SWA Size question
 

Hi,

We would like to run some SWA from the house to a separate garage, total run
is about 35 meters, mostly buried underground.

Using the TLC calculator, here
http://www.tlc-direct.co.uk/Technica...Min+Cable+Size

or here for a short URL http://tinyurl.com/2coh6or

Do I need to say it is a lighting circuit, as there will be some lighting on
it (standard 6A, for a light in the garage).
The main power will be the 20A radial.

If I say it is lighting, then it wants me to use 10mm cable for a 32A
circuit, which seems overkill to me?

Thanks!

SWA Size question
 

17th Regulations require...

#1 Voltage drop within limits for the final circuit type.
Hence cable size must be suitable for the 35m length so as to minimise
voltage drop.

#2 All cables must have RCD protection unless Surface Run, 50mm
depth, SWA or BS8436 or Steel Conduit.
If you run FTE in the house to a jn-box then SWA up the garden, that
FTE must be surface run or 50mm depth or in steel conduit or have RCD
protection at the house-end rather than shed end. Having the RCD at
the house-end means an earth fault on the garage power would take out
the lights so you may want an emergency light in the garage if using
circular saw.
Some people run SWA all the way to the house CU, but in some cases
this can be very difficult (solid floors, space).

There is a uk.d-i-y FAQ available - worth reading.

35m is quite a long way - I assume that includes the run inside the
house to the CU.
If you dropped to 16A radial in the shed and 6A lighting you could
design for a 16A supply (16A RCBO in the house) which would probably
make 6mm SWA doable. 16A is quite reasonable unless your garage is
actually a workshop with lots of power tools in use. For 35m the
garage is most likely going to be a TT supply, ie, 2 core SWA with
garage local earth rod.

SWA Size question
 

"js.b1" wrote in message
...
17th Regulations require...

#1 Voltage drop within limits for the final circuit type.
Hence cable size must be suitable for the 35m length so as to minimise
voltage drop.

#2 All cables must have RCD protection unless Surface Run, 50mm
depth, SWA or BS8436 or Steel Conduit.
If you run FTE in the house to a jn-box then SWA up the garden, that
FTE must be surface run or 50mm depth or in steel conduit or have RCD
protection at the house-end rather than shed end. Having the RCD at
the house-end means an earth fault on the garage power would take out
the lights so you may want an emergency light in the garage if using
circular saw.
Some people run SWA all the way to the house CU, but in some cases
this can be very difficult (solid floors, space).

There is a uk.d-i-y FAQ available - worth reading.

35m is quite a long way - I assume that includes the run inside the
house to the CU.
If you dropped to 16A radial in the shed and 6A lighting you could
design for a 16A supply (16A RCBO in the house) which would probably
make 6mm SWA doable. 16A is quite reasonable unless your garage is
actually a workshop with lots of power tools in use. For 35m the
garage is most likely going to be a TT supply, ie, 2 core SWA with
garage local earth rod.

Thanks for the reply..

I was planning on MCB protection only in the house, as it is going to be SWA
from the CU in the house to the CU in the garage (Well, the SWA would
connect to an adaptable metal box under the CU, then I would connect a 10mm
earth cable from this using, a piranha nut, to the main earth terminal
Then run the L&N to the CU in round plastic conduit (Probably about 15cm).

I would then have a small CU in the garage containing a main switch, a 6A
MCB for the lights (run surface, in round conduit) and a 20A RCBO for the
sockets (Again run surface, in round conduit) - I don't see the need for an
RCBO in the house, as that means the whole supply would go off, including
the lights, if there was a problem with sockets, and a trip to the house
would need to be made to reset it.

I would also add a 16A MCB for a possibly future freezer.

The distance between the house and the garage is about 15m, so I was
planning on exporting the earth, using the amour of 2 core SWA, is that
unreasonable for an exported earth?

Toby...

SWA Size question
 

On May 7, 2:11*pm, "Toby" wrote:
The distance between the house and the garage is about 15m, so I was
planning on exporting the earth, using the amour of 2 core SWA, is that
unreasonable for an exported earth?

It comes down to...
#1 SWA armour cross-sectional area - which should be ok.
#2 No incoming services, garage floor dry, class-2 tools

If you have incoming services you need 10mm 3-core to bond them back
to the MET, you can not use the steel wire armour because its cross-
sectional area converted from steel is not equivalent to 10mm copper.

Personally I would use 3 core SWA because it means you are not reliant
on the armour integrity. The cost difference is probably not too
different. Conversely if you are not exporting the house earth, and
garages can have damp concrete floors or class-1 tools, then 2-core is
fine with a TT supply & local earth rod.

