Cables in Trefoil

[Jereb]

Hello All,
I have a situation that i'm seeking some advice on. Basically we have a ~1500kVA transformer supplying power to our Motor Control Centre. The run is only short, around 20-30 meteres. However total current is rather high as each phase is made up of 3 400mm2 single cores unarmoured run in non trefoil. (i.e each phase is bunched like so Red-Red-Red, Blue-Blue-Blu, White-White-White). Having looked around the build i could not find anything ferrous enclosing the cables like tray etc. but the guys are still concerned about heat issues.
I tend to think that because these cables are bunched and run next to each other, could i not model them as single conductors of some arbitraty large size run flat next to each other with no enclosed ferrous materials around? Will it still cause all kinds of issues?

 

[dpc]

jghrist,

If the six cables are in the same conduit I agree. I was simply trying, obviously not successfully, to explain use of very large conductors is not usually cost effective.

I'm going to bow out since we already have at least two separate discussions going on at the same time.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[7anoter4]

If it is about an open-top cable tray and as the distance between groups is very small you could calculate this only according to J. Stolpe theory for open-top randomly filled cable tray:
"Ampacities for Cables in Randomly Filled Trays" IEEE Transactions on Power Apparatus and Systems, Volume: PAS-90, Issue: 3/1971.
If the cables are not exposed to direct sun-shine[a raised hood -6 inches above the cables will be a suitable protection] the ambient air 40oC, the maximum permitted load will be 540 A per one cable. In order to keep balanced load in a bunch have to be included one cable of each phase and in the order as shown:

 

[burnt2x]

7anoter4,
Nice picture there. But I am scratching my head why the cables should be arranged like that! Gimme some time to digest.

 

[7anoter4]

Thank you, burnt2x. Sometime I did nicer picture, I think.
This is the phase position in order to avoid the currents' unbalance. If you change this order you may get up to 30% percent unbalance and this may heat one cable more than other.