The Lights Dim When My Air Compressor Starts

It is a bit disconcerting when the lights dim when my air compressor starts.

To understand why this may happen is to better understand how the electric motor that drives your air compressor starts.

By and large, the power needed to start the electric motor on the air compressor is pretty much all the power that’s available in a typical house-type 15 amp circuit. If your air compressor  has a multi-horsepower motor – say 2 HP or more – and you are trying to power that with a 120 VAC x 15 amp circuit, odds are good that the power needed to start that motor exceeds the supply in your circuit.

Why the Lights Dim

If, when the compressor motor tries to start, the current needed by the compressor motor is at or exceeds the power available in the circuit, the motor will draw power from anywhere, including the lights. The power going to the lights is reduced, and they dim or flicker.

Compressor Motor Capacitors Help Start the Compressor Motor

In order to ensure that the compressor motor can start even when the draw of the motor is near the capacity of the circuit providing power, the compressor motor comes equipped with capacitors.

The lights dim when my compressor starts - image of compressor motor with red dots marking capacitors
Image of compressor motor with red dots marking capacitors

The start capacitor adds a boost of power when the compressor motor tries to start.

Some compressor motors have run capacitors as well, that continue to help the motor run within the available power supply. Or, other compressor motors come equipped with a single capacitor that fulfills both functions, boosting the motor on start, and enabling the motor to run continuously.

If the start capacitor becomes worn out or starts to fail, the compressor motor doesn’t get the start power boost it needs and the result is dimming or flickering of the lights that may be on the same circuit as the air compressor as all available power in that circuit is drawn to the motor.

Another Reason Why the Lights Dim?

Yes, if a failure in the compressor pump, particularly if the compressor pump is starting to seize up, occurs, this increases the load that the compressor motor has to overcome to start, and you may get the same result, the lights dim when my air compressor starts.

How to Check Capacitors?

Know that a capacitor is a fast discharge device, so be careful that you discharge the capacitor before you inadvertently touch the terminals and get shocked. The video will provide details, and please, be very careful!

Rather than taking a thousand words, here is an excellent video about using a multi-meter or a volt meter to check your capacitor to be sure it’s sound.

The folks in the video give you some idea how to safely check the air compressor capacitor, and the information you need from the existing one to find a new one.

Since testing the capacitor is fairly straightforward, that will typically be the first check if ever the lights dim when my air compressor starts.

Comments on your experiences testing your compressor capacitor are welcome. Only comments about compressor motor capacitors will be included on this page.

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Ed Cannon

My compressor starts but seems to run slow. It eventually builds up speed, but it seems to labor. It has 2 capacitors. I think that the run capacitor may be failing, does that sound logical?