Equipment grounding conductors for conducors in parallel

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thunder15j

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We are designing a 1600 amp,480 volt 3 wire plus ground (no nuetral) underground direct buried circuit using 535.5 MCM DLO/RHW cable for a temporary feed at a mining quarry. There will be four conductors per phase.

We plan on installing in a trench-4 groups of 3 wires-AB&C phases with a little spacing between groups to avoid 310-15 (b)(2(a) derating.I'm not certain of how much spacing is required, but one might interpret spacing as not touching another conductor.

Article 250-122(f)(1) adresses running a full sized equipment grounding conductor in each conduit or cable. However in the case of direct buried cable, nothing is mentioned. I was wondering what the intent of the code is when requiring a full sized ground in each conduit. I assume it has something do with inductive reactance and/or unequal division of current as noted in FPN in 310-4.

I am uncertain of how to handle the equipment grounding conductors in this installation. I could run a total of 1 egc per phase giving a total of 4. This does seem excessive.
I could just run one equipment grounding conductor and ziz-zag it from one side of the trench to the other to be in close proximity to all runs at one time or antother. I was also thinking of running 2 egc's layed between group #1&#2 and between #3&#4. This would keep a egc in close proximity to alll hot/phase conductors at all times.

Please add comments on our proposed installation especially about the equipment grounding conductors. thnx

Notes: 535MCM DLO is rated at 445 amps @ 90C ;we're replacing existing parallel 2 x 535MCM per phase that is currently fed by a 1200 amp breaker. We feel that this is a safety issue and also the existing feed is buried too shallow and is in constant danger of being uncovered. The rating of 535MCM DLO in free air is 725 amps and that is where the previous installer probably goofed.
 
Re: Equipment grounding conductors for conducors in parallel

250.122(F)(1). I'm pretty sure you need to run an equipment ground with each cable/conduit even if direct buried, so you would need 4 EGC's. If you are using a 1600 Amp OCD, then I believe you would need a 4/0 CU or 350 MCM Al EGC. Spacing of the conduits for proper derating (mutual heating) would be found by performing a Neher-McGrath calculation, or if applicable, check out Appendix B in the NEC.
 
Re: Equipment grounding conductors for conducors in parallel

Thunder that is an interesting question, I am not sure of the answer. I would, if possible simply run 4 full size grounding conductors spaced in between the other conductors to cover my +*#.

That said I do not see it as a requirement as you do not have a raceway or cable assembly.

This picture IMO illustrates the reason for the full size grounding conductor in each raceway or cable.

NB3.jpg


This was a 4" RMC with a set of 600 Kcmil copper.

As you can see a fault in the raceway (or cable) could sever the grounding conductor, while the other sets of parallel conductors continue feeding the downstream equipment.

All of that is just my 'guess' I do not know for sure the reason for the full size grounding conductor requirement.

I also do not know if direct buried cables are subject to the same risks.
 
Re: Equipment grounding conductors for conducors in parallel

Thinking more on this how are you handling each end where it comes out of the ground?

It must be going into raceways at each end I imagine.
 
Re: Equipment grounding conductors for conducors in parallel

I think that what you are describing would be a violation of 300.5(I). If the conductors are in a trench, then they must be ?in close proximity in the same trench.? You are not permitted to separate direct buried cables, in an effort to avoid the derating factors of 310.15(B)(2)(a). If you wish to avoid the derating, then you must pay the extra costs to install the four sets in conduits.
 
Re: Equipment grounding conductors for conducors in parallel

Is DLO/RHW suitable for directly buried applications? Also RHW is only 75?C wire in wet locations unless it is marked RHW-2.
I don't think that 250.122(F) applies to this installation and a single EGC should be code compliant.
Don
 
Re: Equipment grounding conductors for conducors in parallel

I am confused. If parallel cables are run in conduit, are you required to run an EGC in each conduit per 250.122(F)? Should it be sized for what is in the conduit, i.e. for one cable or sized for the OCPD device?

And if parallel cables are direct buried, then you can run one EGC sized for the OCPD, which in this case is for 1600 Amps, so from table 250.122 is a 4/0 Cu? How is this allowed?

And to my understanding, the intent of running an EGC in each conduit is so that each EGC can carry the available fault current back to the source. :confused:
 
Re: Equipment grounding conductors for conducors in parallel

Originally posted by wirenut1980:
I am confused. If parallel cables are run in conduit, are you required to run an EGC in each conduit per 250.122(F)? Should it be sized for what is in the conduit, i.e. for one cable or sized for the OCPD device?
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This part of your question I am sure of, each EGC in EACH raceway is required to be sized for the OCPD per Table 250.122.

