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-   -   Grounding pigtails (http://www.diychatroom.com/f18/grounding-pigtails-177274/)

SquishyBall 04-16-2013 09:34 AM

Grounding pigtails
 
Had a thought while reading another post...

Why is it that when you use NM cable with metal boxes, you need to pigtail the grounds to the box and to the device... but when using conduit, the device does not need to be grounded to the box other than via the screws?

If the screws on the device are good enough to make the ground connection in a conduit setup, why are they not sufficient in an NM setup?

al_smelter 04-16-2013 10:02 AM

Conduit is considered the ground path (unbroken metal clear from box to panel). With NM, that metallic path is not there, so it uses the ground wire as the path.

Jim Port 04-16-2013 10:14 AM

From 2011 NEC 406.4(C) Methods of Grounding.
The equipment grounding conductor
contacts of receptacles and cord connectors shall be
grounded by connection to the equipment grounding conductor
of the circuit supplying the receptacle or cord connector.

SquishyBall 04-16-2013 12:13 PM

Quote:

Originally Posted by al_smelter (Post 1160375)
Conduit is considered the ground path (unbroken metal clear from box to panel). With NM, that metallic path is not there, so it uses the ground wire as the path.

Of course. But that's not the question.

So with NM, and say two wires in and out. ground 1 and ground 2 are joined with two pigtails - one to the metal box and one to the device.

With conduit, the metal raceway is the ground, fine. Yet you do not need to ground your device. The connection via the screws is sufficient.

Why is that screws connection not sufficient with NM with a grounded metal box?

Quote:

Originally Posted by al_smelter (Post 1160375)
contacts of receptacles and cord connectors shall be grounded by connection to the equipment grounding conductor

But with conduit it's ok to just use the screws for that. Why are the screws less effective w NM in a metal box. The metal box is just as grounded (from the pigtail) as it is w conduit (from the metal raceway).

mpoulton 04-16-2013 12:54 PM

I don't get it either, and have often wondered about this. It seems to me that a grounding connection should only be required in one location at each box. Bond the box, and the devices should be effectively bonded too. Bond a device, and the box should be effectively bonded too (and probably any other devices in that box). Pigtailing everything seems totally redundant. I would understand it if the device-to-box connection were never considered acceptable for bonding. But it is allowed if the box is grounded through conduit, so the box-to-device connection has been tested and found adequate under those circumstances. Why is that a special case? Why isn't that connection always considered good enough? I don't ever expect answers to these questions. The NFPA often seems like a phony psychic, handing out dubious advice with no explanation.

Dana56 04-18-2013 10:38 AM

Always pigtail grounds to box and device no matter what it dont take much and its good practice. Dont think anybody relies on conduit as ground anymore, u always pull a ground in pipe

stickboy1375 04-18-2013 05:17 PM

Quote:

Originally Posted by SquishyBall (Post 1160361)
Had a thought while reading another post...

Why is it that when you use NM cable with metal boxes, you need to pigtail the grounds to the box and to the device...



What makes you think you have to?

stickboy1375 04-18-2013 05:22 PM

Quote:

Originally Posted by Dana56 (Post 1161917)
Always pigtail grounds to box and device no matter what it dont take much and its good practice. Dont think anybody relies on conduit as ground anymore, u always pull a ground in pipe

Thats a nice opinion you have....but not required.

stickboy1375 04-18-2013 05:26 PM

Quote:

Originally Posted by mpoulton (Post 1160471)
I don't get it either, and have often wondered about this. It seems to me that a grounding connection should only be required in one location at each box.

And in some cases that's all you are required.


Quote:

Originally Posted by mpoulton (Post 1160471)
Bond the box, and the devices should be effectively bonded too.

This is true if you are using self grounding receptacles, switches are allowed to be bonded with the 6-32 alone....


Quote:

Originally Posted by mpoulton (Post 1160471)
Bond a device, and the box should be effectively bonded too (and probably any other devices in that box).

You cannot have the device solely bond the box, think about it, what if the device is removed?

stickboy1375 04-18-2013 05:33 PM

http://i171.photobucket.com/albums/u...75/graphic.jpg

http://i171.photobucket.com/albums/u...receptacle.jpg



I know the pictures show EMT as the wiring method, but any wiring method can be applied to this.

mpoulton 04-18-2013 06:30 PM

Quote:

Originally Posted by stickboy1375 (Post 1162201)
You cannot have the device solely bond the box, think about it, what if the device is removed?

That is a good point. I suppose the code writers are concerned that someone would remove the device that is bonding the box, and replace it incorrectly.

Jim Port 04-18-2013 09:02 PM

Quote:

Originally Posted by stickboy1375 (Post 1162206)
http://i171.photobucket.com/albums/u...75/graphic.jpg

http://i171.photobucket.com/albums/u...receptacle.jpg



I know the pictures show EMT as the wiring method, but any wiring method can be applied to this.

Note that only applies to surface mounted boxes and certain covers. Flush mount needs both to be bonded together.

Dana56 04-18-2013 10:03 PM

Quote:

Originally Posted by stickboy1375

Thats a nice opinion you have....but not required.

Glad u like it... Like to go a little more than "minimum requirement" never know if set screws are properly tightened to ensure good ground thanks for input

k_buz 04-18-2013 10:35 PM

Quote:

Originally Posted by Jim Port (Post 1162330)
Note that only applies to surface mounted boxes and certain covers. Flush mount needs both to be bonded together.

This is true, however, they do make listed self-grounding receptacles that would not need a ground run to it.

Dana56 04-18-2013 11:19 PM

Bonding bushings or locknuts mite be required too if the box has concentric knockouts just another nice opinion :))


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