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I am attempting to replace an outside porch light. After removing the old light, I found the metal electrial box had the ground supply attached to a screw. The new light has a green ground wire and I was wondering if attaching it to the green screw on the bracket would be sufficient enough for grounding, or, do I need to remove the ground supply from the screw and pigtail it to the ground wire from the new light fixture? See the attached photo of the electrical box. enter image description here

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You're going to want to connect the fixture grounding conductor directly to the supply grounding conductor, using either a twist-on wire connector or crimp connector. You'll also want to use a pigtail, to connect the supply grounding conductor to the grounding screw on the metal box.

You should not rely on the support straps attachment means to provide an adequate grounding path.

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  • Agreed that pigtail is best. Should the support strap also be grounded, however? It is metal, and it is supplied with a grounding screw.
    – mac
    Nov 26 '13 at 14:27
  • If the strap should indeed be grounded.
    – Tester101
    Nov 26 '13 at 14:55
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The Grounding Wire can be seen in the background of the image, (you can see it has been painted white), generally, by attaching the mounting bracket to the housing, you create an earth connection, this is not the case, when the casing is made of plastic or has a non-conductive coating.

You can connect your earth to the green screw provided that there is a good earth connection, this can be tested relatively easily by using a multi-meter with a continuity test setting

enter image description here

Based on the image above, you can check that you have a good earth connection by connecting one lead to the earth wire(coming from the electrical outlet) and the other to the point which you will be using for an earth.

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You can just attach the green ground wire for the new light to the green screw. The green screw is connected to the bracket which is connected to the box that the ground supply is connected to. So, you'll have a good ground connection for your light.

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    This may not be true. It looks like the box is painted, so conductivity may be inhibited. Ideally, the fixture grounding conductor would be attached directly to the supply grounding conductor.
    – Tester101
    Nov 26 '13 at 12:02
  • The guy said he had a metal box. Enough said.
    – getterdun
    Nov 26 '13 at 23:00
  • Also, how do you account for the wire already screwed into the box if it isn't a ground? On the other hand, it would be wise to ask him to verify the box is metal, and, testing the ground from the strap to the existing ground with an ohmmeter would be prudent.
    – getterdun
    Nov 26 '13 at 23:47
  • DManning: I "gots to know". Is it really a metal box? And, if you can test continuity between the green screw and the wire screwed to the box(scrape off a little paint where the wire connects, but make sure you test to the wire instead of the screw), I'd like to know the results. TKs.
    – getterdun
    Nov 27 '13 at 0:21
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    @D.Manning My answer still stands, and that's why it's done that way. The grounding path should not rely on the conductivity of the box, strapping, fixture, or other "removable" piece. Each item that needs to be grounded should be bonded directly to the grounding conductor.
    – Tester101
    Nov 27 '13 at 22:02

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