[Nick]

I'm planning to have wiring extended from an existing external socket
into a small garden for the purpose of lighting, water features and
use in a garden room, and would appreciate any advice on cable choice
and routing. I put a rough sketch with some dimensions and layout at
http://64.191.63.245/garden-wiring.gif

So far there is a 20m run of 2.5mm SWA cable from the CU to an
external socket. The electrician put a 32A RCB on the circuit, which
is also covered by the RCCD that some other house circuits are on. I
was thinking of SWA running from the end of the existing circuit,
under a path to one corner of the garden, with a junction box to run
to the room corner (D) and up to the far corner (C). Cable runs for LV
circuits could fan out from the corners and shouldn't be excessively
long. Lighting will be LED's with possibly some LV halogens, computer
controlled (the house already uses DMX), and along the garden
periphery, in trees etc. Pumps will be LV if possible around areas P1
and P2 but we might need a mains feed for those (or for any UV unit).
I'd also considered a short run from either another junction box to
point A or from the planned junction box to point B for LV circuits
back along the path and trees.

I'm not sure what the max likely current demand would be, but as
there's 20m of 2.5mm already from the CU, when calculating voltage
loss (based on worst case) I figured that 4mm or even 6mm SWA would be
prudent so that there'd be at least a 13 amp circuit for the room. The
house supply is TN-C-S and I'd assumed that extending the earth from
the house would be ok and that there'd be no need to have any
complications of a separate earth.

There's a debate about how to feed the room. One suggestion is first
to take the feed to point D directly into the room through the
concrete base (not yet laid), to the circuit in the room and then out
from the room to an external socket at the back or side of the room.
That socket would be used for any garden power tools. The LV control
gear could be in the room and feed out separately to circuits on that
part of the garden. Another idea was to take the feed to point D first
to an external junction box, and then to feed the room as a spur from
the junction box in that corner; basically just a decision of where to
first take that mains spur.

As the garden is small, I'd also wondered whether it would make sense
simply to extend the mains feed only into the room, with an external
socket as spur from the room, and then house the LV control gear for
the whole garden either in the room or outside at corner D and fan out
all LV wiring from that point. But as LV cable runs could then be up
to 15 to 20 meters or so, I'd thought taking mains to at least two
opposite corners would be prudent.

So to sum up, I'm not sure about cable choice although am leaning
towards 6mm, and also any suggestions and thoughts on routing the
mains would be welcome as we've ripping up the garden with diggers
now

Nick

[John Rumm]

Nick,

There are a few aspects of taking power outside like this that can get a
bit complex. You may find having a read through this:

http://wiki.diyfaq.org.uk/index.php?...ricity_outside

may help.


Nick wrote:

I'm planning to have wiring extended from an existing external socket
into a small garden for the purpose of lighting, water features and
use in a garden room, and would appreciate any advice on cable choice
and routing. I put a rough sketch with some dimensions and layout at
http://64.191.63.245/garden-wiring.gif

So far there is a 20m run of 2.5mm SWA cable from the CU to an
external socket. The electrician put a 32A RCB on the circuit, which

If this run is inside the house, why is it in SWA?

I assume by RCB you mean MCB...

You are going to have to take care here since it sounds like a design
decision has been made that could have implications that may come and
bite you later!

Circuits need two types of protection: overcurrent and fault current.
The former is designed to clear a sustained overload that results in too
much current being drawn, and the latter is designed to operate when you
have an outright fault like a short circuit that results in a massive
fault current flowing. Normally both types of protection are provided by
the MCB at the head of the cable. However this does not appear to be the
case in your existing setup. I would guess the designed was relying on
the 32A MCB to provide only the fault current protection for the SWA
cable, and was relying on the fuse in the plug connected to the external
socket to limit the maximum current since the 32A MCB is of a too high a
rating to do this itself.

is also covered by the RCCD that some other house circuits are on. I

This is generally a poor design choice. A fault in the garden will trip
power in the house, and also make nuisance trips more likely since
garden circuits are more likely to give rise to problems.

See the nuisance trip section he

http://wiki.diyfaq.org.uk/index.php?title=RCD

was thinking of SWA running from the end of the existing circuit,
under a path to one corner of the garden, with a junction box to run
to the room corner (D) and up to the far corner (C). Cable runs for LV
circuits could fan out from the corners and shouldn't be excessively
long. Lighting will be LED's with possibly some LV halogens, computer
controlled (the house already uses DMX), and along the garden
periphery, in trees etc. Pumps will be LV if possible around areas P1
and P2 but we might need a mains feed for those (or for any UV unit).
I'd also considered a short run from either another junction box to
point A or from the planned junction box to point B for LV circuits
back along the path and trees.

