I am renovating and temporarily have access to run new cables from my
CU to the rear of the house. So, it's now or never for power in my
garden wooden shed !

I'd like a light in the shed and a single socket outside for use with
hand power tools and garden equipment. I've read several posts on this
topic but wonder if the following would comply with the regs:

1. New MCB (20A or less) on RCD side of CU.
2. From there run 2.5mm T&E to the rear of the house (approx 15m
cable).
3. At the rear connect the 2.5mm supply to a switch (indoor) which
supplies a 3 core SWA underground to the shed (approx 10m).
4. At the shed connect the SWA to the external socket and continue to
supply the light (via a 6A fuse).

Notes:
As the socket is outside I think it's ok that the light goes out with
an RCD trip. The shed is small and not suitable for work inside. With
this in mind is it ok to rely on the main RCD for earth fault
protection?

Most urgent is whether a 2.5mm T&E is sufficient from the CU to the
rear of the house. I can figure out the rest later but the CU access
will be blocked in a few days. The max load should be 60W light and 1kw
power tool.

Any advice appreciated.

Cheers,
Declan.

First of all Part P which i recommend you read up on.

I'm thinking of doing this, what i'm doing it getting 6mm 30amp T&E
from the CU, wired to a RCD in the shed from that run the 2.5mm T&E to
parts around the garden and then 1.5mm for lighting.

On Sun, 08 Jan 2006 15:11:56 -0800, Declan wrote:

I am renovating and temporarily have access to run new cables from my
CU to the rear of the house. So, it's now or never for power in my
garden wooden shed !

I'd like a light in the shed and a single socket outside for use with
hand power tools and garden equipment. I've read several posts on this
topic but wonder if the following would comply with the regs:

1. New MCB (20A or less) on RCD side of CU.

RCBO (MCB/RCD combined on the non RCD side would be better. You are more
likely to get an RCD trip in the shed (damp, dodgey lawnmower etc) and you
don't want that taking out most of the house. The regulations mention that
a fault on one circuit should not affect other circuits (where practicable).

The other option (I'm not qualified BTW so it's upto you verify this
yourself) is non RCD feed to the shed and local RCD for socket. This has
teh benefit of not blacking out the shed when you have the 9" angle
grinder in your hand spinnign away.


2. From there run 2.5mm T&E to the rear of the house (approx 15m
cable).

You could run bigger in case someone wants to upgrade the circuit later.
How are you runnign the cable - will it be bunched with any other cables
or buried in insulation - both lower the rating of the cable.

3. At the rear connect the 2.5mm supply to a switch (indoor) which
supplies a 3 core SWA underground to the shed (approx 10m).
4. At the shed connect the SWA to the external socket and continue to
supply the light (via a 6A fuse).

Have a look at this:

http://www.iee.org/Publish/WireRegs/...s_outdoors.pdf

Also, what type of earth have you got?

HTH

Tim

Based on the info so far I've modified my design to this:

1. Install a 20A MCB on the non-RCD side of the CU.
2. Run 6mm T&E from the CU to the back of the house. The cable will be
bunched with other cables going through the ceiling joists. Can I
connect this to a switch which connects to the underground SWA ? This
is important now as I can run the cable to the back now and figure out
the other details later.
3. Run the SWA underground from the switch to the shed (terminating
inside the shed).
4. Fit a small CU inside the shed which has a 6A MCB for the light and
16A for the socket.

You've all convinced me that an RCD inside the shed would be more
convenient and that I might regret not running 6mm.

I'm not sure what kind of earth I have. I live in an urban suburb in
Ireland, I can only guess the earth goes back to the distribution
supply. How can I check this?

Is there anything that would mean I can't run 6mm T&E from the CU to
the back of the house? e.g. If there was an earthing requirement of
running a continuous earth from the shed to the house CU then I'd be
screwed as access to the CU will be gone.


Thanks for the help,
Declan.

On 9 Jan 2006 05:37:59 -0800 someone who may be "Declan"
wrote this:-

2. Run 6mm T&E from the CU to the back of the house. The cable will be
bunched with other cables going through the ceiling joists.

Then your new cable may need to be de-rated, depending on the loads
in the other circuits and how far they are bunched together.

Can I
connect this to a switch which connects to the underground SWA ?

Of course, or any other suitable cable. If this is SWA then it has a
rather large gland that needs to be attached to a suitable hole in
the enclosure with space to work around it. Metal boxes are often
used to terminate SWA. You don't even need a switch at this point,
but the enclosure needs to be suitable to terminate the "outdoor"
cable.

In addition, if you are reducing the cable size at this point you
need to consider how you will protect this reduction in cable size
and how you will pass on this point to others.

Is there anything that would mean I can't run 6mm T&E from the CU to
the back of the house? e.g. If there was an earthing requirement of
running a continuous earth from the shed to the house CU then I'd be
screwed as access to the CU will be gone.

Does the shed have a water tap in it, or other extraneous conductive
part? Unless it does you do not need to do more on earthing than use
the protective conductor of the supply.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Tim S wrote:

RCBO (MCB/RCD combined on the non RCD side would be better.

An RCBO might be "better" but I suggest you're over-egging the pudding
here, he only wants a light and socket for occasional use, and it sounds
as if he already has a split-load CU with a 30 mA RCD side.

You are more likely to get an RCD trip in the shed (damp, dodgey
lawnmower etc) and you don't want that taking out most of the house.

There will be an isolating switch in the house and most of the time the
shed feed won't be live. Mostly Class 2 equipment will be used, so
trips due to earth leakage are unlikely to occur, except in the case of
a cut-flex mishap. Anyway, an RCD trip should not "take out most of the
house," it should only kill the socket circuits (nothing else should be
on the RCD side). Critically, it will not take out the house lighting.

