Parallel EGC

[nhee]

250.122(F) requires an individual EGC be run with each set of parallelled phase conductors where conductors are run in parallel in multiple raceways.

What is the technical/theoretical basis for this requirement. For instance in parallel underground, non metallic raceways, what is the danger of installing a single EGC in its own non-metallic raceway (other than a violation of 250.122F)?

 

[Pierre C Belarge]

If there is a fault in one of the sets of conductors that are installed in parallel , the EGC can potentially carry the fault current for the OCPD ahead of the parallel conductors.

 

[nhee]

Thanks Pierre.

If it is only phase conductors in the individual conduits, which are non-metallic, what are the possibile causes of a line-ground fault (in the parallel runs where the EGC would be missing)? I guess a dig-in?

 

[petersonra]

Thanks Pierre.

If it is only phase conductors in the individual conduits, which are non-metallic, what are the possible causes of a line-ground fault (in the parallel runs where the EGC would be missing)? I guess a dig-in?

Good question. In such a case it is doubtful the connection to earth itself would cause the OCPD to open. Of course, a L-L fault would.

The general rule is that you have to run a EGC with conductors.

And when you run parallel conductors you have to run them substantially the same. the same length, same # and type of conductors, same raceway. Allowing you to run one of the parallel sets of conductors without an EGC would make the parallel runs different, and might result in slightly different impedance between the runs. That could result in current overloading one of the parallel runs.

So the answer is that it would be safe with respect to a ground fault condition, but potentially might result in an imbalance of current between the two runs leading to a hazard.

 

[charlie b]

Allowing you to run one of the parallel sets of conductors without an EGC would make the parallel runs different, and might result in slightly different impedance between the runs. That could result in current overloading one of the parallel runs.

Except that the EGC does not carry current (except during a fault) and that it is not electrically in parallel with the phase conductors. It would have no influence on the impedance of the other conductors in the conduit.

I can't think of a good reason why this would matter if the conduit were PVC. The only "not so good reason" I can come up with is that requiring it for all parallel runs is easier than making exceptions for certain types of raceways.