I would suggest a SWA cable submain to the outbuilding, and a consumer
unit installed there to supply any other circuits required.

I'm not sure what the max likely current demand would be, but as
there's 20m of 2.5mm already from the CU, when calculating voltage
loss (based on worst case) I figured that 4mm or even 6mm SWA would be
prudent so that there'd be at least a 13 amp circuit for the room. The

You may want to consider running a larger cable through the house and
then converting to SWA for the external run.

house supply is TN-C-S and I'd assumed that extending the earth from
the house would be ok

No - absolutely not! This is just the situation you really don't want to
extend a TN-C-S earth outside. If it were just going into the
outbuilding then it may have been a acceptable solution if implemented
correctly. However since you also need feeds into the garden itself for
lights and water features itself, using the TN-C-S earth is technically
not feasible, and could pose a serious safety hazard if you did it.

and that there'd be no need to have any
complications of a separate earth.

This is what you need to do. The outbuilding wants to be a TT install
with its own earth spike and RCD protection.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

[Nick]

Hi John

Many thanks for the reply.

There are a few aspects of taking power outside like this that can get a
bit complex. You may find having a read through this:

http://wiki.diyfaq.org.uk/index.php?...ricity_outside

I was there earlier today and found it very helpful. I'll be looking
back again in the light of some of your comments.

So far there is a 20m run of 2.5mm SWA cable from the CU to an
external socket. The electrician put a 32A RCB on the circuit, which

If this run is inside the house, why is it in SWA?

A good question indeed given the relative cost and inflexibility
compared to the obvious alternative. I had the place rewired as part
of a major refurb, and at the time it wouldn't have been so hard to
sling the cable in, but with hindsight and now that I've audited and
looked more closely at the job that the electricians did, that would
be one question that I'd ask along with querying a number of
discrepancies between the schedule of test results and reality. They
weren't cowboys and were helpful with some unusual things that I
needed done at the time for the lighting, but now that I've had a
closer look, some of the wiring on the lighting circuits was sloppy
and careless.

I assume by RCB you mean MCB...

My typo.

Circuits need two types of protection: overcurrent and fault current.
The former is designed to clear a sustained overload that results in too
much current being drawn, and the latter is designed to operate when you
have an outright fault like a short circuit that results in a massive
fault current flowing. Normally both types of protection are provided by
the MCB at the head of the cable. However this does not appear to be the
case in your existing setup. I would guess the designed was relying on
the 32A MCB to provide only the fault current protection for the SWA
cable, and was relying on the fuse in the plug connected to the external
socket to limit the maximum current since the 32A MCB is of a too high a
rating to do this itself.

Right. A 32A breaker seems very high given what the circuit is for;
I'm not planning to run a few kilns!

is also covered by the RCCD that some other house circuits are on. I

This is generally a poor design choice. A fault in the garden will trip
power in the house, and also make nuisance trips more likely since
garden circuits are more likely to give rise to problems.

Good point. As we're not having a chest freezer or anything that you
don't want tripped this didn't seem too bad, but if there were trips
from the garden then that could become much more than just a nuisance.
I feel that it should be possible to have a garden install where a
trip was a rare, but if it didn't turn out that way then...

I would suggest a SWA cable submain to the outbuilding, and a consumer
unit installed there to supply any other circuits required.

Thanks.

You may want to consider running a larger cable through the house and
then converting to SWA for the external run.

I'd definitely do this if it were feasible but due to how the wiring
needs to route in the building it would mean ripping up a large floor
area in several rooms and cost ££££. I could route a new SWA out the
front of the building and then either around the house on the wall,
which would be unsightly, or bury it for some of the way, but really
neither would be good.

house supply is TN-C-S and I'd assumed that extending the earth from
the house would be ok

No - absolutely not! This is just the situation you really don't want to
extend a TN-C-S earth outside. If it were just going into the
outbuilding then it may have been a acceptable solution if implemented
correctly. However since you also need feeds into the garden itself for
lights and water features itself, using the TN-C-S earth is technically
not feasible, and could pose a serious safety hazard if you did it.

This is what you need to do. The outbuilding wants to be a TT install
with its own earth spike and RCD protection.