If price is high on the SWA always worth checking Ebay for a cheap
offcut - does happen, you may have to wait a while.

SWA Size question
 

"John Rumm" wrote in message
o.uk...
On 11/05/2010 20:40, Toby wrote:

Thanks John,

This is actually for my brother's house, and after going round there, I
realised he was being a total plank, and the cable run is just 18 meters,
I
have no idea where he got 35m from!

The garage does not have other services.

If it did have a water supply, in plastic, I assume that would not affect
things would it?

No it would not change anything. On two counts - firstly being plastic it
would not be capable of introducing a potential into an equipotential
zone, and secondly, not being TN-C-S there is no need to export the EQ
zone in any case! ;-)

(It doesn't, I was just contemplating running a plastic water pipe to my
own
garage)

The earth in the house is TN-S (Provided through the amour of the
incoming
cable).

That makes it simpler then. You could go for local TT setup in the garage
(thought it was a wooden workshop?), or at that distance with a TN-S head
end you could just export the house earth.

With the revised figures, the TLC calculator shows that 4mm cable is fine
for a 32A lighting circuit (just)!

If you are sure that is enough capacity for any future needs then go with
that (cable is cheap - laying it etc is not!)

So, with a 32A MCB in the house, would it be best to used 3 core cable
and
earth the core just at the house end, or just connect to earth both ends?

If you go with an exported earth, then use two core, and connect the
armour to earth at both ends - it will be your earth[1].

If you go with TT at the remote end, then connect the armour to earth at
the head end, but isolate it at the far end, and instead use your earth
stake for the local earth.


[1] either with metal clad box at the head end, and a metal CU at the
remote end, or the TLC piranha nuts to make a connection if using plastic
boxes/CUs. (note you should not use a metal CU if making the workshop TT)

--

Thanks John,

He is digging the trench now, but isn't sure about the depth - it will run
under some slabs of a patio, then under the back of a flower bed - am I
correct in thinking there isn't a finite depth required, it just needs to be
at a sufficient depth to avoid being damaged by any reasonably foreseeable
disturbance to the ground?

If so, would laying it just under the slabs (so it is then visible if they
are removed) would be OK (The slabs are bedded in cement, so we will
probably angle grin a channel in it), and then two spade depths at the back
of the flower bed be sufficient, with warning tape at 1 spades depth?

We would also take photos and make sure this is kept in an envelope near the
CU.

As for the termination, I would drill a hole through the wall of the house,
then mount a plastic box over this, then thread the cable in the bottom of
the box, then out the back, so no glands are used here, the box just
protects the cable from bending too sharply. Then it would run under the
kitchen cabinets to the under stairs cupboard, and terminate in a metal
adaptable box - then run the earth to the main earth terminal near the
service head, and the L&N to the CU via a short piece of round conduit.

In the garage, the SWA will terminate directly in a 4 or 5 way metal clad
CU, with a main switch, RCBO for the main radial, and MCB's for the freezer
(potentially) and an MCB for the lights - all cables run surface, in round
conduit

Does that sound OK?

The roof of the garage has some large metal triangle supports - do we need
to bond these all together, and then tie that into the earth in the CU too?

Thanks again!

Toby...

SWA Size question
 

On Wed, 12 May 2010 10:30:16 +0100 someone who may be "Toby"
wrote this:-

The roof of the garage has some large metal triangle supports - do we need
to bond these all together, and then tie that into the earth in the CU too?

Are they liable to introduce a potential, including earth potential,
into the building? How large is large?

I have no idea, but it sounds like they are not liable to introduce
a potential. If that is true then they do not need to be bonded.




--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000...#pt3-pb3-l1g54

SWA Size question
 

Bury SWA at a depth suitable for the likely use of the ground.
450mm depth generally, you may want 600mm depth thro a flower bed or
cultivated area.

Cable laying method is important.
Clean the trench of stones, lay riverwashed sand bed, lay cable on
top, cover with more riverwashed sand, tamp down, then yellow cable
marking tape with some sand on top, then fill in. TLC direct do the
yellow cable marking tape by the cut metre. Riverwashed sand is used
so a) no stones b) provides a colour change to help identify the
cable.

You can bury SWA in a flexible underground duct.
Perforated land drain pipe (80mm) is about £25 delivered for 25m;
proper twinwall (smooth interior) electrical ducting is much better,
much stronger - but about £75; flexible nylon or polypropylene conduit
(not PVC) of 25mm would give a little protection against exploring
spade although not much.

