The background is that the old house at the front originally had a 60amp fuse box (now, after 2 upgrades, a 100/80 amp split load CU) + off-peak heaters (removed after oil CH arrived). Then an extension was built at the back. Building the extension entailed moving the incoming elec supply. The fuse box at the front was left in place & a 60amp cable taken to it. A separate RCD 100amp consumer unit was installed for the extension. A supply to the garage comes off via a fuse box. A 16amp 3 phase supply feeds a table saw (2kw) in an old store.
The details are:
Supply: 3 phase TN-C-S(PME) (3 x 100amp) via 25mmsq tails from meter white box [there isn't an elec co cutout].
stage 1: MEM 100amp 3 phase & N switch fuse from which go 25mmsq tails to:
stage 2: BILL 4 way distribution board (max 63A per way), off which via MCBs:
1. 63A MCB via 25mmsq tails to Wylex 1 phase 2 pole switch fuse 80amp (but fitted with 60amp fuse) Output of this goes to a link box which splits into a 16mmsq FTE cable to the old house CU (feeds the kitchen, lights & front rings) + a short tail into a fuse box from which a 4mmsq cable goes to the garage.
2. 63A MCB via 25mmsq tails to Wylex 100A RCD CU for the extension which feeds extension rings, lights, summer water heating & the CH.
3. 16A 3 phase MCB into a 32A Wylex switch fuse from which a 3 phase cable goes to the saw.
I'd like to sort this out & bring it up to date, but need advice on the best method of doing the job.
A. The BILL distribution board: This limits each ongoing way to 63A: is there a better way of splitting the output to the CUs so that a full 100A is available at each CU? Looks to me as if replacing it by several link boxes would do the trick in less space, provided the onward cables are all 25mmsq (ie 100Amp) - ie no need for fuse/MCB. Would that be allowed?
B. Replacement for the 16A FTE cable. The length is about 8 metres. Can 25mmsq tails be used to upgrade the link to 100A for this length? I appreciate the current earth in this cable may be insufficient & would use a 16mm sq earth cable.
C. Ideally it should be possible to isolate each CU separately near MEM switch. 100A switch fuses are large. Is a fuse necessary here as the MEM switch has 100A fuses? ISTM that all that's wanted is a simple 100A unfused cutout which I don't see as needing to be any larger than the switch in a CU. But are these available?
D. The extension CU is all RCD which SFAIUI is out of line with the latest practice which prefers some circuits (lights+heating etc) to be non-RCD protected. Can I retain the existing RCD CU and add a second 100A non-RCD CU in parallel (via a link-box) & reconnect the circuits as appropriate? That would be the cheaper than buying a split load CU & provide more outgoing ways.
E. The saw is being moved to the garage, so the 3ph needs to go to the garage with enough capacity for lighting and a 32A ring for powered handtools. My guess is that a 32Amp link is enough - distance is around 7m allowing for up/down wall. I would use 4mm 4 core SWA cable for this link, but what is the best or correct 3ph switch gear to use at the house & at the garage ends? Clearly it is going to need a small RCD CU chained off one phase of the 3ph switchgear in the garage.
Any advice, opinions, appreciated
BTW I am aware of earthing requirements - it is the power cabling & switchgear I need advice on first.