UK Garage consumer unit in an outbuilding swa 2 or 3 core

[GoBobGo]

Hi all
I'm installing an Garage consumer unit in an outbuilding being converted to a studio for a photographer just 1 ring main and lighting circuit.
supply cable 50M length SWA 10mm due to volt drop. No room where Consumer unit is to bring SWA in and terminate so was going to put IP65 box on outside wall terminate cable in there then bring either the tails of the swa cable in flexible conduit to a box above CU then into CU.
I was going to use 3 core to make sure i had a good earth but then don't think ill have room to terminate the swa armouring and the 10mm earth cable in the CU.
Did think of useing 10mm twin and earth from CU to IP65 box and connect to SWA cable but the the earth size and cable type are 2 different types.
could just use 2 core and use armouring as main earth then only 1 earth to connect into CU.
Advise please.
This could be used for An NAPPIT inspection so as to be right

 

[westward10]

Nothing wrong with using the armour as a cpc assuming no main protective bonding is required at the garage end.

 

[Strima]

What size cable do your calculations state for earth conductor?

 

[GoBobGo]

Nothing wrong with using the armour as a cpc assuming no main protective bonding is required at the garage end.

Yea thanks.

I think ill just terminate in IP65 box on outside wall then bring tails of swa and earth from banjo in through wall in conduit to box above CU then go in behind CU is spaced off wall slightly as other cable go in behind.

 

[westward10]

If it is for a scheme assessment then you should be able to prove the armour is suitable for use as the cpc by using the adiabatic equation in Regulation 543.1.3 but the chances of it not being suitable are probably zero.

 

[timhoward]

I've found this website very handy in the past to fairly quickly and easily prove such things.

CPC Sizing Tool

As above, practically zero chance the armour won't be sufficient.

I suspect the only possible problem area depending on the Ze and what fuse/breaker is protecting the SWA, is what size cpc is used between the banjo and the CU. The 4mm cpc in 10mm T+E might possibly be too small. If you stick 10mm in to match the line (reverting to table 54.7) then all bases should be covered. (but do check, and ask more questions if not sure)

(btw 50m of 2 core SWA = R1+R2 of 0.39 ohms)

Hope the assessment goes well, just had mine with NAPIT. It's also got me wondering how far the bloke is travelling to you on the IoW.... !

 

[pc1966]

As above: as an assessment you should be ready to provide your design measurements / calculations to establish the 2-core SWA choice is acceptable in the context of the supply Ze, chosen OCPD, design load voltage drop, etc (though it almost certainly is), as well as the the inspector seeing the quality of your handiwork.

Take multiple photos of the job, in fact always do that, as it serves as a reminder of what you did for billing as well as if the work is ever disputed or monkeyed with!

 

[pc1966]

Also remember to document how you determined the supply type (TT, TN-S, TN-C-S) and any need for extraneous bonding, as that can apply another factor on minimum CPC equivalent size.