SWA Size question
 

On Wed, 12 May 2010 04:15:12 -0700 (PDT) someone who may be "js.b1"
wrote this:-

Clean the trench of stones, lay riverwashed sand bed, lay cable on
top, cover with more riverwashed sand, tamp down, then yellow cable
marking tape with some sand on top, then fill in. TLC direct do the
yellow cable marking tape by the cut metre. Riverwashed sand is used
so a) no stones b) provides a colour change to help identify the
cable.

That is how it should be done and how it is usually done by those
with time. If it is being done by "professionals" a good way to
check is to ask to see the hand tampers and the hands of the people
who have been using the tampers on the sand.

More usually they pick a time when nobody is looking, heave the
earth/stones in the trench and run the wheel of a JCB over the earth
to firm it up. The end result is often penetration of the outer
sheath by stones, followed a decade or more later by water ****ing
out of the joints and other related problems.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000...#pt3-pb3-l1g54

SWA Size question
 

On May 12, 4:10 pm, David Hansen
wrote:
More usually they pick a time when nobody is looking, heave the
earth/stones in the trench and run the wheel of a JCB over the earth
to firm it up. The end result is often penetration of the outer
sheath by stones, followed a decade or more later by water ****ing
out of the joints and other related problems.

Sub-contractors :-)

Reminds me of the gas crowd running new driveway runs under other
people's driveways to avoid "pattern concrete" and then going bust.
Quite why they didn't shove it up the existing cast iron pipe as
Transco had done is beyond me, but hey-ho, such is sub-contracting.

SWA Size question
 

"John Rumm" wrote in message
o.uk...
Does that sound OK?

Yup.

The roof of the garage has some large metal triangle supports - do we
need to bond these all together, and then tie that into the earth in the
CU too?

I would not have thought so. It does not sound like they are capable of
bringing a potential (even if that potential is "earth") into the zone.
Also as mentioned before, since you are TN-S there is no requirement to
extend the main equipotential zone of the house out to the outbuilding
anyway.

Thanks for your advice John, always appreciated!

Toby...

SWA Size question
 

"js.b1" wrote in message
...
On May 12, 4:10 pm, David Hansen
wrote:
More usually they pick a time when nobody is looking, heave the
earth/stones in the trench and run the wheel of a JCB over the earth
to firm it up. The end result is often penetration of the outer
sheath by stones, followed a decade or more later by water ****ing
out of the joints and other related problems.

Sub-contractors :-)

Reminds me of the gas crowd running new driveway runs under other
people's driveways to avoid "pattern concrete" and then going bust.
Quite why they didn't shove it up the existing cast iron pipe as
Transco had done is beyond me, but hey-ho, such is sub-contracting.


Thanks for your advice js.b1!

Toby...

SWA Size question
 

Toby wrote:



He is digging the trench now, but isn't sure about the depth - it will
run under some slabs of a patio, then under the back of a flower bed -
am I correct in thinking there isn't a finite depth required, it just
needs to be at a sufficient depth to avoid being damaged by any
reasonably foreseeable disturbance to the ground?


The minimum depth, as you say, is a moving target depending on a
risk-based likelihood of the cable being disturbed.

The minimum specified by DNO's for service cables is 450mm *to the top
of the buried cable or the duct containing the cable*, so you will need
to dig deeper than this to lay sand under the cable/duct and allow for
the diameter of the cable/duct.

Under agricultural land (where there is a likelihood of ploughing
activity) the depth requirement can be as much as 1000mm to top of
cable/duct.

see he http://preview.tinyurl.com/d8lswg

Pages 9-10

SWA Size question
 

Dave Osborne wrote:
Toby wrote:



He is digging the trench now, but isn't sure about the depth - it will
run under some slabs of a patio, then under the back of a flower bed -
am I correct in thinking there isn't a finite depth required, it just
needs to be at a sufficient depth to avoid being damaged by any
reasonably foreseeable disturbance to the ground?


The minimum depth, as you say, is a moving target depending on a
risk-based likelihood of the cable being disturbed.

The minimum specified by DNO's for service cables is 450mm *to the top
of the buried cable or the duct containing the cable*, so you will need
to dig deeper than this to lay sand under the cable/duct and allow for
the diameter of the cable/duct.

Under agricultural land (where there is a likelihood of ploughing
activity) the depth requirement can be as much as 1000mm to top of
cable/duct.

see he http://preview.tinyurl.com/d8lswg

Pages 9-10

Just found another DNO document which specifies a trench depth of 530mm
for domestic new connections. They say that for a direct burial they lay
their cable in the bottom of this trench, but I suspect they are
allowing for an 80mm dia. duct to bring the top of the duct to 450mm
below finished level.