A 16 A circuit will be fine here, 20 A is only necessary if two or more
sockets are to installed. I'd suggest using a Type C MCB to handle tool
starting currents (and tool transformer inrush current, if using hired
110 V tools).

The max overall circuit lengths from Table 7.1 of the OSG are as follows:

MCB C16, cable 2.5 (1.5 CPC) 29 m for TN-S earthing or 33 m for PME
MCB C20, cable 2.5 (1.5 CPC) 17 m for TN-S earthing or 27 m for PME
MCB C20, cable 4.0 (1.5 CPC) 19 m for TN-S earthing or 43 m for PME

--
Andy

On Mon, 09 Jan 2006 13:45:37 +0000, Andy Wade wrote:

Tim S wrote:

RCBO (MCB/RCD combined on the non RCD side would be better.

An RCBO might be "better" but I suggest you're over-egging the pudding
here, he only wants a light and socket for occasional use, and it sounds
as if he already has a split-load CU with a 30 mA RCD side.


Possibly. But with a shed it's not beyond the bounds of possibility to get
water ingress running into the electrics, or bugs depositing webs/crap
inside fittings which then get damp. If I were doing it I would use a
different RCD provision from that of the one covering the house sockets -
cost difference is likely minimal anyway. It's true that he would have an
isolator, but sod's law would be that he'd be on holiday, the shed roof
would develop a leak, the isolator would be closed and the fridge/freezer
would suffer assuming they are on the house RCD.

It's all debateable I know.

Cheers

Tim

Thanks Andy, I'm so easily influenced I'm thinking that your suggestion
meets my requirements :-)

You're right in that it is for occasional use and I would keep it
isolated when I'm not using it (probably use the light more than
anything).

"and it sounds as if he already has a split-load CU with a 30 mA RCD
side."

Thats correct, though I need to buy a bigger busbar as the RCD busbar
is full.
So sounds like using 2.5mm sharing the RCD with the house sockets would
be ok regardless of what earth I have once the cable does not exceed
29m (dont think it will but I'll check).

Declan.

Just one question before I run the cable tonight. I figured I'd go with
6mm in order to future proof the wiring at the rear of the house.
Based on the info Andy provided below it seems that the thicker the
cable the shorter the max length (for earthing). So with 6mm I am more
likely to need an earth rod in the garden shed than with 2.5mm, is this
correct ?

MCB C16, cable 2.5 (1.5 CPC) 29 m for TN-S earthing or 33 m for PME
MCB C20, cable 2.5 (1.5 CPC) 17 m for TN-S earthing or 27 m for PME
MCB C20, cable 4.0 (1.5 CPC) 19 m for TN-S earthing or 43 m for PME

Declan wrote:

Just one question before I run the cable tonight. I figured I'd go with
6mm in order to future proof the wiring at the rear of the house.
Based on the info Andy provided below it seems that the thicker the
cable the shorter the max length (for earthing). So with 6mm I am more
likely to need an earth rod in the garden shed than with 2.5mm, is this
correct ?

MCB C16, cable 2.5 (1.5 CPC) 29 m for TN-S earthing or 33 m for PME
MCB C20, cable 2.5 (1.5 CPC) 17 m for TN-S earthing or 27 m for PME
MCB C20, cable 4.0 (1.5 CPC) 19 m for TN-S earthing or 43 m for PME

I don't follow what you're saying. All other things being equal, a
larger cable will always allow you a longer run. There's not really any
need to go to a separately earthed TT installation here, given the very
simple nature of the installation and the likely use.

--
Andy

On Wed, 11 Jan 2006 23:13:48 +0000,it is alleged that Andy Wade
spake thusly in uk.d-i-y:

Declan wrote:

Just one question before I run the cable tonight. I figured I'd go with
6mm in order to future proof the wiring at the rear of the house.
Based on the info Andy provided below it seems that the thicker the
cable the shorter the max length (for earthing). So with 6mm I am more
likely to need an earth rod in the garden shed than with 2.5mm, is this
correct ?

MCB C16, cable 2.5 (1.5 CPC) 29 m for TN-S earthing or 33 m for PME
MCB C20, cable 2.5 (1.5 CPC) 17 m for TN-S earthing or 27 m for PME
MCB C20, cable 4.0 (1.5 CPC) 19 m for TN-S earthing or 43 m for PME

I don't follow what you're saying. All other things being equal, a
larger cable will always allow you a longer run. There's not really any
need to go to a separately earthed TT installation here, given the very
simple nature of the installation and the likely use.

I am just seeing '4.0 with a 1.5mm cpc' there and thinking 'yes, the
length would have to be bloody short'. I suspect for conduit wiring or
SWA with equal or oversized earth conductors, the lengths would be
more in tune with intuition.

--
You cannot shake hands with a clenched fist.
- Indira Gandhi

I was looking more at the TN-S side and got the wrong impression. The
extra 4A drops the length from 29m to 17m and the 4.0mm cable only
increases the length by 2m. I find this surprising but I'm sure there's
a good reason for it. I need to read up on the different earthing
types.
I tried to find table 7.1 online but didnt have any luck. Can you post
a link to it please, or send the info for 6mm ?

Thanks again for the help,
Chris.

Declan wrote:

I am renovating and temporarily have access to run new cables from my
CU to the rear of the house. So, it's now or never for power in my
garden wooden shed !

Another thought: install FOUR core cable as a contingency so that (say)
you can
monitor the state of the shed PIR tripping from within the house?

My old man always suggested that an outside PIR sensor installed for
security
(deterrent) purposes ought to switch on a light WITHIN the house (to
make it
appear that the householder has heard a noise and is coming to
investigate).

HTH

Mungo