ground wire light switch

unglaublitchApril 16, 2013

Hello all. I have a set of light switches, five across.

They were all two-wire switches. The metal box that houses the switches had a ground wire attached to a screw at the back of the box, and all the rest of the wires were attached properly to the switches.

I've changed all five switches to dimmers. The dimmers all have green grounding wires.

Can I connect these five ground wires from the dimmers to another screw on the back of the metal box? Or do they have to connect to the ground wire already coming into the box?

This post was edited by unglaublitch on Tue, Apr 16, 13 at 11:27

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brickeyee

If the boxes are properly fastened together they should all be grounded.

Arguably pig-tailing everything would provide a better long term connection since the screws used to join metal boxes do not have lock washers to make a gas tight connection.

    Bookmark   April 16, 2013 at 12:31PM
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Ron Natalie

A lock washer isn't required for a gas tight connection. There are no lockwashers on the device screws nor on the tapped screw used the bond the box.

You can't use just any screw. It has to be one designed to ground (i.e., the tapped screw hole in the box). Unless your box has provision for multiple grounds, you don't have any choice but to pigtail. Given how tight things are likely in that box, you'll probably need to pigtail to pigtail to get everything to reach/fit.

Note that you better check the rating on those dimmers if you've got 5 of them in the same box. Many of them require derating in this

    Bookmark   April 16, 2013 at 12:37PM
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brickeyee

Machine screw crew threads when torqued correctly are gas tight.

The problem is the clamping action produced by the box fastening from one box to another is NOT gas tight unless you use a lock washer under the screw head fastening the boxes together.
The screw is reliably grounded by the machine threads, the screw head by conduction to the threads, but the contact from the tab of the next box under the screw head is not as good.

Luckily there is a lot of area, and the connection remains low enough impedance to do the job at 120 V.

In a dry interior wall location the grounding to the zinc plated box remains good.

Excessive dampness over many years can cause enough corrosion of the box surface the gangs do not remain well bonded.

I have seen enough of these in bathrooms (especially) and in damp outdoor locations (porch ceilings and protected porch walls not requiring wet treatment) over the years to now pigtail the boxes together and not rely solely on the clamp screws.

When a digital meter says open, it really means open.

It is just one more pigtail per section for ganged boxes in damp locations.

Dry interior locations, I just rely on the ganging screws.

This post was edited by brickeyee on Thu, Apr 18, 13 at 13:03

    Bookmark   April 17, 2013 at 12:42PM
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