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I have several receptacles with two cables: one has black and white wires, the other - black and red wires. Can I replace these receptacles with GFCIs? If affirmative, how should I interpret the red wire? Thanks!

Followup: The black wires are originally twisted together, and the red and white wires are twisted together, too.

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    A picture or diagram showing the wiring would help. – friedo Jul 7 '15 at 18:16
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    Not black/white/red? Pictures or diagrams or both of how it's wired now might be helpful, as would your location in the world and the voltage of the receptacles. – Ecnerwal Jul 7 '15 at 18:17
  • Friedo, there's not much to add on a diagram, it is what it is: two cables sticking out of the wall, one with B & W wires and the other with B & R wires. North America, Massachusetts, the house built in 1956. – DYZ Jul 8 '15 at 18:47
  • Are the receptacles; or part of the receptacle, switch controlled? How were the receptacles originally wired, i.e. how were/are the wires attached to the original receptacle? – Tester101 Jul 8 '15 at 19:02
  • Tester101, no, they are not switch controlled. Originally, the black wires were joined, and the white and the red wire were joined. – DYZ Jul 9 '15 at 4:56
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This really isn't that odd and yes you can replace them with GFCI's. It is common with multi-wire branch circuits, or in cases where one outlet is switched and another isn't.

In the case of the black and red wire, one is hot and one is neutral. You can identify which one is hot with a multi-meter or non-contact tester (though NC testers can sometimes be misleading). Do yourself a favor and mark the neutral with white electrical tape wrapped around it once you identify which wire is which.

Don't get too caught up with the wire colours - while you hope people adhere to the standard, there are lots of cases when it's perfectly OK to deviate.

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    @dzinoviev - Suggest that after you determine which is line and which is neutral, you add colored tape around the end of the insulation, black on hot, white on neutral, if necessary. This way, the next time you (or someone else) opens up this outlet, they won't have to figure it out. – DoxyLover Jul 7 '15 at 19:44
  • I've added that to the answer, though still not sure why it got down voted. – Steven Jul 7 '15 at 20:49
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    I don't think I'd jump to the conclusion that one of the red or black wires must be "neutral". The black and white could be "hot" and "neutral" feeding the receptacle, and the other black and red could be a switch loop to make a half switched receptacle. There's not really enough information provided by the OP, to provide such a confident answer. – Tester101 Jul 7 '15 at 23:29

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