what is the best way to run parallel conductors

[dougjl]

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

 

[rbulsara]

See thread238-74022 and NEC 310.

 

[jbartos]

Suggestion:
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.

 

[hsj]

Best to run R-Y-B-N together. Thant means three bundles in your case. Each bundle containing one conductor of each phase and one neutral conductor. This arrangement will keep the electomagnetic field to a minimum.

 

[alehman]

I agree with jbartos and hsj regarding grouping of 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

 

[dougjl]

Sorry I was not clear about the 12 conductors, I meant 12 conductors per phase at 480 volts 4500A, 4 wire. I am limited to 16 conductors per phase at a maximum conductor size of 750 MCM at the transformer connection supplied by the utility.

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

 

[jghrist]

Keeping 3 phases together will minimize both the total impedance and the phase imbalance of the impedance. Twelve 3ø cable groups can be arranged to optimize cost and mutual heating.

 

[jbartos]

Suggestion/clarification:
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.

 

[busbar]

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.

 

[alehman]

For 12 conductors per phase, you will need to carefully plan the duct configuration for the derating to be tolerable. For a 6 by 2 configuration of bundles or ducts you might be looking at somewhere around 50 to 70% of the NEC 310-16 capacity. 4 by 3 would be much worse. Use insulation with a "-2" suffix (e.g. THWN-2) to allow use at 90C in the wet environment.

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.

 

[jbartos]

Suggestion to the previous posting: The 6 by 2 duct bank configuration has a better thermal dissipation than 4 by 3. Also, 2 conduits in duct bank run will be spare, which is a good practice.

 

[dougjl]

alehman,
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

 

[alehman]

My comment was based on past experience with N-M calcs which is also used for calculating the tables in Appendix B of the NEC. Results depend highly on thermal resistivity, ambient temperature and load factor.

 

[Shortstub]

Dougil,

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.

 

[Shortstub]

Dougil,

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!

 

[dougjl]

shortstub,
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

 

[jghrist]

The Neher-McGrath paper is the basis for most calculations, but is dated and may be difficult to find. I lent my ancient photocopy to someone (don't remember who) years ago and never got it back.

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.

 

[Shortstub]

Many years ago, I used encapsulated bus-bar and concrete trench system. Just had to include specialized civil engineering to preclude water entry. Trench dimensions much smaller than equivalent cable system.

 

[jbartos]

Suggestion: Visit

http://www.nema.org/help/docno.html

http://www.nema.org/index_nema.cfm/1427/4613F7A0-ABCC-4B80-AB7D5B32E29B8436/

etc for more info on ampacity calculations

 

[sthapathi]

Dear Doughil,
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