Thanks! I do need to get a cable laid to the outbuilding soon as the
concrete base is going down (probably tomorrow), and planned to take
the cable up through the concrete base so that it'll be there to feed
into the building when that's built. I'll go for a beefy SWA cable to
minimise losses on the run into the outbuilding, and would 2 core SWA
be fine for that or should it be 3 core?

Nick

[John Rumm]

Nick wrote:

Circuits need two types of protection: overcurrent and fault current.
The former is designed to clear a sustained overload that results in too
much current being drawn, and the latter is designed to operate when you
have an outright fault like a short circuit that results in a massive
fault current flowing. Normally both types of protection are provided by
the MCB at the head of the cable. However this does not appear to be the
case in your existing setup. I would guess the designed was relying on
the 32A MCB to provide only the fault current protection for the SWA
cable, and was relying on the fuse in the plug connected to the external
socket to limit the maximum current since the 32A MCB is of a too high a
rating to do this itself.

Right. A 32A breaker seems very high given what the circuit is for;
I'm not planning to run a few kilns!

In fact my bad possibly...

Just looking at table 4E4A in BS7671, the current rating for two core
SWA if it has a thermosetting insulation[1] capable of withstanding a 90
degree temperature (i.e. XLPE) is actually 36A (and 31A for three core).
So it would be adequately protected by the breaker you have (although
you may want to change that for a 32A HRC fuse to better discriminate
with downstream MCBs).

[1] Most SWAs are like this, however PVC insulated versions also exist
and the max current ratings for these are lower at 28A/25A (2/3 core).
To be sure check the markings on the sheath of the cable.

is also covered by the RCCD that some other house circuits are on. I
This is generally a poor design choice. A fault in the garden will trip
power in the house, and also make nuisance trips more likely since
garden circuits are more likely to give rise to problems.

Good point. As we're not having a chest freezer or anything that you
don't want tripped this didn't seem too bad, but if there were trips
from the garden then that could become much more than just a nuisance.
I feel that it should be possible to have a garden install where a
trip was a rare, but if it didn't turn out that way then...

It should be possible - certainly in the outbuilding anyway. However
there is greatly enhanced scope for problems, and as you say, if you are
unlucky it might prove to be a PITA. If you have a split load CU in the
house and there is space, I would be inclined to move the existing SWA
to the non RCD protected side. As a minimum I would include a double
pole switch such that you can completely isolate the garden feed.

I would suggest a SWA cable submain to the outbuilding, and a consumer
unit installed there to supply any other circuits required.

Thanks.

You may want to consider running a larger cable through the house and
then converting to SWA for the external run.

I'd definitely do this if it were feasible but due to how the wiring
needs to route in the building it would mean ripping up a large floor
area in several rooms and cost ££££. I could route a new SWA out the
front of the building and then either around the house on the wall,
which would be unsightly, or bury it for some of the way, but really
neither would be good.

Your limitation with the existing cable is going to be one of voltage
drop. At full load it is already out of spec at over 12V drop at the far
end (19mV/A/m). (obviously if it only has a socket on it at the moment
you can't get it to full load and it is ok for its current use)

No scope for using the existing cable to pull through a new one?

house supply is TN-C-S and I'd assumed that extending the earth from
the house would be ok
No - absolutely not! This is just the situation you really don't want to
extend a TN-C-S earth outside. If it were just going into the
outbuilding then it may have been a acceptable solution if implemented
correctly. However since you also need feeds into the garden itself for
lights and water features itself, using the TN-C-S earth is technically
not feasible, and could pose a serious safety hazard if you did it.

This is what you need to do. The outbuilding wants to be a TT install
with its own earth spike and RCD protection.

Thanks! I do need to get a cable laid to the outbuilding soon as the
concrete base is going down (probably tomorrow), and planned to take
the cable up through the concrete base so that it'll be there to feed
into the building when that's built. I'll go for a beefy SWA cable to
minimise losses on the run into the outbuilding, and would 2 core SWA
be fine for that or should it be 3 core?

2 Core is fine. You would need to connect the armour to earth in the
house end, but then isolate it from the CU in the outbuilding. There was
a recent thread that covered the issues here quite well:

http://groups.google.com/group/uk.d-...d959cb137922ec
http://tinyurl.com/3k4py3


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

[[email protected]]

Just a note. XLPE insulated (BS 6943X) SWA has max CCC for 90C
conductor temperature.
Where cables are connected to equipment/accessories designed to
operate at 70C, the current ratings need to be derated to 70C such as
by use of CCC specified in BS6004 BS6346.