SWA carrying two circuits?
 

Hi,

I am going to be running SWA cable out to the garage soon (Total run is
a little under 20em from the house CU to the garage CU)

I will also be installing a generator point on the garage, which will be
connecting to a proper transfer switch in the house.

It will be easier and cheaper if I just run a single SWA cable with 4
cores, two for the generator and two for the feed into the garage.

Is this permitted, or do I have to have two separate cables?

The SWA will be terminated in the house and garage in a metal box, then
the two circuits taken from there to there respective places.

Thanks!

Toby...

SWA carrying two circuits?
 

Toby wrote:
Hi,

I am going to be running SWA cable out to the garage soon (Total run is
a little under 20em from the house CU to the garage CU)

I will also be installing a generator point on the garage, which will be
connecting to a proper transfer switch in the house.

It will be easier and cheaper if I just run a single SWA cable with 4
cores, two for the generator and two for the feed into the garage.

Is this permitted, or do I have to have two separate cables?

The SWA will be terminated in the house and garage in a metal box, then
the two circuits taken from there to there respective places.

The main complexity sounds like it is going to be dealing with earthing
and possibly main equipotential bonding. What earthing scheme do you
have in the house?

--
Cheers,

John.

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

SWA carrying two circuits?
 

Toby wrote:
Hi,

I am going to be running SWA cable out to the garage soon (Total run is
a little under 20em from the house CU to the garage CU)

I will also be installing a generator point on the garage, which will be
connecting to a proper transfer switch in the house.

It will be easier and cheaper if I just run a single SWA cable with 4
cores, two for the generator and two for the feed into the garage.

Is this permitted, or do I have to have two separate cables?

The SWA will be terminated in the house and garage in a metal box, then
the two circuits taken from there to there respective places.

Thanks!

Toby...

I have to say, I wouldn't recommend it on principle (i.e. run both
services on a single cable). From a safety point of view, it would not
be usual or expected that you would have a source of energy at each end
of a single cable. If you do choose to go this route, then make sure
your labelling is permanent, clear and unambiguous.

SWA carrying two circuits?
 

John Rumm wrote:
Toby wrote:
Hi,

I am going to be running SWA cable out to the garage soon (Total run
is a little under 20em from the house CU to the garage CU)

I will also be installing a generator point on the garage, which will
be connecting to a proper transfer switch in the house.

It will be easier and cheaper if I just run a single SWA cable with 4
cores, two for the generator and two for the feed into the garage.

Is this permitted, or do I have to have two separate cables?

The SWA will be terminated in the house and garage in a metal box,
then the two circuits taken from there to there respective places.

The main complexity sounds like it is going to be dealing with earthing
and possibly main equipotential bonding. What earthing scheme do you
have in the house?


The idea was to use the SWA armour as the earth for both and have an
additional earthing rod next to the generator point connected via 10mm
cable (Generator is only 7.5KVA).

The generator transfer switch This one -
http://www.briggsandstrattongenerato...ertransfer.htm
does not have the facility to switch the earthing, so I am assuming all
the earth connections can stay connected all the time?

My main earth is supplied by the armour of the incoming supply cable (TN-S)

Is there any reason the earth connection to this can't be left connected
in the event of a power failure with the generator running?
(I assumed it is OK as the transfer switch does not have the facility to
switch the earth connection)
The generator has one leg of it's output tied to ground, so there will
be no potential difference between neutral and earth in the system. If
live found it's way to earth, this would just create a short, tripping
the MCB, so, as far as I can see it, there is no risk to anyone working
on the power grid here (just like the power company ties their neutral
to earth everywhere)

Also any reason the earth rod can't stay "in circuit" during normal
operation (considering the lead water pipe is connected to the earthing
system 24/7 via 10mm cable and this will be acting the same, if not better)

In reality the generator won't be running very often, as the mains power
supply should be reliable, but I already have the generator, so am just
making provision for it's use "just in case" - If I move out, it will be
coming with me too, so nothing to worry about later on either, it will
simply be disconnected.

Toby...

SWA carrying two circuits?
 

Toby wrote:
John Rumm wrote:
Toby wrote:
Hi,

I am going to be running SWA cable out to the garage soon (Total run
is a little under 20em from the house CU to the garage CU)

I will also be installing a generator point on the garage, which will
be connecting to a proper transfer switch in the house.

