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OK, say I forget the black run and breach joint. It's unprofessional. Run a new 4-core 35mm or 50mm as below in white?
It's a crazy run but is the only machine-diggable routing which does not involve damage to a finished garden, or crossing deep ditches.
This is probably worth doing as I could also put an SWA fibre in the same trench to get decent internet to B & D which has always been a problem.
But working with cables that size and length is beyond my experience.
I'd disconnect and abandon the old 10mm yellow run.
The new supply would end inside B in a big isolator, with two parallel 16mm 3P feeds out: one set to DB5 locally (for building B), and one set to the green final run to DB11 in building D.
Question 1: Is it OK to reduce CSA like that locally?
Question 2: Conversely, may I 'supply' the feed end of the (say) 50mm SWA via a switchfuse fed from the main switch with say 4 x 25mm D/I singles? If so, could these come from the supply side, via a new hole in the RHS of the existing isolator, and be protected only by the DNO's 100A cutouts?
Pic of the main switch below. There's no way of terminating another big SWA into that, as the top already has 2No. 25mm 4-core SWAs going out, and there's not much space around.
Question 3: Or is it permissible to 'back-protect' a poorly-protected cable run by having the switch fuse only at the much easier-to-install far end, say at 40A per phase? Mechanical damage to the cable would blow the board's cut-outs, long-term overload from buildings B and/or D would trip the local switchfuse. I'm warming to this idea. Might that be OK?
What would others do? (Apart from say they don't want the job )
What is the design current of this distribution circuit and the length if it follows the white route?
Q1. You can reduce the cable size to make the final connections from a larger cable size as long as the smaller is suitably protected from overcurrent by the ocpd for the circuit. Alternatively a suitable ocpd may be installed after the reduction in cable size as long as certain conditions are met
Q2. Yes you can use smaller cables at the supply end if necessary, subject to the same requirement for them to be protected from overcurrent.
There shouldn't be 2x 25mm outgoing cables connected in that isolator anyway, the terminals are designed for one cable only.
Q3. No you cannot use the cutout fuses to protect a circuit nor can you install the ocpd at the far end of a distribution circuit. This is a very basic part of the wiring regulations.
Some of the questions you are asking are very worrying from someone who is designing electrical installations.