SWA 2 or 3 core to garage?
Mr Sandman
should i go for 2 or 3 core swa down to my garage? its about 40 meters away
and i am going for 16mm3
cheers
steve
John Rumm
> hi all,
>
> should i go for 2 or 3 core swa down to my garage? its about 40 meters
> away and i am going for 16mm3
Two, unless you are exporting a PME earth as well (and hence the main
bonding conductor).
http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside
--
Cheers,
John.
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John Rumm
> On 18/05/2011 09:09, Mr Sandman wrote:
>> hi all,
>>
>> should i go for 2 or 3 core swa down to my garage? its about 40 meters
>> away and i am going for 16mm3
>
> Two, unless you are exporting a PME earth as well (and hence the main
> bonding conductor).
Pardon me talking to myself, but its just been pointed out to me that
you were talking about 16mm^2 SWA. The armour on that has enough copper
equivalent CSA to be used as a main bonding conductor as well - so you
will only need two core regardless.
ARWadsworth
> On 18/05/2011 11:26, John Rumm wrote:
>> On 18/05/2011 09:09, Mr Sandman wrote:
>>> hi all,
>>>
>>> should i go for 2 or 3 core swa down to my garage? its about 40
>>> meters away and i am going for 16mm3
>>
>> Two, unless you are exporting a PME earth as well (and hence the main
>> bonding conductor).
>
> Pardon me talking to myself, but its just been pointed out to me that
> you were talking about 16mm^2 SWA. The armour on that has enough
> copper equivalent CSA to be used as a main bonding conductor as well
> - so you will only need two core regardless.
>
>> http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside
Sorry John, I was working when I pointed that out to you and I needed to
check before posting to the newsgroup.
I would say a 16mm two core is fine for a 40m run using a 40A MCB. This
should cover voltage drop for a lighting circuit in the garage and allow any
mains bonding in the garage to meet the regs.
--
Adam
Bruce Burnett
says...
16mm2 armour is fine in copper equivalence terms to act as a cpc for
fault protection, but NOT to act as a main protective bonding conductor.
See note at bottom of 544.1.1. For bonding it is the equivalent
conductance that matters.
Regards
Bruce
ARWadsworth
The CSA of the armour of a 2 core 16mm SWA cable is 42mm^2.
That gives it the copper equivalent of 18.6mm^2.
For most domestic installations that will be fine.
--
Adam
BruceB
says...
I agree it gives a copper equivalence in K terms which includes specific
heat. That is relevant for temperature rise in the adiabatic equation
in relation to sizing a cpc. But that is the wrong equivalence for a
bonding conducter which has to be done in pure conductance terms. I
said above it was at the bottom of 544.1.1, it is actually at the bottom
of Table 54.8 immediately below that regulation.
You need to be around 70mm2 cable before the armour reaches an
equivalence of 10mm2 copper in conductance terms.
Regards
Bruce
John Rumm
Yup, I think I should go back to what I said in my first post - before I
got misdirected ;-)
In the wiki we have said:
"Note however that the calculated copper equivalent area is only for use
in the adiabatic equation described above. If you need to compute the
actual armour resistance then use the resistance figures
quoted instead."
and
"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 of copper (or an appropriate CSA of another metal offering
equal conductance), it will preclude the sole use of the armour wires as
a combined CPC / Bonding Conductor, since the armour resistance is
typically too large (this is generally true for all SWA cables less than
70mm²). In these cases a separate bonding conductor operating in
parallel with the armour can be used. "
ARWadsworth
I get you.
With an 8 times greater resistivity for the steel than the copper then yes
it will need to be 70mm^2 2 core SWA (as that is where the armourings
reaches 80mm CSA)
--
Adam
John Rumm
I am no fan of exporting PME earths and equipotential zones anyway in
most cases. So you would be better off making the far end TT and
sticking with two core SWA and isolating the earth at the far end IMHO.
Andy Wade
>
> With an 8 times greater resistivity for the steel than the copper then yes
> it will need to be 70mm^2 2 core SWA (as that is where the armourings
> reaches 80mm CSA)
Quite right - as debated previously at length both here and in the IET
forum. I do wonder whether it's what the committee intended to say
though, or whether it's just badly worded.
After all there's no upper limit allowed on the resistance of a copper
main bonding conductor, so why limit the conductance of a steel one?
Logically it should be either the adiabatic (I^2*t) capacity of the
conductor or its continuous current carrying capacity that should matter
for main bonding.
--
Andy
BruceB
spamb...@maxwell.myzen.co.uk says...
> Logically it should be either the adiabatic (I^2*t) capacity of the
> conductor or its continuous current carrying capacity that should matter
> for main bonding.
I was wondering the same as neither maximum resistance nor maximum
length is specified. But I think it was probably deliberate because in
addition to adiabatic capacity and current withstand there is another
factor. The bonding is there to hold the touch voltage down during a
fault and hence the conductance/resistance of the bonding conductor
compared to say the cable introducing a fault is relevant.
Regards
Bruce
Andy Wade
> [...] The bonding is there to hold the touch voltage down during a
> fault and hence the conductance/resistance of the bonding conductor
> compared to say the cable introducing a fault is relevant.
