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Can someone please clarify for me when exactly it's safe/acceptable to use wire nuts?

Regional building codes withstanding, I think the general rules of thumb are:

  • Only use wire nuts in enclosed/covered junction boxes.
  • Only use/combine same gauge wiring
  • Only use wire nuts that are rated for handling the gauge, quantity and type (Al or Cu) of wire involved.

THESE ARE MERELY MY ASSUMPTIONS AND ARE LIKELY INCORRECT

Can I get clarification?

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  • Not sure about the wire gauge (you could use a wire of higher capacity than the circuit) but you should never mix wire types, e.g. Al vs Cu, without a connector specially designed for the junction. And even with a connection between the same time, your connector should be designed for the type of wire you're using.
    – BMitch
    Commented Feb 19, 2012 at 2:16
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    I don't think the different gauge thing is a problem - every light fixture I've ever seen uses wire nuts to connect the small gauge stranded wire of the fixture to the larger gauge solid house wiring.
    – Steve Fallows
    Commented Feb 19, 2012 at 3:20

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Wire nuts come in a lot of different sizes and styles, just look at this PDF. This brochure is from Ideal and go to page 20 for the wire size chart. I used information from Ideal not because I think they are better than others but I'm more familiar with them.

@BMitch is right about mixing AL & CU, but there are wire nuts for those applications as well. Most of these say that you CANNOT use with aluminum to aluminum. Weird.

While I don't think its a problem on this site, you can use most wire nuts with 600 volt maximum. Normally the colors dictate what sizes you can use with what wires and a lot of times these will overlap one another. Yellow, tan, red, and the large grey will allow you to use 2 - #12's with each. Yellow allows 1 to 3 #12's while the large grey allows 2 to 6 # 12's. The colors and the represented wire sizes accepted is pretty much accepted by most manufacturers.

Direct burial wire nuts are available. They are 'gel' filled and have a thicker shell. I don't know if I would direct bury one except for landscape lighting, but they are ideal for being used in damp, moist areas like pole lights.

Anything larger than the listed size should be used with split bolts, which gets treated completely different.

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