It will be easier and cheaper if I just run a single SWA cable with 4
cores, two for the generator and two for the feed into the garage.

Is this permitted, or do I have to have two separate cables?

The SWA will be terminated in the house and garage in a metal box,
then the two circuits taken from there to there respective places.

The main complexity sounds like it is going to be dealing with
earthing and possibly main equipotential bonding. What earthing scheme
do you have in the house?


The idea was to use the SWA armour as the earth for both and have an
additional earthing rod next to the generator point connected via 10mm
cable (Generator is only 7.5KVA).

The generator transfer switch This one -
http://www.briggsandstrattongenerato...ertransfer.htm
does not have the facility to switch the earthing, so I am assuming all
the earth connections can stay connected all the time?

My main earth is supplied by the armour of the incoming supply cable (TN-S)

Is there any reason the earth connection to this can't be left connected
in the event of a power failure with the generator running?
(I assumed it is OK as the transfer switch does not have the facility to
switch the earth connection)
The generator has one leg of it's output tied to ground, so there will
be no potential difference between neutral and earth in the system. If
live found it's way to earth, this would just create a short, tripping
the MCB, so, as far as I can see it, there is no risk to anyone working
on the power grid here (just like the power company ties their neutral
to earth everywhere)

Also any reason the earth rod can't stay "in circuit" during normal
operation (considering the lead water pipe is connected to the earthing
system 24/7 via 10mm cable and this will be acting the same, if not better)

In reality the generator won't be running very often, as the mains power
supply should be reliable, but I already have the generator, so am just
making provision for it's use "just in case" - If I move out, it will be
coming with me too, so nothing to worry about later on either, it will
simply be disconnected.

Toby...

I think John is alluding to concerns about "exporting" the earth from
your TN-S main supply out to the garage and "re-importing" the generator
earth back from the garage to the house.

see here for more info:

http://wiki.diyfaq.org.uk/index.php?...i ng_an_earth

Given that you have a TN-S supply and are proposing properly buried SWA
cabling and this garage is presumably a building of substance rather
than a rickety shed, then I would probably be inclined to (permanently)
connect all the earths together and run an additional main-bonding
conductor alongside your SWA cables. You should consider getting a duct
to run your cables rather than direct burial, because you would need to
upgrade your main bonding earth wire further if it was direct buried.

e.g.
http://www.polypipe.com/polypipe/con...ategoryID=7598

SWA carrying two circuits?
 

In article ,
Toby wrote:
Also any reason the earth rod can't stay "in circuit" during normal
operation (considering the lead water pipe is connected to the earthing
system 24/7 via 10mm cable and this will be acting the same, if not
better)

Using a water pipe to provide the earth isn't allowed these days.

--
*Plagiarism saves time *

Dave Plowman London SW
To e-mail, change noise into sound.

SWA carrying two circuits?
 

Dave Osborne wrote:
Toby wrote:
John Rumm wrote:
Toby wrote:
Hi,

I am going to be running SWA cable out to the garage soon (Total run
is a little under 20em from the house CU to the garage CU)

I will also be installing a generator point on the garage, which
will be connecting to a proper transfer switch in the house.

It will be easier and cheaper if I just run a single SWA cable with
4 cores, two for the generator and two for the feed into the garage.

Is this permitted, or do I have to have two separate cables?

The SWA will be terminated in the house and garage in a metal box,
then the two circuits taken from there to there respective places.

The main complexity sounds like it is going to be dealing with
earthing and possibly main equipotential bonding. What earthing
scheme do you have in the house?


The idea was to use the SWA armour as the earth for both and have an
additional earthing rod next to the generator point connected via
10mm cable (Generator is only 7.5KVA).

The generator transfer switch This one -
http://www.briggsandstrattongenerato...ertransfer.htm
does not have the facility to switch the earthing, so I am assuming
all the earth connections can stay connected all the time?

