When you need to use an appliance or a tool in a location too far for the cord to reach, you can utilize an extension cord. That's what extension cords are for, but they do have their limitations. An undersized cord that's too long for the job can adversely affect the performance of your heavy-demand tools and appliances and could overheat. In addition, a long extension cord gets in the way, and depending on where you route it, is vulnerable to damage. You can prevent fires and electrical hazards by using a cord rated for the load. Also, remember the recommendation of multiple safety organizations that extension cords are only for temporary use.
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Electricity coursing through the wires of an extension cord obeys Ohm's Law, which states that voltage equals current times resistance. Wire resistance increases with the length of a conductor and decreases with the diameter, and it can be significantly different at the end of a long, thin extension cord than at the power source. This phenomenon is a voltage drop, and it has consequences for your tools and appliances. Voltage drop also depends on current, becoming more of a factor when the current is greater.
For example, pass 15 amperes of 120-volt current through a 100-foot, 14-gauge extension cord, and the voltage at the end of the cord is only 112 volts. That will make your circular saw run more slowly. Some appliances, such as refrigerators, draw more current when they cycle on, and they may not get it through a long, undersized cord. The voltage drop through a 12-gauge cord of similar length is slightly less. The final voltage when conducting 15 amps is 115 volts, which may still be insufficient to do the job. Operating more than one tool or appliance on a long extension cord only exacerbates the problem.
More About Resistance
Besides causing voltage drops, wire resistance also produces heat. The longer the cord you use, the more heat the wires generate, especially when the current is large. If you pass a long cord underneath a carpet or through some dry brush in your yard and use the cord to operate a power-hungry appliance, the cord could get hot enough to start a fire. The likelihood of this happening increases if the cord has damage at any place along its length.
Long Cords Are More Likely to Get Damaged
In its journey from the plug to your tool or appliance, a long extension cord can pass any number of places where it could get damaged. These places include driveways, walkways, basements, crawl spaces and underneath doors. Cords that pass through a window and down to the ground can also have damage. Even small nicks and gouges that expose the wires in the cord are dangerous. Standing water conducts electricity, so if the ground around a damaged cord is wet, someone working or walking nearby could get a shock. Also, cords that pass through doors and over walkways are tripping hazards.
Observe Extension Cord Safety Rules
You should use extension cords in accordance with safety recommendations established by fire associations and groups like the Electrical Safety Foundation International (ESFI). Always choose a cord rated to handle the load you need it for, and don't use a longer cord than you need. Route the cord safely and remember to unplug it when it isn't in use. Keep in mind that extension cords are only for temporary use and should not take the place of permanent wiring. When you're done with your cord, wrap it up and put it away to prevent you or anyone in your family or neighborhood from being one of the 4,000 victims of accidents caused yearly by improper extension cord use.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker and Family Handyman.