Triplexing of Cables in Cable Tray

[Redskinsdb21]

Often I see power cables in cable tray tri-plexed. If there a reason for this like makes the power flow more efficient?

Thanks!

 

[FacEngrPE]

Electrically the trifoil arrangement balances the impedance of the three phase legs. Most of the time unbalanced impedance is small at 50 and 60 Hz.

More importantly the "Cable Cleats" anchor the cables against short circuit forces.
The trifoil arrangement is mechanically efficient, when securing three phase legs with no neutral conductor. Other arrangements are available.

https://www.powerandcables.com/product/cable-cleats/

https://nasco-corp.com/products/cable-cleats/

 

[bacon4life]

Any ferrous (i.e. iron/steel) metal loop around a single phase conductor will cause the metal to heat up. When running nontriplexed cables, this can happen when running each cable through a separate hole in a box or through a separate metal conduit. Even if the PVC ducts are used, there could be steel rebar in the duct bank that forms a loop. See

https://www.eng-tips.com/viewthread.cfm?qid=114338

for more discussion.

For circuits with multiple cables per phase, it is important that all wires are run as equally as possible. Very small differences in impedance can cause very large differences in how much current flows. A run placed as AAAABBBBCCCC would have very unequal currents in each wire due to how the magnetic fields from each wire interact. The paper at

https://www.researchgate.net/public..._overheating_of_the_power_cable_tray/download

has some neat graphics on the temperature rise of various configurations.

For circuits with 1 cable per phase, impedance is minimized by have the cables as close together as possible. Delta is slightly smaller than flat configuration.

Large forces are generated by fault currents. Maybe triplexed cables need less anchoring to resist short circuit forces?

 

[7anoter4]

The peak short-circuit current[ip] depends on the supplying system [See IEEE 551 or IEC 60909-0]. However, according to IEC 61914, the force between single core[ cables or cores] it has to be: F=0.17*ip^2/s where s it is center-line distance.[F in N,s in m]. So, the minimum distance will produce the maximum force for the same short-circuit current.

 

[JezNZ]

The unbalanced impedance can be significant if running parallel sets of cables.