My main earth is supplied by the armour of the incoming supply cable
(TN-S)

Is there any reason the earth connection to this can't be left
connected in the event of a power failure with the generator running?
(I assumed it is OK as the transfer switch does not have the facility
to switch the earth connection)
The generator has one leg of it's output tied to ground, so there will
be no potential difference between neutral and earth in the system. If
live found it's way to earth, this would just create a short, tripping
the MCB, so, as far as I can see it, there is no risk to anyone
working on the power grid here (just like the power company ties their
neutral to earth everywhere)

Also any reason the earth rod can't stay "in circuit" during normal
operation (considering the lead water pipe is connected to the
earthing system 24/7 via 10mm cable and this will be acting the same,
if not better)

In reality the generator won't be running very often, as the mains
power supply should be reliable, but I already have the generator, so
am just making provision for it's use "just in case" - If I move out,
it will be coming with me too, so nothing to worry about later on
either, it will simply be disconnected.

Toby...

I think John is alluding to concerns about "exporting" the earth from
your TN-S main supply out to the garage and "re-importing" the generator
earth back from the garage to the house.

see here for more info:

http://wiki.diyfaq.org.uk/index.php?...i ng_an_earth


Given that you have a TN-S supply and are proposing properly buried SWA
cabling and this garage is presumably a building of substance rather
than a rickety shed, then I would probably be inclined to (permanently)
connect all the earths together and run an additional main-bonding
conductor alongside your SWA cables. You should consider getting a duct
to run your cables rather than direct burial, because you would need to
upgrade your main bonding earth wire further if it was direct buried.

e.g.
http://www.polypipe.com/polypipe/con...ategoryID=7598


Thanks for the reply.

The garage is indeed a proper building (brick) and is only about 6m from
the house, so this is why I am choosing to export the house earth there
- if it were down the other end of the garden, then I expect a separate
TT system would be better for the garage. - It will have an earthing rod
for the generator too, so it will be both TN-S and TT I suppose!

I was planning on using 4mm XPLE SWA cable, and having the feed to the
garage via a 32A MCB (Not RCD protected) and then have a 5 way CU in the
garage, with a main switch, RCBO's for sockets and exterior lights and a
MCB for the freezer circuit (All the wiring in the garage will be done
in surface mount conduit, so nothing will be buried in the walls)

How do I find out the CSA of the armour in the 4mm SWA cable - is there
a chart somewhere showing the values?, I am happy to use 6mm if I need
to, to get the CSA up on this cable, and save having to run another duct
(I will be running a green duct for comms cables anyway, but would
rather not run another duct just for a separate earth cable if I can
help it.

Toby...

SWA carrying two circuits?
 

Dave Plowman (News) wrote:
In article ,
Toby wrote:
Also any reason the earth rod can't stay "in circuit" during normal
operation (considering the lead water pipe is connected to the earthing
system 24/7 via 10mm cable and this will be acting the same, if not
better)

Using a water pipe to provide the earth isn't allowed these days.


I realise that, but as the lead water pipe is bonded, it will be acting
as an earth (but not the primary one!) too!

SWA carrying two circuits?
 

Toby coughed up some electrons that declared:

How do I find out the CSA of the armour in the 4mm SWA cable - is there
a chart somewhere showing the values?, I am happy to use 6mm if I need
to, to get the CSA up on this cable, and save having to run another duct
(I will be running a green duct for comms cables anyway, but would
rather not run another duct just for a separate earth cable if I can
help it.

Toby...

Send me your email address (mine's valid) - I have a datasheet I can send
you on XLPE with actual and copper-equivalent CSAs.

Can't find the source though, otherwise I would have just posted a link...

Cheers

Tim

SWA carrying two circuits?
 

Dave Osborne wrote:

I think John is alluding to concerns about "exporting" the earth from
your TN-S main supply out to the garage and "re-importing" the generator
earth back from the garage to the house.

I was more concern about the difficulty that may occur should one end up
with a TN-S system that has to become a TT for use with the genset. That
would them mean the need for RCD protection on all circuits to maintain
the fault protection.

However - more info from Toby suggests that the genset will be
configured to supply a TN-C-S style supply.



--
Cheers,

John.

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

SWA carrying two circuits?
 

John Rumm wrote:
Dave Osborne wrote:

I think John is alluding to concerns about "exporting" the earth from
your TN-S main supply out to the garage and "re-importing" the
generator earth back from the garage to the house.

