The electrical supply here has been chopped and changed over the last
15 years & needs sorting out before Part P puts a spanner in the
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
The details are:
Supply: 3 phase TN-C-S(PME) (3 x 100amp) via 25mmsq tails from meter
[there isn't an elec co cutout].
stage 1: MEM 100amp 3 phase & N switch fuse from which go 25mmsq tails
stage 2: BILL 4 way distribution board (max 63A per way), off which
1. 63A MCB via 25mmsq tails to Wylex 1 phase 2 pole switch fuse
80amp (but fitted with
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
Any advice, opinions, appreciated
BTW I am aware of earthing requirements - it is the power cabling &
switchgear I need advice on first.