Originally posted by wirenut1980:
And if parallel cables are direct buried, then you can run one EGC sized for the OCPD, which in this case is for 1600 Amps, so from table 250.122 is a 4/0 Cu?

How is this allowed?
Click to expand...
I am not sure what you mean, the one 4/0 Cu is large enough to operate the OCPD

Originally posted by wirenut1980:
And to my understanding, the intent of running an EGC in each conduit is so that each EGC can carry the available fault current back to the source. :confused:
Click to expand...
I agree.

My thought on the full size EGC in each separate raceway is this and it is only opinion.

If in a paralleled installation we where allowed to run an EGC that was sized only in relation to the conductors in the conduit a fault could happen in one raceway that severs the EGC in that raceway leaving the feeder with an undersized EGC while the other raceways are still intact supplying the load and the fault.

Remember that when a fault happens in paralleled raceway current is still coming into the fault from both directions until the OCPD opens.

[ February 11, 2005, 12:36 PM: Message edited by: iwire ]
 
Re: Equipment grounding conductors for conducors in parallel

I agree with Bob. But as is my style, I will say it in another (and more verbose) way. :roll:

I believe that the reason for a full-sized EGC in each conduit is related to the failure scenario of a fault within one of the conduits. Here?s why: Suppose you had three parallel conduits. Suppose each of the parallel EGCs were sized as one third the area of the EGC that is required for the OCPD. Therefore, the total resistance of the three parallel EGCs would be one third the resistance of any single conductor. Thus, the parallel EGCs would have the same resistance as a single EGC sized per Table 250.122. Now suppose you have a fault at the load. Then the fault current returning to the source will pass through the parallel set of EGCs as though they were a single conductor. This is why it seems strange to require a full-sized EGC in each conduit. It would seem that the resistance value would be just fine, if each of the parallel EGCs were smaller.

But now let?s consider a fault, specifically a fault from a phase to the EGC, internal to one of the conduits. This could happen if the conduit is cut by a digging machine. The resistance of the fault path is not the same as it is in the first scenario I described. The parallel EGCs will no longer share the fault current equally. The one that experiences the fault will see a higher share of the fault current. If it is not ?full-sized,? then it might not survive the experience. If this causes it to fail open before the OCPD trips, then the fault current will now pass through the remaining EGCs. If they are not full sized, they also might not survive the experience. This could cause them to fail open, and it could happen before the OCPD trips.

So how big do you need to make each of the parallel EGCs, before you can be sure that any one can handle the fault current imposed by a fault internal to the conduit? No one knows. But we can be sure that a full-sized EGC would do the job. So rather than calculate the minimum size (as a percentage of the full-sized EGC), it is simpler to just require a full-sized EGC in each conduit.
 
Re: Equipment grounding conductors for conducors in parallel

Thanks fellas, that really clears things up for me. And just to clarify, in the opening post, the paralleled cables are to be direct-buried and only require a single 4/0 Cu conductor equipment ground? :)
 
Re: Equipment grounding conductors for conducors in parallel

Guys, thanks for the input. More info on our layout. We propose digging a three foot wide trench about five feet deep. We will sand the bottom then lay in our conductors. Then we will put about a foot of sand on top of the conductors. The next step is to aid in future recovery of the conductors. We will lay in some type of identification/stop digging material, probably some abandoned belting material. Then we will top off the trench with sand, laying a metallic locating warnin tape made by Ideal industries near the top of the trench.

Conductors will be landed on busbar on the supply side, and probably a terminal block on the load side. Conductors will be protected by a PVC sweep line side and run up from the ground in free air into a portable elctrical control-distribution trailer that contains all the MCC equipment.

Nobody mentions anything about inductive reactance in the reasoning for all the required full sized EGC's. Sounds as though that probably isn't an issue.

The DLO cable is similiar to SOW, types "G" and "W" and is a single conductor rated for running above ground and underground.
 
Re: Equipment grounding conductors for conducors in parallel

The DLO cable is similiar to SOW, types "G" and "W" and is a single conductor rated for running above ground and underground.
Click to expand...
I just don't find anything in the manufacturers information that says any of those cables are suitable for directly buried applications.
Don
 
Re: Equipment grounding conductors for conducors in parallel

Don, my mistake in implying that SOW, G, and W are rated for being installed in the earth. I was refering to the above ground similarities only, and I need to verify the exposed above ground part for DLO. I have seen this cable used many times in this application in mining situations and portable AC plants. Only DLO is rated for placement in the earth. The product description mentions that when it is installed in a raceway or earth, then RHW ampacities are to be used. http://www.generalcable.com/North_America/NorthAMCat.html
 
Re: Equipment grounding conductors for conducors in parallel

I would like to bring the discussion back to one aspect of the original question and to my first comment. But I don?t have my code book at home. So can someone tell me if the proposed layout is legal?