I was more concern about the difficulty that may occur should one end up
with a TN-S system that has to become a TT for use with the genset. That
would them mean the need for RCD protection on all circuits to maintain
the fault protection.

However - more info from Toby suggests that the genset will be
configured to supply a TN-C-S style supply.



This is correct.

The transfer switch can also have an RCD fitted if I need it - do I (I
would rather not if I can get away with it to be honest) but if I do
need one, can I fit a 100mA one?

Thanks again!

SWA carrying two circuits?
 

"Tim S" wrote in message
...
Toby coughed up some electrons that declared:

How do I find out the CSA of the armour in the 4mm SWA cable - is there
a chart somewhere showing the values?, I am happy to use 6mm if I need
to, to get the CSA up on this cable, and save having to run another duct
(I will be running a green duct for comms cables anyway, but would
rather not run another duct just for a separate earth cable if I can
help it.

Toby...

Send me your email address (mine's valid) - I have a datasheet I can send
you on XLPE with actual and copper-equivalent CSAs.

Can't find the source though, otherwise I would have just posted a link...

Cheers

Tim

Here is a link to a table
http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

FWIW my view is that thought through properly then 2 circuits in one SWA
cable is fine when suitably labeled. Also, provided you gland properly then
it should not be necessary to run a separate earth wire. And finally, SWA
is suitable for direct burial - ducting should not be necessary if you think
everyrhing through.

Regards
Bruce

SWA carrying two circuits?
 

Toby wrote:
Dave Osborne wrote:
Toby wrote:
John Rumm wrote:
Toby wrote:
Hi,

I am going to be running SWA cable out to the garage soon (Total
run is a little under 20em from the house CU to the garage CU)

I will also be installing a generator point on the garage, which
will be connecting to a proper transfer switch in the house.

It will be easier and cheaper if I just run a single SWA cable with
4 cores, two for the generator and two for the feed into the garage.

Is this permitted, or do I have to have two separate cables?

The SWA will be terminated in the house and garage in a metal box,
then the two circuits taken from there to there respective places.

The main complexity sounds like it is going to be dealing with
earthing and possibly main equipotential bonding. What earthing
scheme do you have in the house?


The idea was to use the SWA armour as the earth for both and have an
additional earthing rod next to the generator point connected via
10mm cable (Generator is only 7.5KVA).

The generator transfer switch This one -
http://www.briggsandstrattongenerato...ertransfer.htm
does not have the facility to switch the earthing, so I am assuming
all the earth connections can stay connected all the time?

My main earth is supplied by the armour of the incoming supply cable
(TN-S)

Is there any reason the earth connection to this can't be left
connected in the event of a power failure with the generator running?
(I assumed it is OK as the transfer switch does not have the facility
to switch the earth connection)
The generator has one leg of it's output tied to ground, so there
will be no potential difference between neutral and earth in the
system. If live found it's way to earth, this would just create a
short, tripping the MCB, so, as far as I can see it, there is no risk
to anyone working on the power grid here (just like the power company
ties their neutral to earth everywhere)

Also any reason the earth rod can't stay "in circuit" during normal
operation (considering the lead water pipe is connected to the
earthing system 24/7 via 10mm cable and this will be acting the same,
if not better)

In reality the generator won't be running very often, as the mains
power supply should be reliable, but I already have the generator, so
am just making provision for it's use "just in case" - If I move out,
it will be coming with me too, so nothing to worry about later on
either, it will simply be disconnected.

Toby...

I think John is alluding to concerns about "exporting" the earth from
your TN-S main supply out to the garage and "re-importing" the
generator earth back from the garage to the house.

see here for more info:

http://wiki.diyfaq.org.uk/index.php?...i ng_an_earth


Given that you have a TN-S supply and are proposing properly buried
SWA cabling and this garage is presumably a building of substance
rather than a rickety shed, then I would probably be inclined to
(permanently) connect all the earths together and run an additional
main-bonding conductor alongside your SWA cables. You should consider
getting a duct to run your cables rather than direct burial, because
you would need to upgrade your main bonding earth wire further if it
was direct buried.

e.g.
http://www.polypipe.com/polypipe/con...ategoryID=7598


Thanks for the reply.

