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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Here’s links to a collection of our other blog posts about ELECTRICAL WIRING:

Which house appliances need a dedicated electrical circuit?

Can a short circuit cause a high electric bill?

Can you use a light switch for a water heater disconnect? 

• When did the requirement for two 20-amp kitchen counter appliance circuits begin?

What is the maximum spacing requirement for securing NM-cable (nonmetallic-sheathed cable)?

Is it alright to just put wire nuts on the end of unused or abandoned NM-cable or wiring?

What causes copper wires to turn green or black in an electric panel?  

 Is the latest edition of the National Electrical Code (NEC) the standard used for the electrical system of new homes?

What are typical aluminum service entrance wire/cable sizes for the electrical service to a house?

Why is it unsafe to bond neutral and ground wiring at subpanels?

When was it first required that neutrals and grounds be separated (not bonded) on any panel past the main service panel? 

Should I get a lightning rod system to protect my house?

Why is a strain relief clamp necessary for the cord connection to some electric appliances?  

Does a wire nut connection need to be wrapped with electrical tape?

What is the minimum clearance of overhead electric service drop wires above a house roof?

What is the difference between a UL rating for dry, damp, and wet locations? 

What is three phase electric service?

What is a reliable way to tell if the electrical service is 3 phase or single phase? 

Can NM-cable (Romex®) be used to make a cord and plug to connect an appliance?

What are the requirements for NM-cables entering an electric panel box?

What is the color code for NM cable (Romex®) sheathing?

Can I remove a 240-volt range receptacle and hard-wire the range? 

Why do some wires in an electric panel have tape wrapped around them near their connections?

Why is undersize electric wiring in a house dangerous? 

Why is bundled wiring in an electric panel a defect?

What causes flickering or blinking lights in a house?

What is the voltage rating of a house electrical system? 

Why are old electrical components not always "grandfathered" as acceptable by home inspectors?

What is tinned copper wiring?

What is a conduit body or condulet? 

How can I find out the size of the electric service to a house?

What could cause an extremely high electric bill?

Can old electrical wiring go bad inside a wall? 

What are the code requirements for NM-cable (nonmetallic-sheathed cable or Romex®) in an attic? 

What is the difference between "grounded" and "grounding" electrical conductors? 

What does it mean when a wire is "overstripped" at a circuit breaker?

What is an open electrical splice?

What are the most common electrical defects found in a home inspection? 

What is the life expectancy of electrical wiring in a house? 

Do any pre-1960 houses have aluminum wiring?

What is the gooey stuff on some of the wire connections in the electric panel? 

How much does it cost to rewire a house?  

What is an "open junction box"? 

What are the clearance requirements for an overhead electric service drop that is directly over or near a swimming pool? 

How dangerous is old electrical wiring? 

What is a ground wire? 

What are the most common homeowner electrical wiring mistakes?

I heard that aluminum wiring is bad. How do you check for aluminum wiring? 

What is "knob and tube" wiring?

     Visit our ELECTRICAL and HEATING AND AIR CONDITIONING pages for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 



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