Why do the lights dim when the air conditioner starts up?
Tuesday, July 10, 2018
The compressor in the outdoor unit of a central air conditioner requires a brief, big surge of electricity to start it moving each time the system cycles on. The burst needed to get it going is called the LRA, an acronym for “Locked Rotor Amperage.” It is also sometimes informally called the “inrush current”—a good way of visualizing it. If you remember Newton’s First Law of Inertia from high school physics, “an object at rest tends to stay at rest and an object that is moving tends to continue moving, unless acted on by an outside force.” It takes extra energy to start any object moving, which is why trucks start in a low gear and compressors need that extra kick.
The LRA rating of a 2-1/2 ton heat pump condenser (outdoor unit) is circled in the red box above, along with the RLA, which is the amperage it draws after start-up and stands for “Rated Load Amperage.” As you can see, the LRA startup amperage is five times the RLA running amperage. This burst of electricity usage can drop the voltage serving the rest of the house, and briefly dims the lights by starving them of electricity.
Dimming is an annoying problem, but not necessarily dangerous. You might just choose to live with it. We see it most often in older homes with 100-amp service and a large central air conditioner. A 4-ton air conditioner, for example, can briefly exceed the rating of the electrical system at start-up, but not long enough to trip a breaker. Plus, the LRA grows further as the compressor ages.
In newer homes with 150 or 200-amp electric panels, it may require some detective work by an electrician to figure out why the lights are dimming. Our electrician friend, Craig Eaton, suggests to start by checking the wire connections at the panel and the condenser circuit. Repairing a loose or deteriorated connection could solve the problem. Also, running heavier gauge wiring would definitely fix it, but that’s an expensive and labor-intensive solution that is impractical.
There are two different devices that an air conditioning contractor or electrician can install to eliminate light dimming. One is called a “hard start kit,” which is a capacitor on steroids. Every heat pump or air conditioning condenser unit has a capacitor built-in that adds an extra surge of current needed at start-up. A hard start kit is a super capacitor that stores and then releases a larger bump. It is the cheaper of the two solutions.
The second choice is the “soft start kit,” a more sophisticated system that uses a programmed circuit board to measure the characteristics of the compressor motor. It then optimizes the electrical input of a start capacitor over several start cycles, causing the starting inrush current (LRA) to be significantly reduced. We suggest you ask your trusted electrical or air conditioning contractor which is best for your system.
To learn about other defects that can cause lighting fluctuations, see our blog post “What causes flickering or blinking lights in a house?”
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