The garage is indeed a proper building (brick) and is only about 6m from
the house, so this is why I am choosing to export the house earth there
- if it were down the other end of the garden, then I expect a separate
TT system would be better for the garage. - It will have an earthing rod
for the generator too, so it will be both TN-S and TT I suppose!

I was planning on using 4mm XPLE SWA cable, and having the feed to the
garage via a 32A MCB (Not RCD protected) and then have a 5 way CU in the
garage, with a main switch, RCBO's for sockets and exterior lights and a
MCB for the freezer circuit (All the wiring in the garage will be done
in surface mount conduit, so nothing will be buried in the walls)

How do I find out the CSA of the armour in the 4mm SWA cable

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

- is there
a chart somewhere showing the values?, I am happy to use 6mm if I need
to, to get the CSA up on this cable, and save having to run another duct
(I will be running a green duct for comms cables anyway, but would
rather not run another duct just for a separate earth cable if I can
help it.

Toby...

I've done a bit more homework...

For a line CSA of 4mm2, the minimum earth CSA is 10mm2 if using the
steel armour of a buried SWA cable, provided that there is mechanical
protection[1]. (Table 54.1).

For a 4mm2 2-core SWA cable, the CSA of the armour is about 20mm2, so
you are OK in principle and don't need an additional earth wire.

[1] - dig a trench at least 550 deep. Lay about 50mm of sand
blinding[2]. Lay SWA on sand blinding (if laying two cables, keep them
separated laterally[3]). Check that top of cable is at least 450 deep.
Add 150mm sand blinding on top of cable; place mechanical protection on
top of sand blinding (roof slate, concrete tiles, clay half pipe, etc),
back fill another 150 or so; Place yellow "Electrical Service Below"
tape; Backfill to top.

[2] Use soil in lieu of sand if soil is "free of sharp stones and other
materials that might damage the cable".

[3] Note that two SWA cables sharing the trench incurs a current rating
factor of:

(Table 4C2)
0.8 (one cable diameter lateral separation);
0.85 (125mm lateral separation);
0.9 (250mm lateral separation);

Two-core 4mm2 70deg armoured cable buried direct has current capacity of
37A (Table 4D4A) so you should up-rate your line conductors to 6mm2 or
have a minimum lateral separation of 250mm to get 32A current rating.

SWA carrying two circuits?
 

On Wed, 3 Jun 2009 11:55:31 +0100 someone who may be "BruceB"
wrote this:-

Also, provided you gland properly then
it should not be necessary to run a separate earth wire.

Indeed. The armour of such cables is used in just this way for the
electrical supply to many buildings.


--
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

SWA carrying two circuits?
 

Tim S wrote:
Toby coughed up some electrons that declared:

How do I find out the CSA of the armour in the 4mm SWA cable - is there
a chart somewhere showing the values?, I am happy to use 6mm if I need
to, to get the CSA up on this cable, and save having to run another duct
(I will be running a green duct for comms cables anyway, but would
rather not run another duct just for a separate earth cable if I can
help it.

Toby...

Send me your email address (mine's valid) - I have a datasheet I can send
you on XLPE with actual and copper-equivalent CSAs.

Can't find the source though, otherwise I would have just posted a link...

Wot like this:

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



--
Cheers,

John.

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

SWA carrying two circuits?
 

BruceB wrote:
"Tim S" wrote in message
...
Toby coughed up some electrons that declared:

How do I find out the CSA of the armour in the 4mm SWA cable - is there
a chart somewhere showing the values?, I am happy to use 6mm if I need
to, to get the CSA up on this cable, and save having to run another duct
(I will be running a green duct for comms cables anyway, but would
rather not run another duct just for a separate earth cable if I can
help it.

Toby...
Send me your email address (mine's valid) - I have a datasheet I can send
you on XLPE with actual and copper-equivalent CSAs.

Can't find the source though, otherwise I would have just posted a link...

Cheers

Tim

Here is a link to a table
http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

FWIW my view is that thought through properly then 2 circuits in one SWA
cable is fine when suitably labeled. Also, provided you gland properly then
it should not be necessary to run a separate earth wire. And finally, SWA
is suitable for direct burial - ducting should not be necessary if you think
everyrhing through.

