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Q: electrical extension cord usage ( Answered,   1 Comment )
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 Subject: electrical extension cord usage Category: Miscellaneous Asked by: cwanda-ga List Price: \$10.00 Posted: 14 Apr 2006 12:19 PDT Expires: 14 May 2006 12:19 PDT Question ID: 718931
 ```Why not plug one extension cord into another? I bought a 40' cord to plug into a 50' cord to use for an electric mower. Both cords have the same rating (3-wire, 16 guage, 13 amps). Mower draws 9 AMP's, 120 VAC. New cord says Do Not Plug One Extension Cord Into Another. Is this a safety issue? Fire hazard? Just aimed at people who might use a lower-rated cord with a higher-rated cord and expect higher-rated performance? I am looking for something from electricians or electrical engineers, underwriters labs, or the like.```
 ```Hello cwanda, I am not an electrical engineer. However, I did start in EE before changing to mechanical. I do feel like I am capable of answering your question. The fact that your 16 Ga. cords have 13 amp ratings is based on the resistance of 16 Ga. wire. The whole thing is related to "Ohm's Law" which states: E = I x R or Voltage = Current x Resistance In your case the resistance of 16 Ga. wire is 4.016 ohms per 1,000 feet: http://www.powerstream.com/Wire_Size.htm So, the longer the wire (extension cord) the more voltage drop you get. This does two bad things. The loss in voltage generates heat and causes a reduction in performance of what ever is plugged into the extension cords. Too much voltage drop will damage the equipment you are using. The amperage rating of each of the extension cords you have is based on the particular length of the individual cord. When you plug one cord into the other you have doubled the voltage drop and cut the effective amperage capacity in half. Here is a good article on extension cords: http://www.jlab.org/ehs/manual/PDF/6210T2ExtensionCords.pdf You will notice that for a 100 foot 16 Ga. cord (which is about what you have when you plug the 50 and 40 foot cords together) they rate the cord at a maximum of 5 amps. According to this chart you should have 12 Ga. wire for your cord. Now, from personal experience, you may be able to use your two existing cords without any problem. However, it will very likely reduce the life of what ever tool you have plugged in because it will be operating at a reduced voltage. This will cause more heat to build up in the tool and could cause a premature failure. Of course everything fails eventually and whether you would notice the difference or not is debatable. If you operate the tool for extended periods, you can get enough heat in the extension cords to actually melt the insulation. I suppose after enough time a fire could result. As to the safety issue. The junction of the two cords can be a source for problems. If the connection is partially pulled apart, it can either cause a very high resistance, start a fire, or cause a shock. I was using an extension cord with a screw gun on a hot humid day and sweating profusely. My sweat ran into the connection and I got a pretty good shock. So, the fewer connections you have the better. If you should use a higher rated cord with a lesser rated cord, you would want to use the larger gauge cord closest to the receptacle. To be safe you should probably invest in a 100 foot 12 Ga. cord. I think I have covered everything, but in the event that I haven't, please ask for a clarification. Enjoy your mowing, Redhoss```
 ```You may want to have a look at voltage drop and other calculation software and freeware at http://www.mc-group.ca```