Sub-panel Ground not to Service Equip

[shaner]

There are 4 wires coming into the sub-panel in question (from the service panel on exterior of townhouse), but there is another bare copper wire connected to the ground bar that leaves the bottom of the sub-panel. Where it goes is a mystery. I'm assuming it is going to a water line connection.

Otherwise, the ground/neutral separation in the sub-panel appears correct, and service panel is properly grounded.

Can a sub-panel have a bare copper ground wire running to a water pipe connection, or some other type of secondary electrode that is separate from the service panel ground?

Thanks for your help!!

 

[al hildenbrand]

Take a look at 250.54.

 

[shaner]

So, if I'm reading correctly, if the main ground rod is bonded to the water pipe, then the sub-panel can be bonded to the water pipe? And if so, how close do these bonding connections have to be relative to one another?

 

[macmikeman]

That water pipe might have some pvc stuck in the middle of it somewhere withing the building someplace. That would cause one section to be unbonded unless a separate bond was established.

 

[jwelectric]

That water pipe might have some pvc stuck in the middle of it somewhere withing the building someplace. That would cause one section to be unbonded unless a separate bond was established.

If there was a non-metallic piece of pipe installed why would the other part of the metal pipe need bonding?

 

[George Stolz]

While not required, it may have been desired for some reason. So long as such a connection is not prohibited, they can bond almost anything they want.

250.104(B), FPN: Bonding all piping and metal air ducts within the premises will provide additional safety.

 

[LarryFine]

Could ths panel have originally been the main panel?

 

[jamesguy10]

It sounds like whoever installed it doesn't understand grounding, and thought he was doing a world of good by grounding the subpanel again

 

[al hildenbrand]

shaner,

Your opening question is different from the later two questions, as I read them. I'm focusing on the first question.

The NEC Handbook has the following explanatory text for 250.54:

Grounding electrodes, such as ground rods, that are connected to equipment are not permitted to be used in lieu of the equipment grounding conductor, but they may be used for supplementary protection at electrical equipment locations. For example, grounding electrodes may be used for lightning protection or to establish a reference to ground in the area of electrically operated equipment. Sections 250.4(A)(5) and 250.4(B)(4) also specify that the earth not be used as the sole equipment grounding conductor or effective (ground) fault current path. Supplementary grounding electrodes are not required to be incorporated into the grounding electrode system for the service or other source of electrical supply.

In the subpanel you describe in your opening post:

there is another bare copper wire connected to the ground bar that leaves the bottom of the sub-panel. Where it goes is a mystery.

It is unfortunate that we don't know what the EGC (equipment grounding conductor) connects to. It has two probable purposes, (1) bond to a conductive assembly, as George points out with 250.104(B) FPN, or (2) bond to some kind of electrode. Neither will be part of the GES (grounding electrode system) required for the service.

 

[smithacetech]

Is it a #8 solid wire? Perhaps it goes to a jetted tub? I remember an argument that the ground/bond could go to the feeding panel instead of the service.

 

[benaround]

Could also be used for the telco system.

 

[George Stolz]

I dunno, Frank - I just glanced over 250.94, which sent me back to 800.100, which didn't seem to allow a random metal water pipe inside the structure to bond the telco system.

But I am stepping out on a very precarious edge here, and I can smell crow from where I'm standing.

 

[al hildenbrand]

In support of your point, George, the NEC Handbook includes two exhibits and a lot of explanatory text for 250.94. Exhibit 250.39 shows a communications grounding electrode seperate from the GES (grounding electrode system) and says that it "does not comply" with 250.94.

In the same body of Handbook explanatory text one also finds Exhibit 250.40 showing the same communications system as in Exhibit 250.39 with a bonding jumper added between the communications grounding electrode and the GES. The bonding jumper allows the system to comply with 250.94.

To follow on Frank's idea, the bare copper coming from shaner's subpanel could be extending the feeder EGC to the communications ground, thus bonding the communications equipment back to the MBJ (main bonding jumper) and the GES.