Having your lights flicker or dim slightly for a short period when your air conditioning first turns on is quite common and, in most cases, not something you really need to worry about. However, there are also times when this issue can indicate some problem with your outdoor AC unit or your home’s electrical system. Here, we’re going to explain why it’s normal for lights to flicker when your AC turns on as well as what other issues can cause this to happen and how to know when flickering or dimming lights indicate a problem.

Why an AC Causes Lights to Flicker When It Turns On

Central air conditioners and heat pumps require a huge amount of energy and typically use much more electricity than any other appliance or device in the home. These units always use lots of electricity when running, but the power draw is typically anywhere from three to five times when the unit first starts up. The additional current needed for the unit to start is what often leads to the lights slightly flickering each time your AC turns on.

Your home’s electrical system has a finite supply of energy, which means it can only power so many things at one time. Whenever lots of additional power is required, such as when your AC starts, it can result in some power being temporarily diverted from other things like your lights. This is why it’s completely normal for your lights to slightly dim or flicker for an instant as your AC turns on and draws power away from them.

One easy way to understand why this happens is to think about it like your home’s plumbing system. The main water line that feeds your home can only supply so much water at one time. Whenever more water is being used than the system can supply, it will result in reduced water flow from some or all of your fixtures.

This is something you’ve surely experienced many times. For instance, if someone flushes the toilet while you’re showering, you’ll notice that your shower is momentarily weaker. This is because the water pressure is now being shared by two fixtures instead of just being used for the shower. This is exactly what happens when your AC turns on as all of the power it draws means that there is less power available to keep your lights fully lit.

Whenever your AC starts, your lights will often dim and lose around 5% of their brightness for a second or two. When you have an issue is if the lights lose much more of their brightness or go out completely. Lights that flicker for longer times or continue flickering once the AC unit is fully up and running can also be a sign of a potentially serious problem. We’ll now look at some of the issues that can cause these things to happen.

Loose Electrical Connections or Damaged Wiring

Loose electrical connections and damaged wiring can often cause a short circuit. This leads to additional current flowing through the circuit that your AC is on or voltage loss in your light fixtures. Short circuits can cause the circuit breaker to trip if more power flows through the circuit than it is designed to handle. However, sometimes the additional current is not enough to trip the circuit breaker. It may still be enough to cause your lights to dim much more or more or cause the lights to completely turn off for a second.

If only one light fixture is affected, you can be fairly certain that the problem lies with a loose connection or damaged wiring in that fixture itself. Similarly, if only the fixtures in one room or part of the home lose power when your AC starts, the issue most likely lies with the circuit that those light fixtures are on. If all of your light fixtures are affected, then the problem most likely is related to your AC unit, blower or thermostat or to the wires that feed the circuit your HVAC system is on.

No matter whether one or all of your lights are affected, you definitely don’t want to ignore the problem. This is because damaged wires or loose connections can be a major fire hazard and also because a short circuit has the potential to cause major damage to your electrical system or your air conditioner.

Faulty or Worn-Out AC Capacitor

All cooling units have components known as capacitors that supply additional energy to the unit. The start capacitor stores a powerful charge that it releases to help the unit’s fan and compressor motors start up. Then the run capacitor takes over and supplies additional current to help the unit keep running without overloading your electrical system.

If you notice that your lights often continue flickering when your AC is running, it typically means that the run capacitor is failing and the unit is drawing more power than your electrical system can provide. If you notice that your lights dim by 30-40% or more when your AC first turns on, it often indicates that the start capacitor is beginning to wear out and can no longer store a sufficient charge. When this happens, the AC unit will then draw additional power from your electrical system leading to your lights and potentially other appliances temporarily losing power.

If the start capacitor is completely worn out or fails, your AC unit usually won’t start at all since your electrical system may not be able to supply enough power for the motors to start. A worn-out start capacitor will often also lead to the circuit breaker tripping as the AC will then try to draw more current than the circuit can handle.

Overloaded Circuit

The National Electrical Safety Code states that all major appliances like your air conditioner have to be on their own dedicated circuit. However, some older homes may have other appliances, outlets or light fixtures wired to the same circuit. This can lead to overloading and cause anything else on that circuit to lose power whenever the AC turns on or is running. When this happens, you will often also hear an audible humming or buzzing noise coming from the circuit or your electrical panel. Again, this is a serious issue as overloading can cause major damage and create a huge risk of fire.

Another common issue in older homes is that their electrical system simply isn’t designed to withstand the high amperage draw from a modern air conditioner. Modern AC units draw far more amps than older units. This can also result in overloading and cause lights to flicker dramatically or the circuit breaker to trip frequently when the AC turns on. In this case, the only effective option is to have your electrical panel upgraded and potentially the circuit re-wired so that it can supply sufficient power to the AC unit without overloading the circuit.

AC Unit Is Too Old

As AC units get older, the compressor motor often starts to have issues that lead to it drawing more power. The more power that the motor draws when it starts up and runs, the more likely your lights are to flicker or greatly dim. In this case, your only options are to learn to deal with the flickering lights or to have the unit replaced. Replacing the unit is obviously the better option since it can also improve your overall comfort and help lower your summer energy bills.

If you’re having any issues with your AC or need to have a new unit installed, you can trust the team at Hitchcock Heating & Air. We specialize in all types of cooling services for both residential and commercial customers in the Rome area. Our team can take care of all of your heating and indoor air quality needs as well. Contact us today if you need to schedule an AC inspection or if you have any questions.

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