What Causes House Lights to Dim?

You may become concerned when your lights periodically dim for no apparent reason. Often this is nothing to be concerned about, but it can be a sign of a more serious problem. If you notice your lights dimming, it is important to find the source of the problem and fix it as soon as possible.

...
There are many reasons your lights may occasionally dim.

Problems With the Power Grid

...
A local transformer may have blown.

The problem may be at the local utility substation. A transformer may have blown — a common occurrence during a storm — or the service feed coming into your house may have been interrupted. The area could be experiencing brownouts or possibly blackouts due to either a limited supply of power or an unusually high demand for power.

Overloaded Circuit

If your lights dim when an appliance comes on, such as the air conditioner, microwave or dryer, check to see if the lights are on the same circuit as the appliance. This is the most common reason for your lights dimming. Most fixed appliances — those that are not mobile — should be on a dedicated circuit on the circuit breaker. Large appliances such as these use up a lot of energy when they first start up, so if you notice the lights dimming when your dryer kicks on, this is most likely your problem.

Improper Wiring

...
See if your electrical wires are the right gauge.

Your lights may dim if the wiring in your home is not a large enough gauge. When there is a large current in the wire, the voltage will drop. Due to this lower voltage, your lights will dim. Many older houses have this problem and require new wiring. If you have continual trouble with dimming lights, call a professional electrician to inspect your wiring.

A Corroded Neutral Wire

If the light in your house fluctuates between being bright and dim, it could indicate a more serious problem. You may have a loose or corroded neutral wire in the main circuit breaker. Electricity is brought to your home through three wires. The first is black and is called the "hot" wire. The second is called the "return" wire and is white. The third wire is either green or bare and is "neutral." This type of problem could lead to an electrical fire or other safety issues.