I currently have power to my garage which is fed from a 30amp fuse on the
main consumer unit to another fuse box in the garage. This fuse box has 2
feds one for a small socket ring (3 double sockets) and the other to provide
lights for inside the garage and security lights. One of the double sockets
has a built-in circuit breaker and is used to supply power to a small pond
pump and light (this is the only constant draw).
In addition to the garage I plan to build a big shed to be used as a small
scale workshop. Rather than take a new feed off the main consumer unit I was
planning on extending the ring and lighting circuits from the garage to the
workshop. I will be using a few power tools but it is unlikely that the will
be all used at the same time. Possibly use a dust extractor & saw or router
at the same time. Can not see that there would be any greater a load than
would be found in a modern kitchen.
Do anyone think there is anything fundamentally wrong with what I planning ?
This is fine in principle and there are a couple of ways you could do
a) Put a fused connection unit (FCU) on the garage ring and fit it
with a 13A fuse. Wire from there to your shed using 2.5sqmm cable
and then into as many sockets as you like in the shed. For lighting,
put a second FCU in the shed with a 5A fuse and run the lighting from
that in 1sqmm cable.
b) If you have a spare way in the CU in the garage, you could put in a
20A breaker and wire again in 2.5sqmm cable to the shed. Use an FCU
with 5A fuse for the lights as before. If the garage CU doesn't have
a spare way, then you could consider moving it to the shed and putting
a new CU into the garage. This should give you all the current you
need for what you describe.
Obviously if you are going across ground then you need to either
suspend the cable high from a supporting wire using the rules for
that, or by using buried SWA cable.
If you were to cable back to the main house CU, with appropriate cable
you could deliver more current. For example with a 32A breaker and
4 sqmm cable you can run a 32A circuit directly to BS1363 sockets
without a downstream CU if you like. Personally I prefer to use a
CU in each outbuilding for future expansion.
Don't be tempted to uprate the 32A fuse in your existing main CU to
provide for more current for the installations unless the cable from
there to the garage is up to it.
There is a useful reference to this in the Electricians Guide to the
16th wiring regulations by Whitfield. You can find most of it
conveniently reproduced on www.tlc-direct.co.uk in the technical
To email, substitute .nospam with .gl
Replace the garage fuse box with a modern consumer unit. You will want two
circuits. Firstly a lighting circuit fed from a 6A Type B MCB. Secondly a
radial (or ring) circuit for the sockets fed from an RCBO (B20A/30mA will do
here, although a 32A could be used if you can live with possible lack of
discrimination (the B32A type response may be quicker than the 30A cartridge
This will provide RCD protection to all the sockets. This is essential for
lawnmowers and power tools. The RCD will not cover the lights, so when you
chop through the mains lead of your handheld jigsaw, the lights won't go out
and you can stop cutting before you chop off your arm. You can run a
separate circuit for the shed if you like, but it is probably not necessary.
It may be best to run as a radial (with substantial cable, such as 6mm) as
it probably suits the topography better. Only one cable is required to enter
the shed from the garage on the socket side. (I assume the two are attached
to each other).
The shed is not actually built yet but is likely to be 5 to 10 meters from
the garage. I think I will bury cables rather than suspend overhead. I will
bury using a suitably armoured covering.
Just checked in the garage and what I refered to as a fuse box is actually a
Wylex Powerbreaker RCD unit. It is ftted with a B6 mcb for the lights & a
B32 mcb for the Sockets. It sounds like I can just extend these circuits to
Thanks for the replies guys very useful information.
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