300.5(I) says that the conductors must be in close proximity in the trench. Thunder15j wanted to give them some separation, to avoid the derating factors of 310.15(B)(2)(a). I believe that, at least, is not permitted. But do the derating factors even apply to a direct buried installation? If not, is there another section that deals with ampacity of direct buried cables, and that addresses having more than 3 current-carrying conductors in close proximity?
 
Re: Equipment grounding conductors for conducors in parallel

thunder,
I still don't find any statement the web site you gave or any others that DLO is listed for direct burial applications.
Don
 
Re: Equipment grounding conductors for conducors in parallel

Charlie b, if you go to : http://www.generalcable.com/North_America/NorthAMCat.html and then enter part # 81953 next to general cable part search it will take you to DLO spec sheet. Under the footnote description it is mentioned that when buried in the earth, one is to use RHW ampacities. However, table 310-6 doesn't have a listing for 535, but general cable lists the ampacity at 445 amps.

Don, regarding 300-5 (i); it could be argued that exception 1 could apply even though we are not using a raceway. In fact, this installation would have the same separation that one would have if conduit was used.

330-5 (i) exception 2 is one in which I hadn't considered. I was trained to always cluster phases A, B & C together in each conduit on all parallel runs. If were to use this exception, because of the fact that were are using four conductors per phase, I think we would have to derate to 80% as per table 310-15 (b)2a.
 
Re: Equipment grounding conductors for conducors in parallel

Thunder,
I don't think that the wording in the spec sheet is saying that this cable can be directly buried. The wording is just quoting the name of Table 310.16 as it appears in the NEC. I don't believe that this insulation type is suitable for directly buried applications.
As far as the separation issue, if this were an underground ductbank, I think that you would have to do the engineering calculations or apply the tables in Annex B. The underground duct bank layouts in Annex B call for 7.5" center to center on the raceways.
Don
 
Re: Equipment grounding conductors for conducors in parallel

First of all, I agree with Don?s assessment of the spec sheet. There is nothing that indicates this cable can be directly buried in earth.
Originally posted by thunder15j: Don, regarding 300-5 (i); it could be argued that exception 1 could apply even though we are not using a raceway. In fact, this installation would have the same separation that one would have if conduit was used.
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I disagree. 300.5(I) explicitly states that ?All conductors of the same circuit . . . shall be installed in close proximity in the same trench.? Exception 1 talks about parallel raceways. It does not allow for parallel trenches, nor for separation within the same trench.
330-5 (i) exception 2 is one in which I hadn't considered. I was trained to always cluster phases A, B & C together in each conduit on all parallel runs. If were to use this exception, because of the fact that were are using four conductors per phase, I think we would have to derate to 80% as per table 310-15 (b)2a.
Click to expand...
You cannot use Exception 2 either. It is explicitly addressing isolated phase conductors. That means putting Phase A in a separate raceway than Phase B, and putting Phase C in a third raceway. That is not the situation you are attempting to use.

Please note that 310.15(B)(2)(a) says that you must derate, when you have single conductors that are not in raceway and for which you do not maintain spacing. But 300.5(I) forbids ?maintaining spacing,? by requiring all conductors to be ?in close proximity.?

I still say that the bottom line is that your proposed installation would constitute a pair of code violation:
(1) The ampacity of four conductors is not enough for your load, since you would need to derate to 80%, and
(2) The conductor you propose to use is not rated for direct burial.
 
Re: Equipment grounding conductors for conducors in parallel

Don & Charlie b, thanks for the feedback.

I did find that at American Insulated wire corp.s website, one of their DLO cable had a "USE" marking.http://www.aiwc.com/catalogsection/prodsdisplay.htm then click on power cable on left side; then click on Misc: DLO & Ind motor lead cable.

Regarding the separation issue, one might consider using a triplexed assembly for each of the four runs, then use a 7.5 inch spacing between each run. They are all part of one circuit. Here again, every thing is almost the same as if it were in PVC, except the PVC and of course the full sized euqipment grounding conductor in each run.
 
Re: Equipment grounding conductors for conducors in parallel

thunder,
I see that only the 1000V rated DLO is marked USE. The last time I tried to buy any DLO, my supplier could only provide the 2000V rated version. I wonder why one is marked USE and the other isn't. They use the same construction and materials.
Don
 
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