Why Do My Lights Dim?

I'm asked this question a lot in the summertime. Quite often in the summer, when your central air conditioner kicks on , you may experience lights dimming in your home.

This happens because of a condition called "inrush". When a motor is running, it consumes a rated amount of current. And it's generally constant, unless the load of the machine it's driving changes.

Now when a motor starts up, particularly under a load, it draws inrush current. This current can spike as high as three to six times normal running current. Since current and voltage are inversely related, a spike in current will cause a drop in voltage. This voltage drop is why you're seeing your lights dim. The voltage drop should only last momentarily however. If it lasts for a couple seconds or longer, you may have other conditions present.

Those conditions can be any of the list below:

* Main Electrical Service heavily loaded.

* Main Electrical Service undersized for the electrical loads in your home.

* Main Electrical Service entrance conductors too small, or too long.

* Loose connections on conductors for the main, or conductors for the A/C branch circuit.

* Utility company transformer near or at capacity.

* Utility company secondary conductors too long.

* Incoming voltage too low at service entrance.

* A/C unit itself drawing too much current because it's old, inefficient, mechanically worn, weak or bad capacitor.

Some of these conditions will need to be checked by you, the homeowner. Others will need to be checked by the utility company. If you are in doubt, call a professional electrician to evaluate. If your system gets a clean bill of health, it's time to call the power company.