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Are Paralell Condcutors in Cable Tray Okay 2

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dougjl

Electrical
Sep 14, 2001
44
Hi,
I have an installation where two sets of parallel 400 MCM conductors were installed in a cable tray. They were installed as individual conductors in a single layer. The voltage of the system is 480 three phase and the circuit is feeding from a switchgear to an MCC MLO main.

An engineer at the site claims that this installation is not normally used in industrial environments as this arrangement will cause noise and over heating of the cables. He then said that if the parallel runs were multi-conductor cables that there would be no issues. I am having a hard time understanding how that would make a difference. The individual cable were arranged phase a, phase b, phase c, phase a, phase b, phase c. Each of the conductors were zip tied to the tray and next to one another (no spacing). Code clearly allows this type of installation. Can anyone explain how a the multi-conductor arrangement is better? I really don't understand why there is a problem.

Thanks,
Doug
 
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In a multi-core cable the cores are twisted and change their location in space so the current unbalance between parallel cores is concealed.
The single-core cable advantages are:
Individual cores= less weight easier installation.
More space between cores =better cooling= increased ampacity.
Disadvantages are:
c) More distance more reactance
d) More unbalance between parallel cables
e) More expensive [total assemble of cables].
The recommended run is in trefoil arrangement a,c,b-b,c,a or in straight line A,B,C,C,B,A.
See [for instance]:
 
Big single-core cables on systems with large fault currents should be cleated, not zip-tied. Nylon ties burst under the electro-dynamic forces of a fault. Metal ties cut through insulation and add earth fault or arcing fault to the problems you already have.
 
Thank you for the information.
 
Dear dougjl, The other advantage of single conductors is the bending radius as you can get tigher bends. Without armouring they become lighter. YOu don't get steel wire armouring as this will act as a single turn of a transformer and the cable will cook. This is normally aluminium foil. The terminations of these conductors must also be in special galnds and the gland plate must be aluminium, fibreglass or some other non magnetic material for the same reasons.

THe link to the paper works and is a complex answer. I have used trefoil many times but have never had and answer to the neutral if you are running a reduced neutral because you have an almost balanced load and still require a reference.

Here in Africa we have people that do strange things. They had a four cables, 4 core, SWA, ECC and for some reason decided to use all four cores in one cable for each phase. They earthed the cable armouring on both ends to give them the earthing. They turned on and they cooked the cable. Circualting currents. After the second failure they removed the earth on the Motor Control center end and the third installation did not cook the cables. The problem is that if the secondary PVC insulation on the outer sheath is punctured, the earth circuit returns and the cable will cook AGAIN. They know of this but still they try.

I will now attempt to up load two files for you. If you don't get them but want them, let me know and I will e-mail them to you if I have an address.
 
 http://files.engineering.com/getfile.aspx?folder=762abbfc-d60b-4d03-b07c-837a4c4db034&file=Cables_AEI_single_core_cables.pdf
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