conditioning, electric water heaters, and multiple large appliances need 200-amp service. If your home has a 100-amp panel and a 3-ton or larger AC system, the service may simply not have enough capacity to handle the compressor's inrush current without excessive voltage drop.
Loose Connections at the Panel
The main breaker, the lugs where the service entrance cables connect, and individual circuit breaker connections can all loosen over time. A loose connection adds resistance, which increases voltage drop under load. This is both a performance issue and a safety hazard, as resistance generates heat.
Corroded or Damaged Service Entrance
The service entrance cable runs from the utility meter to your electrical panel. If this cable is corroded, damaged by water intrusion, or has deteriorated connections at the meter or panel, voltage drop increases. This is particularly common in Aiken's humid climate where moisture can enter weatherheads and conduit connections over decades.
Failing AC Compressor
An air conditioner compressor that is beginning to fail draws more starting current than normal. A compressor with worn bearings, a failing start capacitor, or internal winding issues may pull significantly higher inrush current, causing worse dimming than when the system was new. If the dimming coincides with a change in how the AC sounds during startup, the AC system itself may need service.
Missing or Failed Hard Start Kit
A hard start kit (also called a compressor start assist or start capacitor boost) reduces the inrush current of the compressor during startup. If your system had a hard start kit that has failed, or if a hard start kit was never installed, the compressor may be drawing more inrush current than necessary. An HVAC technician can install or replace a hard start kit for $100 to $250.
Solutions
For Normal, Minor Dimming
If the dimming is brief, minor, and consistent, no action is needed. This is a normal characteristic of your electrical system. If it bothers you, a hard start kit on the AC compressor can reduce the inrush current and minimize the dimming.
For Excessive or Worsening Dimming
- Have your panel inspected: A licensed electrician can check all connections, measure voltage drop under load, and identify loose or corroded connections
- Consider a panel upgrade: If your home has 100-amp service and modern electrical demands, upgrading to a 200-amp panel resolves voltage drop issues and provides capacity for future needs
- Verify dedicated circuits: Your AC system should be on its own dedicated circuit with properly sized wiring. If it shares a circuit with other loads, this exacerbates voltage drop
- Have the AC system inspected: An HVAC technician can check compressor amps, capacitor condition, and recommend a hard start kit if appropriate
Unity Power & Light provides electrical panel inspections, panel upgrades, and dedicated circuit installations throughout Aiken. If your lights are dimming excessively when the AC starts, or if the dimming has been getting worse, call us at (803) 220-4491 for a diagnostic visit. We will identify the cause and recommend the right solution for your home.