installation of cables 6

[rmullin]

My cable sizing calculations (to NEC standards) have told me that I need 4 x triplexed circuits in one trench for a particular situation. For ease of installation it would be desirable to have 3 'groups such that the arrangement has the 4 'reds' grouped together, likewise with the other two phases. I think there could be a problem here though in terms of increased reactance, and overheating of cables. If someone could confirm my suspicions that would be appreciated.

 

[DanDel]

Your concern is justified. NEC 300.3(B)(1), 310.4, and maybe 392.8(D) address this; there is an exception for non-metallic raceways in 300.3(B)(1) which may apply.

 

[waross]

If an unbalanced current flows through a "hole" in a magnetic material it will cause heating in the magnetic material. This hole may be a steel conduit or it may be a hole in a steel cabinet.
This will be an issue if the phase bundles enter the switch gear through seperate holes. It will be a more serious issue if the phase bundles pass through steel conduit for mechanical protection.
I have seen serious heating damage to a 400 amp switch with a 200 amp load.
The switch was fed by parallel conductors in rigid steel conduit. "A""A""B" in one conduit and "B""C""C" in the second conduit.
respectfully

 

[davidbeach]

What they said. Any metal around a net current will be trouble. A single steel elbow in an otherwise all PVC run of conduit will create enough heat to destroy the cables in a relatively short time. Unless you can have an entirely non-metallic pathway, it is best to group the conductors such that you have the same number of each phase conductor in each group.

 

[RalphChristie]

http://www.bruggcables.com/pdf/Brugg_Cables_User_Guide.pdf

Not necessarily the same cable or voltage-level you are using, but some of the info might be helpful.

Regards
Ralph

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[ausphil]

RalphChristie, good brochure.

Rmullin, Schneider puts out a good document about heating in cables due to skin and proximity effects. It outlines the eaxample of what you are talking about. We have also seen with IR cameras examples where people run all the same phases together in a flat formation, and wonder why their cables are runnning very hot.

The brochure is at the following site.

http://www.schneider-electric.com/cahier_technique/en/pdf/ect83.pdf

Ausphil

 

[mc5w]

Where your nonmetallic conduits pass through the metal wall of the box you need to cut slots between the holes so that as far as the magnetic field is concerned, what you have is a big hole. Grind off the burrs and then push some red type 1 RTV silcone sealant through the slots and let it harden ( without smoothing it out ) so that the electrical inspector will notice what you did.

If you have 4 current carrying conductors in an underground duct you will only get 1.6 times as much current as 2 conductors. If running 4 conductors in parallel using isolated phase and neutral configuration you are better using 2 conductors per duct such that you have AA AA BB BB CC CC NN NN GG groupings. Isolated phase is frequently used for underground installations as it makes equal length conductors a lot easier to achieve.

However if you are placing 12 current carrying conductors in the same trench in a direct burial configuration, you can only have 1/2 as much current per conductor than with 3 conductors directly buried. This is because if you go over 4 conductors the soil acts as the heat disspation bottleneck hance the derating is in square root proportion to the number of current carrying conductors.

How you can have problems equalizing conductor lengths is that these 2 conduits

* +
* +
* +
* ++++
*
*********

have different lengths but these 2 conduits


* +
* +
* +
**********
+
+
++++

have the same lengths. The second corner requires that the conduits be staggered like this when looking in cross section:


AA AA CC CC GG
BB BB NN NN

or something like that.

If in a direct burial installation you have phase groupings

AAAA BBBB CCCC NNNN GG

you could also transpose your phase groupings so that for 1/3 of the run you have

BBBB CCCC AAAA NNNN GG

and for another 1/3 of the run you have

CCCC AAAA BBBB NNNN GG

which will equalize reactance and reduce induced voltage in the neutral and equipment grouding conductors.

Mike Cole