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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