what is the best way to run parallel conductors
what is the best way to run parallel conductors
(OP)
I have to run 12 - 750MCM service conductors underground into my switchgear. What configuration of the conductors will give me the best performance?
The run will be about 60'. I would like to avoid derating the conductors as much as possible. Code states that parallel conductors installed in non-metallic raceways underground may be arranged to isolate each phase, but that the raceways should be kept as close as possible to one another.
Since I would like to keep to 3 conductors per raceway (if I use raceway) that would mean four raceways per phase. What arrangement would keep the phases close together, 3 bundles of 4 for each phase and treat each bundle as a single conductor?
If I went direct burial with the conductors and laid them all out next to each other in a single layer what derating should be used and which ampacitiy table would I select from.
thanks in advance,
Doug
The run will be about 60'. I would like to avoid derating the conductors as much as possible. Code states that parallel conductors installed in non-metallic raceways underground may be arranged to isolate each phase, but that the raceways should be kept as close as possible to one another.
Since I would like to keep to 3 conductors per raceway (if I use raceway) that would mean four raceways per phase. What arrangement would keep the phases close together, 3 bundles of 4 for each phase and treat each bundle as a single conductor?
If I went direct burial with the conductors and laid them all out next to each other in a single layer what derating should be used and which ampacitiy table would I select from.
thanks in advance,
Doug






RE: what is the best way to run parallel conductors
RE: what is the best way to run parallel conductors
1. For 3-Phase 3-Wire system
4 conduits, 3#750MCM/conduit
2. For 3-Phase 4-Wire system
3 conduits, 4#750MCM/conduit, if there is a small harmonic content in the neutral, thus 750MCM neutral suffices.
RE: what is the best way to run parallel conductors
RE: what is the best way to run parallel conductors
Thermal derating is based on spacing, soil characteristics and ambient temperature. Maximize spacing to reduce required deratings. 3 or 4 ducts normally require fairly minimal derating however.
Thread238-53067
Thread238-9203
RE: what is the best way to run parallel conductors
jbartos did you mean each phase in a seperate raceway or the arrangement hsj suggests. If I do use raceway I do plan on using non-matallic which code then allows each phase to be run seperately. Also is the use of a duct bank versus direct burial done to allow easier amintenance or are there other reasons for that?
thanks all for the replies,
Doug
RE: what is the best way to run parallel conductors
RE: what is the best way to run parallel conductors
1. For 3-Phase 3-Wire system
4 conduits, 3#750MCM/conduit
normally means 3 conductors per conduit, each conductor in the conduit belongs to one and different phase of the three phase system. It is evident since there are not 4-phase systems widely used.
RE: what is the best way to run parallel conductors
As a practical matter for the described cable size, use of increased standing with listed RHw-2/DLO cable including appropriately sized compression terminals may simplify installation.
RE: what is the best way to run parallel conductors
Conduit provides protection against various types of damage that may occur (dig-ins, rodents, vehicle traffic, foundation settlement, etc) and allows for easier replacement if needed. Direct burial of important feeders in industrial environments is unusual.
DLO cable is easier to handle, but has thicker insulation and typically does not have nylon jacket. Also, fine stranding may require special lugs.
RE: what is the best way to run parallel conductors
RE: what is the best way to run parallel conductors
Can you tell me what you are basing the derating of duct bank configuration on? Is it by Neher-McGrath formula or is there a section of NEC code I am overlooking?
thanks,
Doug
RE: what is the best way to run parallel conductors
RE: what is the best way to run parallel conductors
Get a copy of the Siemens Handbook ('66 vintage) on Power Cables. One section is devoted to multiple-cable and phase configuatrions to yield minimum reactance without unnecessarily increasing loss.
RE: what is the best way to run parallel conductors
If your cable project is not "fait accompli" why not consider a busbar system. A bundle (excuse the pun) in material alone can be saved!
RE: what is the best way to run parallel conductors
Good idea but, the installation will be underground and I am not aware of a bus system that can be installed underground.
Can anyone tell me where I can find the Neher-McGrath paper? I can not find it on the IEEE site.
Thanks,
Doug
RE: what is the best way to run parallel conductors
I would suggest instead either the Siemens Power Cable book recommended by Shortstub or "Rating of Electric Power Cables" by George J. Anders, McGraw Hill/IEEE Press, 1997, IEEE ISBN 07803-1177-9. Both are based on Neher-McGrath's method but are more comprehensive than the original paper.
RE: what is the best way to run parallel conductors
RE: what is the best way to run parallel conductors
http://www.nema.org/help/docno.html
http://www.nema.org/index_nema.cfm/1427/4613F7A0-ABCC-4...
etc for more info on ampacity calculations
RE: what is the best way to run parallel conductors
Undergroud busduct(480 V 4500A) can be the best solution .The trench for busduct shall be of sufficient size for easy installation of the busduct .At least 4" gap between busduct outer and cable trench in all directions for better heat dissipation. The trench can be covered with slabs as usual. In My experience once a busduct crossed a road through underground . That was in a cement factory . I remember the rating as 480V 3000A.
The advantages of busducts are plenty it is like 'fit and forget'
Voltage drop problem will not arise as the solid busbars can be always slightly over rated. almost zero failure.
Better contact a busduct manufaturer they can supply your 60 feet busduct in a short time suitablle for bottom entry to switch gear and running underground.
Best Regards
Sthapathi
RE: what is the best way to run parallel conductors
RE: what is the best way to run parallel conductors
RE: what is the best way to run parallel conductors
What we have finally settled on is to set underground vaults below the switchgear and below the utility transformer, with a 4 foot I.D. Concrete pipe connecting the vaults. Inside of the pipe I will run my conductors via cable tray. This allows me to run 9 conductors per phase and not have the huge derating problem encountered by a duct bank. We do not think the temperature inside the pipe will reach 40C, but if it does we will use a fan to ventilate the vaults and pipe.
We have also decided to move the switchgear outside of the building and next to the utility transformer. We will build a building around the switchgear to protect it from the elements. My service conductor runs have gone from 220' to 25'much more manageable.
Feeders from the switchgear will go above ground in cable tray to the various loads.
Any comments or suggestions on the above mentioned approach?
Thanks Again,
Doug
RE: what is the best way to run parallel conductors
RE: what is the best way to run parallel conductors
One other thought- a fault in a the cable tray can damage all of your conductors, whereas in separate ducts it would more likely be confined to one set. Probably not a deal-breaker, but worth considering.