Agreed, so why don't they stipulate a maximum resistance for copper main
bonding? There is a resistance limit for supplementary bonding of
course, fist introduced in the 16th ed., IIRC.
I can only imagine that it's a non-issue in practice, with the length in
practice unlikely to be enough to lead to an excessive resistance.
Large buildings will have larger capacity supplies, mandating larger
main bonding conductors. Supplementary bonding, OTOH, can be as small
as 1 mm^2 if incorporated in a cable or otherwise mechanically
protected, so the possibility of excessive resistance is more likely.
--
Andy
Tim Watts
> I am no fan of exporting PME earths and equipotential zones anyway in
> most cases. So you would be better off making the far end TT and
> sticking with two core SWA and isolating the earth at the far end IMHO.
>
It's a tricky issue. That solution is clean if you are doing a single
submain to a remote shed.
But what would be the best solution for running a lighting circuit round the
garden (gatepost lamps, couple of lamps in the garden sort of thing? The
same circuit might supply a small lamp in a bike shed and other odds and
sods. This would be a single supply circuit with local switching.
I suppose you could make everything class II and worry less (we'll assume an
RCD at the house end of the circuit source).
Or you could TT it - but if it goes to a variety of distant loads would you
have to stake it in several places?
What if it was desirable to use this circuit in the vicinity of another
circuit, eg in the workshop[1] so that if a power tool tripped the socket
circuit, the lights stayed on - you'd have to have the same earthing system
and cross bond them.
I'm still undecided on mine. My original plan was:
20A radial to a few waterproof sockets on the house wall (one on each
corner) - In theory a class I appliance *could* be plugged in outside from
this.
32A supply to workshop for sockets only (re point about tripping and lights
not going out as a result)
10A general lighting supply going all over the place including the workshop
(which would have a pair of DP isolators locally on point of entry)
This seemed like a good idea when I believed I had a TN-S earth, but got a
bit unravelled when the bloke proved EDF's records were wrong by opening up
the cutout housing (I actually have TN-C-S).
Thoughts appreciated :)
--
Tim Watt
BruceB
>
> (snip)
> This seemed like a good idea when I believed I had a TN-S earth, but got a
> bit unravelled when the bloke proved EDF's records were wrong by opening up
> the cutout housing (I actually have TN-C-S).
>
> Thoughts appreciated :)
I know I'll be at odds with some here, but I see very little problem
exporting TN-C-S earth. Or as I describe it extending the equipotential
zone. You do have to watch out on the bonding issue, but the likelihood
of a broken neutral coming into a house are vanishingly small UNLESS the
final few metres of cable is coming in overhead through trees into your
house. I watched my PME supply go in and the final earth electrode was
buried with the cable within a few metres of my house. So virtually
zero chance opf broken neutral in my case. Many believe there is a
general prohibition on exporting a TN-C-S earth. But that is a myth -
no such rule exists.
Regards
Bruce
John Rumm
> In article<7dhka8-...@squidward.dionic.net>, t...@dionic.net says...
>>
>> (snip)
>> This seemed like a good idea when I believed I had a TN-S earth, but got a
>> bit unravelled when the bloke proved EDF's records were wrong by opening up
>> the cutout housing (I actually have TN-C-S).
>>
>> Thoughts appreciated :)
>
>
> I know I'll be at odds with some here, but I see very little problem
> exporting TN-C-S earth. Or as I describe it extending the equipotential
> zone. You do have to watch out on the bonding issue, but the likelihood
I have no problem with exporting it in some circumstances. To a shed or
garage etc might be fine. I would be less keen where there is easy
access to an independent earth such as in a building with its own
incoming services or a greenhouse. Since in these cases one has to take
care to maintain the EQ zone. In the case of a greenhouse this may be
impossible.
> of a broken neutral coming into a house are vanishingly small UNLESS the
> final few metres of cable is coming in overhead through trees into your
> house. I watched my PME supply go in and the final earth electrode was
All the properties round here are wired overhead from a transformer on a
pole. They recently replaced all the LV wiring from the transformer to
the houses and along the road. Its interesting to note that they changed
the wiring style from four independent wires spaced vertically a few
inches apart, to an arrangement with all the wires twisted into a bundle
around a support wire. I presume the latter arrangement also makes
disconnection of the PEN conductor in isolation somewhat less likely.
> buried with the cable within a few metres of my house. So virtually
> zero chance opf broken neutral in my case. Many believe there is a
Probably not that much even on an overhead supply these days... I note
they certified ours for PME when they replaced the drop cable, even
though the house is setup for TT.
> general prohibition on exporting a TN-C-S earth. But that is a myth -
> no such rule exists.
Indeed.
BruceB
see.my.s...@nowhere.null says...
> Its interesting to note that they changed
> the wiring style from four independent wires spaced vertically a few
> inches apart, to an arrangement with all the wires twisted into a bundle
> around a support wire. I presume the latter arrangement also makes
> disconnection of the PEN conductor in isolation somewhat less likely.
>
John Rumm
A quick google would seem to confer.
Probably a better solution where you must use overhead. Only downside is
aesthetic... the cables carry far more visual weight than before even if
there are fewer of them.