John,
Good to see you down here!. [Linked Image]
Personally I'd never bury any cable (Neutral-Screened or what ever) without Conduit over it.
Sure, for an Out-Building, the derating isn't going to make a bit of difference.
But, what will is some idiots shovel or spade that never took any notice of the Signal Strip at 300mm below the surface.
I'd sooner the PVC conduit(Orange) was seen before the shovel hits the cable.
I'm talking purely from a repair point of view here.
Besides, all of our PoCo street cables run through 200mm ducts under the footpaths, so who am I to argue?. [Linked Image]