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Why do my lights flicker when my AC unit turns on?

Electricity makes our modern world possible. Without it, we would have no cell phones or dishwashers or air conditioners. But it can be a frightening foe when it starts to misbehave.  Many homeowners notice that their lights flicker when their air conditioner turns on, prompting them to call an electrician or air conditioner professional in a panic. Does that flickering mean there’s something wrong with the power or the air conditioner?

When the Lights Go Dim

If you think of the electricity in your home’s wiring like the water in your plumbing, it can make it easier to understand why flipping on the air conditioner can make the lights dim briefly. Like the water in your pipes, there is  only so much electricity to go around at any given time, especially in older homes with smaller electrical mains.

When you turn on a high-demand appliance like a modern air conditioner, the initial power cost can put a massive drain on the available electricity. Just like when you flush the toilet while someone’s in the shower and the water pressure has to be split between the two fixtures, when your air conditioner kicks on, it diverts some of the power in your system.

For a normal-functioning air conditioner, this happens for just a split second — but depending on the sensitivity of your eyesight, a split second can be very noticeable. In fact, most people can easily distinguish 10 to 25 flashes of light each second. Many can detect 50 flashes per second. So even if your air conditioner only needs to draw extra electricity for 1/10 of a second to start running, it will seem that something odd is happening.

In reality, what’s happening is probably a very normal response to a limited amount of power being funneled into your home and the  large demand being placed on that supply. When your air conditioner causes the lights to flicker each and every time it comes on, or if you’ve recently had an air conditioner upgrade and suddenly notice this symptom, don’t get nervous.  It’s always a good idea to call in a professional to check things out, but you have very little to worry about.

Flickering Lights Aren’t Always OK

Usually, flickering lights are a normal result of a high demand on a limited power supply, especially in homes 30 years old or older with original electrical boxes. In rare instances, it can indicate trouble, so you should pay close attention to the way the lights behave when they flicker. Observe whether they flicker in particular parts of the house more than others, and what other appliances may cause them to flicker.

Lights that dim for a long time, get brighter before they dim or that only dim on particular circuits may indicate bigger problems than a heavy electrical demand. If they flicker when the air conditioner isn’t on, or only during certain times of the year independent of the air conditioner, you should have a professional evaluate your system for safety. Sometimes old wire connections pull apart causing shorts, breakers malfunction, or animals chew through wiring, creating unsafe conditions.

Once you’ve had your electrical system checked by a professional and are certain that there’s nothing wrong with it, you have a few choices. If you only use your air conditioner seasonally, you can simply choose to ignore it without any consequences. You can have your service upgraded, or a have a soft starter added to your air conditioner to reduce the electrical drain on startup.

When the lights flicker, it’s not always a sign of trouble in the wiring. More often than not, it’s just a heavy load on your electrical current temporarily reducing the amount of electricity available for your light bulbs and other appliances. This small drain isn’t anything to worry about and it’s not going to damage your appliance, but if you’re very concerned, call an air conditioning professional or electrician to check your system for your own peace of mind.