Remember that if you are exporting a TN-C-S earth, then you must also
export the equipotential zone. Hence it must be large enough to support
operation as an earth and a main bonding conductor. That can eat into
your copper equivalent mm of armour CSA quite quickly.

--
Cheers,

John.

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

SWA carrying two circuits?
 

John Rumm coughed up some electrons that declared:
Wot like this:


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




Er yes...

SWA carrying two circuits?
 

John Rumm wrote:

However - more info from Toby suggests that the genset will be
configured to supply a TN-C-S style supply.

That's not allowed - using any combined neutral and earth (CNE)
conductor in a consumer's installation is illegal in the UK, under the
ESQC regulations [1].

Configure the generator to make a TN-S system. Here's a blast from the
past which includes an ASCII wiring diagram:

http://groups.google.co.uk/group/uk....n&dmode=source

I would very strongly discourage the 4-core SWA approach here, because
its unconventional nature could be quite dangerous for the unwary.
Using two 2-core SWAs will bolster up the armour CSA nicely too.


[1] http://www.opsi.gov.uk/si/si2002/20022665.htm#8 - regulation 8(4)

--
Andy

SWA carrying two circuits?
 

"Andy Wade" wrote in message
...
=source

I would very strongly discourage the 4-core SWA approach here, because its
unconventional nature could be quite dangerous for the unwary. Using two
2-core SWAs will bolster up the armour CSA nicely too.


I think the opposite is true from the point of view of CSA size. 4 core SWA
meets the Cu equivalent of 10mm2 from 4mm2 cables upwards whereas with 2
core cables you need to be using 10mm2 cable before the Cu equivalent hits
10mm2. (I am assuming you might want the SWA to act as main equipotential
bonding).
See http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

Unconventional is not necessarily unsafe. Labels and diagrams......

Regards
Bruce

SWA carrying two circuits?
 

BruceB wrote:

I think the opposite is true from the point of view of CSA size.

I was thinking of the total armour CSA, assuming that the two armours
are in parallel. I know there are arguments about how fault current
will divide between the two, but if you design so that each cable is
separately compliant then the parallel path will only help lower the Zs.

4 mm^2 SWA has armour CSA of 19 mm^2 for 2-core and 23 mm^2 for 4-core.

For 6 mm^2 the corresponding values are 22 and 36 mm^2, so in both cases
2x 2-core gives you considerably more total steel area than 1x 4-core.

4 core SWA meets the Cu equivalent of 10mm2 from 4mm2 cables upwards
whereas with 2 core cables you need to be using 10mm2 cable before
the Cu equivalent hits 10mm2. (I am assuming you might want the SWA
to act as main equipotential bonding).

Hmm, the requirement for bonding conductors is "equivalent conductance"
to copper, not equivalent thermal fault capacity. Since the resistivity
of steel is over 8 times higher than that of copper you're looking for
over 80 mm^2 of armour for main bonding on a PME installation, which
takes you up to _much_ larger SWA sizes. A separate (paralleled) copper
bonding conductor is probably a more practical proposition. Of course
this is only relevant if there are actually any
extraneous-conductive-parts in the outbuilding that need bonding.

Unconventional is not necessarily unsafe. Labels and diagrams......

In general, yes. In this case no, IMHO. See the 314 group of
regulations in BS 7671.

--
Andy

SWA carrying two circuits?
 

"Andy Wade" wrote in message
...
BruceB wrote:

I think the opposite is true from the point of view of CSA size.

I was thinking of the total armour CSA, assuming that the two armours are
in parallel. I know there are arguments about how fault current will
divide between the two, but if you design so that each cable is separately
compliant then the parallel path will only help lower the Zs.

4 mm^2 SWA has armour CSA of 19 mm^2 for 2-core and 23 mm^2 for 4-core.

For 6 mm^2 the corresponding values are 22 and 36 mm^2, so in both cases
2x 2-core gives you considerably more total steel area than 1x 4-core.

4 core SWA meets the Cu equivalent of 10mm2 from 4mm2 cables upwards
whereas with 2 core cables you need to be using 10mm2 cable before
the Cu equivalent hits 10mm2. (I am assuming you might want the SWA
to act as main equipotential bonding).

