Replacing 2 prongs outlets with 3 prong


Old 10-25-13, 03:16 PM
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Replacing 2 prongs outlets with 3 prong

Hello I am replaceing some 2 prong outlets with 3 prong outlets.

Here is the situation.

I have 2 wire flex armored cable running to the metal box.

There is no hole in the box that I can attach a ground wire from the new three prong outlet to.

However on the new three prong outlet the mounting screws go through the yoke that the grounding screw is attached to.

And the mounting screws attach to the metal box that the armored cable attaches to.

As I understand it the armored cable acts as the ground back to the breaker box.

I have attached the three prong outlets without a ground wire running to the box and with an outlet tester they test "Good".

Do I need to drill a hole and use self tapping ground screws to attach a ground wire or is the outlet considered grounded as is?
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Old 10-25-13, 03:21 PM
ray2047's Avatar
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You can use self grounding receptacles or use a ground clip that slips over the box. Verify you have ground using a multimeter. Outlet testers can be wrong. Be sure you have ~120 volts.

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Old 10-26-13, 07:31 AM
pcboss's Avatar
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Self tapping screw typically do not have the required number of threads engaged in the metal. The correct screw is a 10-32.

If the box does not a a tapped hole your wiring may be too old to be used as a grounding means. The cable needs a bond strip or to be MC type cable.
Old 10-26-13, 12:41 PM
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Join Date: Mar 2012
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You can either use a self-grounding receptacle plus a ground clip or drill a small hole and use a self-tapping ground screw (which is a 10-32 screw). I prefer the self-tapping screws with pigtails because then the device is bonded to ground even when it's not mounted to the box.

That still leaves the question of whether the cable sheathing will provide an adequate path to ground. Assume it will. One important reason for installing a metal conduit system has always been to provide that path. The bonding strip was added when the better materials that had been used earlier "went to war."

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