Direct Buried Cables 3 Phase

[KB1974]

Hi,

I am a mechanical engineer tasked with some electrical engineering scope of work and not sure where to look for information on the following...

I have 12 x Off 95mm2 SWA 3 Phase cables coming from Solar inverters into an energy building panel. Each cable is carrying a maximum of 160Amps and will be wired directly into suitably rated MCCB's. In an effort to minimise construction costs, i have been asked if we can bury these 12 cables directly in the ground in a trench with backfill and i am struggling to find the relevant information on it within BS7671. Can anyone help with :-

• Is there another industrial standard that relates to this type of work?
• Is it allowed to be done without concrete ducting/casing?
• Is it possible to lay all 12 together or must they be separated by soil/sand etc
• Is it possible to bury in say 4 x rows of 3 cables or 3 x rows of 4 cables?
• is there a specified amount of spacing distance with regards burying cables like this?
• Am i correct in thinking the top layer of cables needs to be a minimum of 700mm from the surface to allow protection of the cables?

I apologise if these are simple questions with simple answers, but i have no experience in the industry and just need some guidance.

All help will be deeply appreciated.

Regards & Thanks,
Kieran

 

[UNG]

There is no reason why you can't, given that you have the cables who did the original electrical design as the installation method should be taken into account when sizing the cable to be used

 

[pc1966]

You can bury SWA cables directly in the ground but often it is better to use twinwall duct or similar so it is possible (with a lot of effort) to replace one later. If you do bury directly, and to some extent with duct, you do need to ensure the surrounding material is free of large/sharp stones. For small/medium jobs then that typically means spending a lot on sand/small gravel, but if a big job you might look at hiring a soil separator to take any big stones out and then use the finer stuff to back-fill.

The wiring regs don't specify a cable depth, just that is has to be deep enough to avoid any reasonable risk of damage. For LV cables (up to 1kV) typically 600mm is considered OK for most ground but if farm land then 1-1.2m might be necessary.

You can have the cables together but need to allow for the total heating effect. With 95mm and 160A they are quite a bit below max operations but if grouping the cables you ought to work out the factor to decide if it is OK or not. I would have to check what is considered sufficient spacing that they are treated as independent, but usually a couple of cable/duct diameters would be reasonable.

Also 95mm is big at around 4cm diameter and heavy around 5-6kg/m (at least for a wimp like myself) so I would not attempt to pull one of, say, 3 cables out from duct as I suspect it would be a major metal snake fest of them being tangled up.

I am sure there are others on here with more experience of wrangling this sort of size!

 

[littlespark]

Remember to lay cable warning tape halfway between where the cables are and the top ground level when backfilling.

There is, I’m sure, reference to burying cables in BS7671

 

[pc1966]

There is, I’m sure, reference to burying cables in BS7671

There is but AFIK nothing specific on the depths.

I think the typical 0.6m sort of thing comes from the DNO guidance on installing cables.

 

[Electro-tech]

 

[Electro-tech]