Chris wrote:
In article ,
John Rumm wrote:
In fact I would suggets that you _should_ extend to a ring since you
are quite likely to exceed the current limit for the cable before the
fuse blows should a fault occur (a 16A or 20A fuse would be more normal
on a radial circuit like this). As a ring however, the 30A fuse will be
fine.

Ok, thanks.

The fuse situation seems a bit unusual to me anyway: The garage is fed
from a 30A fuse in the house by a heavy duty outdoor cable. In the garage
is a small CU, with the workshop feed on a C20 MCB. Finally in the
workshop is a small CU with two circuits; the 30A socket circuit and a 15A
lighting one.

So if I understand correctly, the garage fuses are pretty pointless,
unless/until the MCB (and probably cable) in the garage are upgraded.

That does seem a bit of a 'growed like topsy' setup. It's not
particularly bad or dangerous though. The most significant problem
will be the lack of discimination between the workshop CU and the
garage CU (and/or the lighting circuit, see below).

Given that the whole of the outside wiring is fed from a 20 amp MVB in
the garage you are OK extending the sockets wiring without making it
into a ring as a 20 MCB is the correct rating for a radial circuit of
this sort. I see little point in changing it to a ring unless you
really need to uprate the total load capability.

The 30A MCB (or is it a fuse?) in the garage is, as you say,
superfluous but the lighting one isn't. That's my one worry about the
system, a 15 amp circuit isn't correct for lighting and most lights
are only rated to be used on 6 amp or (sometimes) 10 amp circuits.

Personally I'd retire the CU in the workshop, leave the power circuit
as a 20 amp radial protected by the MCB in the garage and run the
lights from an FCU with a 5 amp fuse in it.

Is the wiring in the garage and workshop protected by an RCD? It
certainly should be. The neatest solution (if it isn't protected)
would be a combined 20 amp RCD/MCB in the garage CU.

--
Chris Green )