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I am replacing a light fixture in my bathroom and have the following setup:

  • The electrical box is plastic
  • The wire coming from supply has black (hot), white (neutral) and copper (ground).
  • The light fixture comes with a metal bracket for attaching to the electrical box that has a green screw
  • The light fixture has black, white and copper wires.

The instructions supplied with the fixture dont give me an example of how to wire it up if there is an existing ground wire coming from supply. It only tells me to attach the copper wire of the light fixture to the green screw. So I'm curious what I should do with the ground from the supply. Looking online, my options appear to be this:

Wiring Options

So I think since the electrical box is plastic, I should just connect the ground of the supply with the ground of the light fixture with the wire nut. Do I need to connect both of them together with the green grounding screw on the light fixture as well, or would that only be if the electrical box was metal?

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The instructions for the fixture are only correct for a metal box. If a metal box was used, the box itself would (should) be grounded. The bracket that holds the light would then be connected to the box, which would make the bracket grounded. Finally the ground wire from the fixture would attach to the bracket, grounding the fixture.

In the case of a plastic box, the box is not grounded. All ground connections must be made by connecting the ground wires together. So in your situation you are correct, you'll connect the ground from the supply to the ground from the fixture using a twist-on connector (or other approved connection).

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Even plastic electrical boxes may have a metal strip for ground that connects to the metal screw receptors. Check that first.

If not, connect the ground from the panel to the fixture ground with a marette (just like the power wires) and twist the fixture ground around the green screw.

While it isn't critical to connect the ground wire from the fixture to the grounding screw on the mount, it's a good idea. I always make sure this ground wire is shorter than the power wires so that if the fixture falls it gets stopped by the ground wire attached to the mount rather than stressing the power wires from the panel.

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    I have never seen a plastic box with a grounding strip. – Brad Gilbert Oct 31 '12 at 4:43
  • Really? At least some of the one's I've bought have them; it's about 1/2" wide and runs from one screw hole along the back to the other. I never really thought much about it and just assumed it was something required. – Brian White Oct 31 '12 at 13:58
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    Interesting point about making the ground wire shorter. The only negative I can see with this is that if all the stress is on the ground, the ground could become disconnected. You now have a dangling fixture with no ground - lets hope it doesn't become energized! – Steven Oct 31 '12 at 15:04
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I would connect both coppers wires together via the green screw. I would also ensure that the copper on supply side is longer than hot and neutral, that way if there is a breakage it is most likely the hot or neutral that snaps and if it shorts to any metal work on the lamp it will go to earth/ground.

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    In general it is ill-advisable and I believe against code to tie more than one wire to any screw down point not specifically made for multiple junctions. The accepted approach would be via pigtail as @Tester101 suggests – noybman Sep 21 '17 at 4:50

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