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

 

[advidana]

you cannot separate phases in each conduit. emf forces will force the cables together causing the insulation to be damage. I saw this type of installion above grade the conduit crushed each other when the power was turned on.

 

[prk]

advidana- it is interesting to know that ducts getting crushed, could you please provide more details like no. of ducts, size of cable runs, configuration etc.

 

[dougjl]

Thank you all for the comments and suggstions.

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

 

[Shortstub]

Good "Out of the box" thinking!

 

[alehman]

Good idea. You might consider putting a fire stop in your duct to help prevent a problem on one end from damaging the other. Avoid aluminum cable tray unless you can keep it very dry.

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.