Hmm, the requirement for bonding conductors is "equivalent conductance" to
copper, not equivalent thermal fault capacity. Since the resistivity of
steel is over 8 times higher than that of copper you're looking for over
80 mm^2 of armour for main bonding on a PME installation, which takes you
up to _much_ larger SWA sizes. A separate (paralleled) copper bonding
conductor is probably a more practical proposition. Of course this is
only relevant if there are actually any extraneous-conductive-parts in the
outbuilding that need bonding.

Unconventional is not necessarily unsafe. Labels and diagrams......

In general, yes. In this case no, IMHO. See the 314 group of regulations
in BS 7671.

--
Andy



The 314 group of regulations are to do with division of installation. What
I am suggesting is that the armour of a multicore SWA covering live
conductors of more than one circuit is perfectly acceptable. Think about
earthed metallic conduit where the conduit forms the protective conductor.
It has always been acceptable to have more than one circuit with the
conduit. The multicore SWA is akin to that. Overall, 2 cables will of
course give you more armour, but more expense, to my mind unnecessarily.

Thank you for the point about equivalent conductance for protective bonding
conductors. I had forgotten you cannot just apply the adiabatic rules as
you can for protective conductors!

Regards
Bruce

SWA carrying two circuits?
 

Andy Wade wrote:

4 core SWA meets the Cu equivalent of 10mm2 from 4mm2 cables upwards
whereas with 2 core cables you need to be using 10mm2 cable before
the Cu equivalent hits 10mm2. (I am assuming you might want the SWA
to act as main equipotential bonding).

Hmm, the requirement for bonding conductors is "equivalent conductance"
to copper, not equivalent thermal fault capacity. Since the resistivity
of steel is over 8 times higher than that of copper you're looking for
over 80 mm^2 of armour for main bonding on a PME installation, which
takes you up to _much_ larger SWA sizes. A separate (paralleled) copper
bonding conductor is probably a more practical proposition. Of course
this is only relevant if there are actually any
extraneous-conductive-parts in the outbuilding that need bonding.

Is there a nuance we have missed in the wiki here?

at the end of the section on:

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

We say "Also note that if one is exporting the equipotential zone, that
probably means the CPC of the submain is also being used as a main
bonding conductor, and so it will will have to meet the minimum CSA size
requirements for a main equipotential bonding conductor. Since this is
often 10mm² copper equivalent, this can preclude the use of some of the
smaller sizes of SWA since the copper equivalent of the armour may be
too small. In these cases a separate bonding conductor can also be used."

Rather than an 8x factor we have: "Note that for ease of use of the
above figures one needs to convert the actual CSA of the steel armour
wires to a "copper equivalent". To do this divide the quoted figure by
2.255. So for example: a 2 core 2.5mm² cable with 70° C PVC cladding has
a copper equivalent armour area of 17 / 2.255 or 7.54mm²"


--
Cheers,

John.

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SWA carrying two circuits?
 

John Rumm wrote:
Andy Wade wrote:

4 core SWA meets the Cu equivalent of 10mm2 from 4mm2 cables upwards
whereas with 2 core cables you need to be using 10mm2 cable before
the Cu equivalent hits 10mm2. (I am assuming you might want the SWA
to act as main equipotential bonding).

Hmm, the requirement for bonding conductors is "equivalent
conductance" to copper, not equivalent thermal fault capacity. Since
the resistivity of steel is over 8 times higher than that of copper
you're looking for over 80 mm^2 of armour for main bonding on a PME
installation, which takes you up to _much_ larger SWA sizes. A
separate (paralleled) copper bonding conductor is probably a more
practical proposition. Of course this is only relevant if there are
actually any extraneous-conductive-parts in the outbuilding that need
bonding.

Is there a nuance we have missed in the wiki here?

at the end of the section on:

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


We say "Also note that if one is exporting the equipotential zone, that
probably means the CPC of the submain is also being used as a main
bonding conductor, and so it will will have to meet the minimum CSA size
requirements for a main equipotential bonding conductor. Since this is
often 10mm² copper equivalent, this can preclude the use of some of the
smaller sizes of SWA since the copper equivalent of the armour may be
too small. In these cases a separate bonding conductor can also be used."

