MadSci Network: Engineering

Re: How can two outlets be used to achieve 240 VAC

Date: Sat Mar 28 07:34:49 1998
Posted By: Lawrence Skarin, Faculty, Electrical Engineering, Monroe Community College
Area of science: Engineering
ID: 891040562.Eg

Thanks for your question, Chris. First, let's say something about Electric Codes and safety. Most houses have as electric service three wires coming in that I will call +120, neutral, and -120. After going through the watthour meter, and the main circuit breakers, the +120 and -120 connect to individual circuit breakers for distribution in the house. It is essential that electric codes be followed exactly. To provide 240 volts to, let us say, an electric clothes dryer, a standard 240 volt outlet is used. The +120 goes to the outlet on a black wire and the -120 goes to the outlet on a red wire. Just as you suspected, a voltmeter connected from red to black will read 240.

To address your two outlets, I am worried about them being wired correctly. The wide side is supposed to be neutral -- the white wire. It's the narrow side that's supposed to be "hot" -- either + 120 or -120, and these usually come on a black wire. Had you described connecting your voltmeter between the two narrow sides and reading 240, I'd feel OK. So please have an electrically competent person check that out! Our household electricity standards have gone through a lot of safety evolution, and we don't want to throw that away by not following the rules!

To sum up, you sure can get 240 out of two outlets where one is wired to + 120 and the other to -120. But this 240 should only be accessed with outlets and plugs conforming to Code.

Larry Skarin

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