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Joined: Jan 2002
Posts: 2
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mlundy Offline OP
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I have been reviewing changes in the 2002 NEC and had a question that I would like to get the Board's input on. Section 404.8(B) notes that if two devices (a snap switch and another device such as a switch or receptacle) are installed in an enclosure and there is greater than 300V potential difference between the adjacent devices, a barrier between the two devices must be installed.

A Code Change book that I was reading then gave an example of a switch (use to switch a 277V lighting circuit) installed next to a 120V receptacle. It indicated that the voltage between the two devices would be greater than 300V (277V + 120V). However if you were to put a voltmeter probe on one of the conductors of the 277V circuit and the other voltmeter lead on a wire from the 120V circuit, the voltmeter would read 0V because the two supplies are isolated from one another. I have read two different "2002 NEC Change" books and they both provide a similar example. What is your take on this?

I could understand that if two, 277V circuits were switched from two switches in one enclosure there would be a potential difference of 480V between the switches assuming that two different phases were used. However I don't interpret 404.8(B) the way that these two Code Change books have.

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Joined: Nov 2000
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You will read voltage becuse the systems are not really isolated. They grounded conductors of both systems will be tied together by the grounding system.
Don(resqcapt19)


Don(resqcapt19)
Joined: Aug 2001
Posts: 7,520
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With a common neutral/ground the voltage you get between a 277V hot and a 120V hot will depend upon which phase of each system is present. It will certainly exceed 300V in some combinations.


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