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Question DetailsAsked on 10/29/2016

Ground wire connection in outside motion light

Hi I am installing a new motion sensor light outside. Should I connect all three ground wires ( the one coming out of the light, the one coming out of the house electric, and the one with the green screw on the mounting plate) together in the same wing nut wire connector? The directions say that connecting the ground wire with the screw in recommended but not necessary. So I just wanted to make sure it was safe and proper to connect all three wires together. I purchased an Ideal 12 AWG solid pigtails with screws for the connection from the mounting plate. So is it correct and safe to connect the ground wire from the light, ground wire from the house electric, and ground wire with screw from the mounting plate together? Also what is the purpose of using the mounting plate ground wire with the screw? Why not just tie together the ground wire from the fixture and ground wire from the house electric? Thank you!

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1 Answer


You did not say whether the box is metal or plastic.

1) If plastic box - wire nut the fixture ground wire AND the stub wire from the mounting bracket green screw to the circuit ground wire (house electric) in the box. Be sure to use a wire nut that is sized to accomodate three wires - the ones that come with fixtures commonly are for 2 wires only, and commonly only for #14 or even #16 or smaller or only for stranded, not solid, wire so be sure the wires are properly twisted and retained in the connector - that it is not too small or too large in internal diameter.

This wiring method gives two grounding routes for the fixture (directly along ground wires, and also circuit ground through stub wire to grounded mounting bracket to fixture (through the mounting screws or threaded nipple), so you don't get zapped if the fixture gets energized - also the direct wire connection so it is still grounded even if the fixture is free of the bracket during installation or partial removal for testing.

2) If metal box - then as it says, the circuit ground to the mounting bracket green screw same as in 1) above is redundant but a very good idea.

As I recall, there are two alternative legal wiring methods (way it commonly used to be done when all boxes were metal) to 1) above for metal boxes a) and for both types (b :

a) for metal boxes only - circuit ground wire connected through the screw-down ground connection in the metal box, then wire-nutted to the fixture ground wire for direct ground connection, and using no stub ground wire to the bracket (on the assumption it is already grounded by direct metal-to-metal contact and by the metal mounting screws to the metal grounded box). The fixture second ground path is light fixture (presumably metal) through the metal center threaded stub nipple or through the mounting screws into the mounting bracket, then through metal contact to the box or through the mounting screws into the grounded box.

b) for metal or plastic boxes - circuit ground wire through box ground clamp per code (to ground the box) as before, then to green ground wire on bracket. Then fixture ground wire to a second green screw on bracket (not provided with light but bracket is almost always threaded for it on other end of bracket from the green screw). You cannot legally put both wires under the green screw without splicing them legally in an eye or hook type lug connector rated for that use - not the dinky ones sold in wiring kit plastic boxes, which are generally aluminum and for low-voltage uses. Then the secondary grounding path is fixture through mounting nipple/screws to bracket to circuit ground from the green screw. The main reason b) would be used is if the fixture were provided with or previously had its ground lead cut short so it is too hard to wire into the box while holding the fixture up or because the box is getting too full of connections - so by connecting to the bracket instead of the circuit ground you clear some of the clutter and hook up closer to the fixture.

In case it was not clear above - the reason for the gorunding of the mounting bracket/strap is so the fixture is still grounded even if the ground wire comes loose from the circuit ground - wire nut falls off, contact in wire nut is bad, not stripped far enough so the wire nut is holding it by the insulation rather than metal-to-metal, ground wire gets pulled out of the wire nut while manipulasting the fixture to get it mounted, etc. By wiring a stub wire to the bracket from the circuit ground that also avoids the question of whether the bracket is actually grounded to a metal box or not - could be hung up on siding or drywall so not making box-to-bracket contact, could be plastic box, electrician might not have grounded the ground wire to the metal box as it should be (this is VERY common), could be the fiber washer on the mounting screw got between box and bracket so its insulating them, etc - just a safer way to try to make sure you don't look like a candidate to play the coyote on the Roadrunner Hour when cleaning fixture or changing light bulb if the fixture somehow becomes "hot". Course, you should not be monkeying with the fixture or changing bulbs with the power on anyway, but hey - we are talking the coyote here, right ? And of course there are photocells and timers that can inadvertantly make the power comes on while working on it.

And if that is not enough - one other reason for the stub ground wire to the mounting bracket. Many lights do not have a ground wire - use 2-wire wiring, and the cewramic light socket is isolated and not connected to the fixture housing at all - so the only grounding protection you get in case the housing gets energized by a frayed or burnt wire making contact with it is from the grounded bracket through the mounting nipple or screws to the fixture.

Answered 3 years ago by LCD

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