On 14/02/2019 14:43, sm_jamieson wrote:

I am putting in SWA cable to the garage, total length max 30 metres,
load just a few sockets for low powered tools, and lights, connected to 32A RCBO in the house CU. Should I use 4mm or 6mm SWA ?

So are we assuming the design load is 32A?

Current capacity for either are fine (the current carrying capacity
"clipped direct" for PVC clad [1] SWA would be 38A/49A (4mm^2/6mm^2))

So we can check voltage drop. Since you have lighting, that ought to be
a 3% or 6.9V. At 11mV/A/m 32A load would give ~ 10.5V, so 4mm^2 cable
will not meet the spec.

Going to 6mm^2, that would be 7.3mV/A/m or ~7V

So you could easily argue that 6mm^2 will get you close enough -
especially as it does not sound like the circuit will likely have
anything near that load on it normally. (you could also argue that the
effects of small voltage drops with modern low energy lighting are
overstated anyway).

You have not said what the earthing system is at the head end, but we
can make a crude disconnection time check: If you are using 2 core 6mm^2
SWA, the core resistance will be about 3 mOhms/m, and the armour
7mOhms/m for a total of about 10 mOhms/m round trip. That gives us 0.3
Ohms. If it were a TN-S head end you could add on 0.8 for Ze, so 1.1 Ohm
total. Low enough to meet then 1.37 Ohms min for a B32 MCB [3].

Prospective fault current = 230/1.1 = 253A, enough to be in the magnetic
part of the trip response.

Adiabatic check, sqrt( 253^2 x 0.1 ) / 115 [4] = 0.7mm^2 minimum CSA
wire required. The copper equivalent 7mm^2 of armour being 3mm^2 if we
use the 2.255 conversion factor. So plenty good enough.



[1] Much SWA will be the 90 deg C XLPE style sheathed stuff anyway,
which has higher limits)

[2]
http://wiki.diyfaq.org.uk/index.php/...our_as_a_C PC

[3]
http://wiki.diyfaq.org.uk/index.php/...g_Voltage_Drop

[4] Appropriate k factor for a PVC cable - but low for a XLPE one - see
[1] above.




--
Cheers,

John.

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