On 19 Feb 2006 11:17:03 -0800, "
wrote:
Hi,
Can I run this past some people to find out if I'm doing this
correctly...
I'm want to run power to an outbuilding that I'm going to use as a
workshop. (ok it's a shed, but outbuilding sounds grander!)
My plan is to run cable from a spare MCB in my house's consumer unit to
the shed (under a patio, so it'll be well protected) On entering the
shed it'll go into an auxilery consumber unit with build in RCD. This
consumner unit will have 3 MCBs to supply the lighting, power points
and a water feature in the garden
I've read around on the net and I like the idea of a consumer unit in
the outbuilding, but I was wondering if this was an ok way of doing it.
If so, what rating should the MCB be at my house's consumner unit end?
and also what do the ratings of RCD's actually mean? I've seen 63A 30ma
and 80A 30ma etc and I'm a tad confused... :-S
Thanks in advance for any help you can give,
Cheers,
Rob
Having an auxiliary CU in the shed is entirely reasonable. The main
points to consider a
- Power requirements at shed
- Cable type and sizing from house
- Earthing arrangements
You need to determine likely current requirements for all equipment
that will be used in or from the shed first and to base the supply
rating on that plus a margin.
For an installation with underground cable, the convention is to use
3way + shield SWA armoured cable. This needs to be sized
sufficiently large to meet current carrying requirements as well as
the maximum allowed volt drop for the length to be used - so you need
to know length as well. The reason for this is in part to make sure
of an adequate supply voltage under load for the equipment, but also
so that in the event of a short circuit at the shed CU or in the
cable, that the MCB at the house end will trip, cutting off the supply
and protecting the cable. Note that there is no problem in
oversizing.
For earthing, there is a choice of exporting the house earth as long
as the distance is short or separating the earth (using an insulating
fitting at the shed CU) and installing a ground rod at the shed.
If you export the house earth, you can fit either an MCB (either side
of an RCD in the house CU if there is one) or an RCBO (combined RCD
and MCB). Note that RCDs, on their own do not trip on excessive
current. Their rating in Amps is only their carrying capacity.
If you use an RCD or RCBO at the house, then any current imbalance
fault in the shed will cause all power to the shed to be cut. You
might not want that - e.g. lights going out or freezer going off. In
that case an MCB only should be used in the house CU and a 30mA RCD in
the shed CU with lights upstream of it etc.
If you go for the local earth rod option, then you are creating an
equivalent to TT earthing of the main supply - i.e. what happens with
overhead supplied power to a house. In this case, an MCB is fitted
at the house CU and a 100mA time delay RCD is used on the supply input
at the shed CU. You then install MCBs and 30mA RCD as before.
It is worth reading through the relevent sections at
http://www.tlc-direct.co.uk/Book/1.1.htm
for more details.
--
..andy