Two phases by duct in underground raceway? 1

[salvadormm]

We’re thinking in taking advantage of an already constructed underground raceway (duct bank) of roughly 1 km (0.625 miles), with 4” ducts build in polyvinyl chloride (PVC), for installing a new medium-voltage circuit (13.8kV) of 5 conductors per phase of 500kcmil in size with feeder configuration 1/C. Three single phases are not allowed to fit in a 4 in duct (in compliance with NEC specifications), only two phases would fit, so we then would have to install phases R and S in one duct, phases S and T in the next duct, and successively. Also, two cables of the same phase should be fitted in one single duct. Otherwise, a new underground raceway with 6 in ducts should be constructed, but this is bounded due to economical constraints.

Using the actual configuration with two cables by duct, we would have higher magnetic fields because phases R, S and T are not longer together and magnetic field cancellation will no longer be achieved. Despite the non-metallic underground raceway, we would like to know if there are any important thermal effects in cables jackets that could have a immediate effect in the normal operation of the system, or any other side effect that could arise because of the use of this non-conventional configuration.

Thanks in advance for the help and analysis in this issue.

Sincerely,

Salvador Martínez

 

[davidbeach]

In that situation I'd go with two conductors of the same phase in each of three ducts. Single conductor per duct is a common installation practice, two conductors of the same phase would build on that but loose the advantages of being able to remove and replace a single conductor; but you've lost that already. A less expensive installation would have been twice as many 2" ducts and one conductor per each.

Regardless of whether you go with the above or with your proposal it is imperative that there be no ferrous metal that forms a complete loop around any group of conductors less than an equal number per phase. That could be re-bar cages in the duct bank, it could be steel elbows in the PVC ducts, it could be any of a number of other things, but if present it will cause enough heat to fail your insulation.

 

[salvadormm]

Thank you very much davidbeach for your help and your quick response.

Salvador Martínez

 

[jghrist]

For this long of a run, I think it would be worthwhile to analyze each option, either with hand calculations of appropriate software to see the effect on circuit impedance and thermal rating.

Generally, the closer spaced the phases, the lower the impedance. There will also be less circulating currents in the cable shields and less heating with closer phase spacing. In the normal single cable per phase application, closer phase spacing also means closer cable spacing and more mutual heating, but in the present case, mutual heating will not be affected.

Alternating phases with different phase in each duct would effectively reduce the phase spacing an may reduce impedance and increase the thermal rating.

 

[BJC]

"only two phases would fit.."
I just looked at my Okonite catalog and 15 KV -500MCCM with 220 mil ( 133% ) insulation has a diameter of 2.033"
4" Sch 40 PVC conduit has a inside diameter of 3.998". That's "jamming" in the extream.
My calculator says to get two cables in a 4" duct the cables have to have a diameter of 1.78" ( that's for a 40% fill). I don't know who makes 15 KV, 500 MCM cable that small.
Am I reading your intentions correctly?
If your using the NEC the cables may have to be derated becasuese of the duct bank configuration ( your going to use 8 conduits?).

 

[resqcapt19]

BJC,
The NEC only permits a 31% fill for 2 conductors. The 40% fill is for 3 or more and it also permits a 53% fill for a single conductor. Chapter 9, Table 1 of the NEC
Don

 

[BJC]

resqcapt19
Your right, I should have written 31%.
Still to get 31% fill or less the cables would have to be about 1.55" dia. Hard to find for 500 MCM 15 KV cable.
I think its time for a backhoe.

 

[jghrist]

Cablec Unishield EPR shielded power cable MV-90 15 kV 100% insulation level 500 kcmil overall diameter = 1.38"

 

[BJC]

If he can get that cable (1.38") he can put three of them in one duct. Cablec is no more, correct? They went to BICC and are not General Cable.

 

[salvadormm]

Thank you very much Don, jghrist, BJC for your comments. The commercially available cables at our disposal have an outside diameter of 1.5" (with two cables filling about 32% of a 4 in duct) approximately, however, your observations have been heard and now we’re evaluating the use of 750kcmil cables, in single cable ducts, in compliance with the NEC specifications.

Salvador Martínez