Rather than an 8x factor we have: "Note that for ease of use of the
above figures one needs to convert the actual CSA of the steel armour
wires to a "copper equivalent". To do this divide the quoted figure by
2.255. So for example: a 2 core 2.5mm² cable with 70° C PVC cladding has
a copper equivalent armour area of 17 / 2.255 or 7.54mm²"

Sorry - hit send too soon...

Out CSA equivalence is presumably applying only the adiabatic fault
withstand requirements - which is appropriate for the original scope of
providing a CPC to an outbuilding. However I get the impression we are
missing a requirement here when said CPC is to also be a main bonding
conductor.

--
Cheers,

John.

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| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

SWA carrying two circuits?
 

"John Rumm" wrote in message
o.uk...

Is there a nuance we have missed in the wiki here?

at the end of the section on:

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

We say "Also note that if one is exporting the equipotential zone, that
probably means the CPC of the submain is also being used as a main bonding
conductor, and so it will will have to meet the minimum CSA size
requirements for a main equipotential bonding conductor. Since this is
often 10mm² copper equivalent, this can preclude the use of some of the
smaller sizes of SWA since the copper equivalent of the armour may be too
small. In these cases a separate bonding conductor can also be used."

Rather than an 8x factor we have: "Note that for ease of use of the above
figures one needs to convert the actual CSA of the steel armour wires to a
"copper equivalent". To do this divide the quoted figure by 2.255. So for
example: a 2 core 2.5mm² cable with 70° C PVC cladding has a copper
equivalent armour area of 17 / 2.255 or 7.54mm²"



Yes there is a nuance missed I think

There are 2 separate calculations or issues.

Fault currents and protective conductor sizing. Adiabatic regime applies
which allows you to use ratio of 'k' values to work out minimum protective
conductor size. Table 54.7 gives an example.

Protective bonding conductors. Reg 544.1 covers protective bonding
conductors. 544.1.1 introduces the phrase 'affording equivalent conductance
in other metals' which is then emphasised in the note at the bottom of Table
54.8. This is much more demanding and as was said in another post by Andy
Wade means you need pretty large (impractically large for domestic) sizes of
swa to comply if you want armour as a bonding conductor.

Regards
Bruce

SWA carrying two circuits?
 

BruceB wrote:
"John Rumm" wrote in message
o.uk...
Is there a nuance we have missed in the wiki here?

at the end of the section on:

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

We say "Also note that if one is exporting the equipotential zone, that
probably means the CPC of the submain is also being used as a main bonding
conductor, and so it will will have to meet the minimum CSA size
requirements for a main equipotential bonding conductor. Since this is
often 10mm² copper equivalent, this can preclude the use of some of the
smaller sizes of SWA since the copper equivalent of the armour may be too
small. In these cases a separate bonding conductor can also be used."

Rather than an 8x factor we have: "Note that for ease of use of the above
figures one needs to convert the actual CSA of the steel armour wires to a
"copper equivalent". To do this divide the quoted figure by 2.255. So for
example: a 2 core 2.5mm² cable with 70° C PVC cladding has a copper
equivalent armour area of 17 / 2.255 or 7.54mm²"



Yes there is a nuance missed I think

There are 2 separate calculations or issues.

Fault currents and protective conductor sizing. Adiabatic regime applies
which allows you to use ratio of 'k' values to work out minimum protective
conductor size. Table 54.7 gives an example.

Indeed - I think we have covered this angle well enough...

Protective bonding conductors. Reg 544.1 covers protective bonding
conductors. 544.1.1 introduces the phrase 'affording equivalent conductance
in other metals' which is then emphasised in the note at the bottom of Table
54.8. This is much more demanding and as was said in another post by Andy
Wade means you need pretty large (impractically large for domestic) sizes of
swa to comply if you want armour as a bonding conductor.

Yup, I think we need to create a new section on exporting equipotential
bonding to cover this requirement.

Not sure how we missed that first time thought - since the same words
are on the end of Table 54H in the 16th edition.

I think it might be wise to modify the tables we have that show the
armour CSA to also include the resistance per m based on table H.2 in
BS5467:1997

--
Cheers,

John.

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| Internode Ltd - http://www.internode.co.uk |
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