Separately derived systems share EGC?

[m. downey]

This is my first post here, as it's the first time I can remember having a question that I could not find an answer to in the NEC or this forum. I am citing NEC 2014.

We are replacing the lights, and the wiring for the lights in an existing GRS conduit installation. The conduit contains 4 conductors, a hot and neutral from two different separately derived systems (two transformers). The lights are fed from the two circuits in alternating fashion. There are no EGCs currently installed (this work was done 40+ years ago). Obviously we must run a new ground back to each panel with the new wiring. Can can this ground be shared at any point in the circuit, such as along the shared GRS conduit run? The reason for doing this is that an additional ground wire would violate the conduit fill limit.

I cannot find a code section that specifically permits or forbids this, the closest thing I can find is 250.30(A)(6), which permits the sharing of a GEC for separately derived systems:

(6) Grounding Electrode Conductor, Multiple Separately Derived Systems.
A common grounding electrode conductor for multiple separately derived systems shall be permitted. (...)

This seems to be functionally equivalent to me, as the grounds from two systems are bonded together, but I don't see a similar rule for EGC. Any insight is appreciated, thanks in advance.

 

[LarryFine]

Welcome to the forum.
Not only is a single shared conductor okay, you can still use the conduit as a shared EGC if it's intact.

 

[m. downey]

Thanks Larry. Is this explicit anywhere in the code?

 

[ActionDave]

This is my first post here, as it's the first time I can remember having a question that I could not find an answer to in the NEC or this forum. I am citing NEC 2014.

We are replacing the lights, and the wiring for the lights in an existing GRS conduit installation. The conduit contains 4 conductors, a hot and neutral from two different separately derived systems (two transformers). The lights are fed from the two circuits in alternating fashion. There are no EGCs currently installed (this work was done 40+ years ago). Obviously we must run a new ground back to each panel with the new wiring. Can can this ground be shared at any point in the circuit, such as along the shared GRS conduit run? The reason for doing this is that an additional ground wire would violate the conduit fill limit......

What is wrong with the GRS conduit being used for an EGC?
What kind of lights and lighting circuit is this that has conduit that only has room for five wires?

 

[ActionDave]

Thanks Larry. Is this explicit anywhere in the code?

250.118(2) lists Rigid metal conduit as an equipment grounding conductor.

 

[m. downey]

What is wrong with the GRS conduit being used for an EGC?
What kind of lights and lighting circuit is this that has conduit that only has room for five wires?

the client (agency) is requesting a wire type.
this is 3/4C with XHHW-2 #8 for voltage drop (~1300').

250.118(2) lists Rigid metal conduit as an equipment grounding conductor.

I agree. Does the code also explicitly permit two or more separately derived systems to share an equipment grounding conductor?

 

[ActionDave]

I don't know of any place in the code that explicitly mentions two or more separately derived systems sharing an equipment grounding conductor, however, in order to be code compliant all equipment grounds in the same structure or fed from the same building including separately derived systems must be bonded together, i.e. shared, so I don't see a need for any specific language.

 

[LarryFine]

Code prohibits rather than permits, for the most part.

 

[kwired]

the client (agency) is requesting a wire type.
this is 3/4C with XHHW-2 #8 for voltage drop (~1300').



I agree. Does the code also explicitly permit two or more separately derived systems to share an equipment grounding conductor?

Client's request makes it a design requirement, NEC is fine with metal raceways used for EGC purposes. Is pretty common for wire type EGC to be specified these days though.

All GEC's and EGC's ultimately are required to be bonded together in different areas of the code - the reasoning is to put them all at same potential, as well as other items that are required to be bonded to the grounding system. If everything is at same potential shock hazards decrease when you touch two different items at same time.