Mike gives some good advice.

To clarify, direct burial (i.e. not in any form of ducting or similar mechanical protection) of this type of cable is not permitted under BS7671, irrespective of what the manufacturer's datasheet says; a suitable level of mechanical protection is required (522.8.10 is not limited to ADS).

It is not uncommon to use string cable in ducts on large scale solar parks. However, most string cable does not provide a guaranteed level of water resistance and sadly many ducts end up with water in them, whether due to faulty installation, condensation, damage etc; the cables will likely resist damp but permanent immersion in a tube of water will result in degrading insulation resistance or even faults and fires. Some cable is available with water ingress protection ratings from the manufacturer over and above the cable standard, for additional cost.

In a domestic setting, I would first consider whether you can avoid buried DC entirely. For example, mounting an outdoor rated inverter on the array framework means you've got AC circuits in SWA with ADS on your side, so anyone digging things up (gardening) in future a) Can isolate cables in a safe manner (per EAWA) and b) Is protected "as normal" against accidents. Of course that brings its own issues.

If ducts are unavoidable then a) Make sure they're specified (e.g. BS EN 61386-24) and installed properly, b) Seal them properly until they are required (not just a plastic baggy taped around the top...) c) Give serious consideration to cables specified with suitable water ingress rating and d) Keep a record of where they are (in a way that can be handed on to the next owner of the property). I might also consider e) where the ends of the ducts come up if there is a possibility that they will bring fire into a dwelling should there be a disaster (and if so, require LSZH cabling or reposition), f) whether it is possible to provide a form of (emergency) isolation on the array side and g) Watching the installer like a hawk to make sure that cables aren't being damaged when pulling into the duct.

Worth noting that the IET's Code of Practice has a few extra notes too, including depth and making sure it's protected against rodents using it as a burrow.

PS - Just noticed you say "series of ducts" and "flexible"... Do consider the practicality of pulling the cable in! A single straight run will be easier, cheaper and a lot less likely to result in damage! Of course that's not always possible but you get the drift.

Jam said:

permanent immersion in a tube of water will result in degrading insulation resistance or even faults and fires.

Agreed . Worth also noting that even most types of SWA, whilst suitable for direct burial not in ducts, is not specified for permanent immersion, and care is required to ensure plastic ducts don't become waterlogged for long periods.

Jam said:

Worth noting that the IET's Code of Practice has a few extra notes too, including depth and making sure it's protected against rodents using it as a burrow.

Definitely, and further guidance on depths for different locations is provided in the relatively new 3rd Edition of the IET CoP for Grid-Connected Solar PV ... along with the advice that if the burial is in soft ground, add 400 mm to the 'minimum depth' for the initial depth of burial.

thanks jam/gkenyon.

• the under ground run is only a 2 meter run between the to building as a number of other services as to be run i may use a accessible duct to run the separate ducts in. 

thanks jam/gkenyon.

• the under ground run is only a 2 meter run between the to building as a number of other services as to be run i may use a accessible duct to run the separate ducts in.