Do metal boxes need to be grounded?

stash-hdyJune 14, 2011

I changed out a variable fan switch that is mounted in a four ganged metal box situation. There is one outlet and two 3way light switches in the ganged boxes along with the variable fan control. Found that the devices all were grounded but there was not a ground wire to the metal box. Is it a requirement to have the box itself grounded?

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normel

yes, the box should be grounded. It should not be grounded through the devices, but by means of a machine screw, clip, or through metal conduit.

    Bookmark   June 14, 2011 at 8:51AM
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stash-hdy

Thanks, just finished grounding the box using the screw on one of the cable clamps.

    Bookmark   June 14, 2011 at 10:32AM
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brickeyee

"Thanks, just finished grounding the box using the screw on one of the cable clamps."

Not suitable.

There should be a 10-32 threaded hole in the back of the box for attaching a grounding pigtail.
If the box is very old the hole may not be threaded, and if even older may not be there.
You can use the slide on clips to ground the box (though relying on spring pressure remains a weak point).

Only machine screw threads are allowed for grounding (no self tapping screws).

    Bookmark   June 14, 2011 at 10:38AM
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bus_driver

It is not clear that the cable supplying the box in this case has an equipment grounding conductor that really provides a dedicated ground. Simply splicing together grounding leads and attaching them to the box is worthless unless there really is a functional ground.

    Bookmark   June 14, 2011 at 6:43PM
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stash-hdy

My house was built in 1995 and this box must have been added later. All other boxes I have done work in have plastic boxes but this one had metal for whatever reason. I will change out the location of the pigtail.

Is the reason why the self tapping screw can't be used is it because it can be stripped out?

Thanks for the feedback.

    Bookmark   June 15, 2011 at 8:22AM
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brickeyee

"Is the reason why the self tapping screw can't be used is it because it can be stripped out? "

Self tapping screws do not provide a gas tight connection for long term reliability as a low impedance connection.

    Bookmark   June 15, 2011 at 9:41AM
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