Grounding Electrode Conductors in Multicore Cables

[cod]

A marina floating dock with a large 1-phase transformer is being fed from shore two multiconductor cables each of which incorporates 4-#1 bare ground conductors (83,690CM each). The transformer secondary neutral must connect to an electrode grounding wire that in this case must be sized as the equivalent of a 3/0 cable (167,800CM). The 4-#1 ground conductors in one multiconductor cable total almost 335,000CM when they are paralleled, and would be more than suitable for a system grounding electrode conductor, sizewise. However, the NEC rule for paralleling conductors states that conductors cannot be paralleled if smaller than 1/0 which seems to rule out using the #1 ground conductors in the cable as grounding electrode conductors. Are there any exceptions to this rule and what is it based on? It doesn't seem logical if all of the paralleled conductors are exactly the same size and length in one common cable.

Any help would be appreciated.

Charlie

 

[waross]

Is it too late to consider upgrading the supply cables so that the grounding conductors will be 1/0.
Another option may be to use a 3 conductor construction for one of the cables and use the spare conductor for a system grounding conductor.
I'm in Canada. I would reread the marina section of the code to see if there are any special considerations.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cod]

Thanks Bill for your reply;

The actual design wasn't fully described in my message. What we are doing to serve the large 1-phase 480-volt transformer is to use a 4/c cable with two of the conductors paralled, then to parallel this cable with a second one that is the same. Thus we have 8 paralleled conductors, 4 for each hot leg.

It appears that the only solution to get a grounding electrode conductor from shore to the float is to run a separate wire which is truly ridiculous since there are 8#1 conductors available that would serve just fine. But the Code is the Code and as far as I can see, there is no way around it.

Thanks.

Charlie

 

[dpc]

Are both these cables in the same conduit?

If you have two separate conduits, each conduit needs to contain a fully sized ground wire.

I'm not really clear on the installation details, or the type of cables. But there are good reasons for the NEC limit of #1/0 for paralleled conductors (in general).

However, you can always apply for a variance and make your case to the local AHJ. Running a ground wire external to the existing conduit is not recommended since the reactance seen by a ground fault can be quite high. The ground wire sizing in the NEC is based on making sure the ground wire will pass enough fault current to trip the phase overcurrent protection (before the ground wire melts).



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[resqcapt19]

DPC,
The conductor in question is a grounding electrode conductor and not an equipment grounding conductor. There is no code or electrical reason to run the GEC in each cable or have a full size one in each cable.

Charlie,
I am not so sure that 310.4, and its 1/0 minimum for conductors install in parallel, applies to the grounding electrode conductor. However you will have to use some or all the grounding conductors as the primary EGC. If the feeder originates at the service equipment, I would ask the AHJ for permission to parallel all of the grounding conductors and use them as both the primary equipment grounding conductor and the secondary grounding electrode conductor.

 

[rbulsara]

Am I missing something?

It appears that the transformer is on the dock. The conductors from shore to the xfmr are primary conductors and the #1 ground conductor in there are the equipment grounding conductors (EGC) for the primary circuit. Hopefully it is sized correctly as primary EGC and bonded to the xfmr frame.

The secondary side grounding electrode conductor (GEC) is only required to be bonded to the frame and if the frame is effectively grounded, it will suffice. But I am not sure primary EGC are can be substituted for it.

In theory if everything (non current carrying parts) on the dock is bonded to the tranformer frame, you would be fine and no need to run GEC to the shore. If this is not acceptable to AHJ, I would think you need to run an separate GEC, which has nothing to do with primary EGC to the grounding electrode on the and can be run independent of the primary cables.


Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

resqcapt19,

You're right - I was thinking about EGC when I wrote that. I've had so many battles with EGC sizing over the years, I gave a knee-jerk response without reading carefully. Thanks for the correction.

Dave

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)