Two phases by duct in underground raceway?
Two phases by duct in underground raceway?
(OP)
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
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






RE: Two phases by duct in underground raceway?
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.
RE: Two phases by duct in underground raceway?
Salvador Martínez
RE: Two phases by duct in underground raceway?
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.
RE: Two phases by duct in underground raceway?
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?).
RE: Two phases by duct in underground raceway?
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
RE: Two phases by duct in underground raceway?
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.
RE: Two phases by duct in underground raceway?
RE: Two phases by duct in underground raceway?
RE: Two phases by duct in underground raceway?
Salvador Martínez