# sub panel ground bars and grounding metal boxes



## jkfox624 (Jun 20, 2009)

2 things i've always wondered about. I'm no electrician but why does a metal box need a ground tap in it? If you attach the circuit ground to the receptacle screw, its attached to the metal strap which you run metal screws through into a metal box. Isnt this in essence achieving the same thing? And why does a sub panel need the grounds seperate from the neutrals but the main panel doesnt? Electricians get tired of answering my questions so i'll spread some out here. :laughing:


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## 480sparky (Feb 1, 2009)

jkfox624 said:


> 2 things i've always wondered about. I'm no electrician but why does a metal box need a ground tap in it? If you attach the circuit ground to the receptacle screw, its attached to the metal strap which you run metal screws through into a metal box. Isnt this in essence achieving the same thing?


Until the device is removed.



jkfox624 said:


> And why does a sub panel need the grounds seperate from the neutrals but the main panel doesnt? Electricians get tired of answering my questions so i'll spread some out here. :laughing:


To provide a positive path to ground (ie, back to the transformer) for any fault current that may be imposed on the system. Otherwise, it can only follow the neutral, which may burn things up.


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## Zinsco (Oct 14, 2009)

1. If the device is self grounding, no grounding pigtail is needed.

2. On a sub panel, if the neutral and ground are connected, current will flow through the metallic parts, conduit, etc between the sub panel and the main panel. This is very hazardous, I've received a bad shock by uncoupling a pipe that was energized as a result of this. At the main panel, the service conductors are present and the metal parts will not have potential.


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## JohnJ0906 (Jan 7, 2007)

jkfox624 said:


> 2 things i've always wondered about. I'm no electrician but why does a metal box need a ground tap in it? If you attach the circuit ground to the receptacle screw, its attached to the metal strap which you run metal screws through into a metal box. Isnt this in essence achieving the same thing?


 As soon as the device is removed, the box is no longer grounded.


> And why does a sub panel need the grounds seperate from the neutrals but the main panel doesnt? Electricians get tired of answering my questions so i'll spread some out here. :laughing:


The proper question is, why do the grounds and neutrals connect at the main service entrance, when they are separate everywhere else?
Neutrals carry current, grounds only carry current when there is a ground fault. That is why we separate them almost everywhere. We don't want a current path through conduits, water pipes, equipment frames, etc. If they are connected at, say, a receptacle, some of the current will use the ground as a path. Generated electricity takes all possible paths back to source, so we want the neutral to be the only possible path.
For the circuit breaker to operate under a ground fault condition, there has to be a circuit, of low enough resistance to have high current flow. That path is through the service neutral back to source. That is why we have to bond the grounds to the neutral at the main service entrance. If, say, the hot wire in an appliance shorts to the frame, the current will flow from the hot, to the frame, back through the ground, all the way back to the service entrance, back through the neutral to the transformer, through the windings and back to the service on the hot. This should be a low resistance path, resulting in high current. The breaker sees this high current, much higher than its rating, and trips.


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## woodchuck2 (Feb 27, 2008)

jkfox624 said:


> 2 things i've always wondered about. I'm no electrician but why does a metal box need a ground tap in it? If you attach the circuit ground to the receptacle screw, its attached to the metal strap which you run metal screws through into a metal box. Isnt this in essence achieving the same thing?


As 480 stated if the device is removed then there is no ground. Also IMO if the device is grounded but the box is not and the short to ground occurs in the box then you are relying on the device screws to complete that short to ground. If the device is loose for any reason then the box itself may remain hot without the breaker tripping.


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## jkfox624 (Jun 20, 2009)

woodchuck2 said:


> As 480 stated if the device is removed then there is no ground. Also IMO if the device is grounded but the box is not and the short to ground occurs in the box then you are relying on the device screws to complete that short to ground. If the device is loose for any reason then the box itself may remain hot without the breaker tripping.


It all makes sense when explained, just sometimes i see this stuff and think wtf without really thinking it through.


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## tombs31 (Apr 18, 2010)

Even if it is a self grounding recept. the box needs to be bonded.


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