Have you ever been in a room and had the lights dim when some major appliance or electric space heater is turned on? People frequently wonder why this happens and what to do about it. Usually, it means that the appliance and the lights are drawing from the same circuit.
Appliances with large motors, such as compressors in air conditioners and refrigerators, draw a large amount of current while the motor is starting. This current surge causes a corresponding voltage drop on the circuit, which will cause lights connected to the circuit to dim.
To avoid the effects of these voltage drops on other electrical devices in the house, major appliances should be connected to dedicated circuits, serving only the appliance. All permanently installed major appliances, such as central air conditioners, heat pumps and clothes dryers, are wired this way.
Also, in relatively new houses, there are circuits dedicated to kitchen and laundry room receptacles, where high current users such as refrigerators, washers, microwaves and toaster ovens will be plugged in. But where homeowners install power-hungry appliances in other locations, there is rarely a dedicated circuit present.
The most common example of this in older houses is window air conditioners. In newer houses, the most likely culprits are laser printers.
Unless an electrician installs a dedicated circuit, a high-current appliance plugged into a 120-volt receptacle will share a circuit with every electrical device in that room and usually several other rooms as well.
Voltage drops don't just cause lights to dim. They can also cause electronic devices, such as computers, to malfunction. Surge protectors installed in outlets used by computers protect only against voltage peaks, not voltage drops.
To avoid the effects of voltage fluctuations on sensitive equipment, don't plug a major appliance into the same circuit as computers and other electronic devices. If you have a laser printer, plug it directly into the wall receptacle, rather than into the same outlet strip as the computer.
An uninterruptable power supply, or UPS, can be plugged in between a computer and the receptacle to even out the power supply. Also consider having dedicated circuits installed.
If lights dim throughout the house, it's a different situation - you may have inadequate power supplied to the house, and an electrical service upgrade may be advisable.
If lights in your house get dimmer and brighter, it might indicate a serious problem and should be investigated immediately by a qualified electrician.
Dean Uhler has been a home inspector for more than 12 years and is president of Baltimore-based Boswell Building Surveys Inc. Uhler is a member of the American Society of Home Inspectors (ASHI) and is the treasurer of the Greater Baltimore Chapter of ASHI.
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