25mm swa armour for cpc

Rockomuir · 2024-09-19T13:36:24+0100

Hi all,
Just planning a sub main to an outbuilding .
It will be 80 metre run, buried direct on a 63amp fuse .

So I am thinking of running 25mm swa but just been doing my calcs on whether I can use armour as Cpc. The outbuilding is going to be TT so no need of a bonding conductor on the submain.

After doing calcs in 2 core swa I get
99/51 x16 = 31.02

What I cannot find is csa of 25mm armoured to see if it is big enough .

If anyone could give me the answer it would be most appreciated

oscar21 · 2024-09-19T14:02:20+0100

Why do you need an earth/cpc if its powering a TT installation.

Rockomuir · 2024-09-19T17:13:14+0100

oscar21 said:
Why do you need an earth/cpc if it’s powering a TT installation.

Sorry I might being completely daft. The swa is going to need a Cpc from the supply end but disconnexted at the outbuilding end as this will become TT. Just wanted to work out if 25mm armoured is big enough to use for cpc

westward10 · 2024-09-19T17:50:06+0100

The csa of the armour is around 42.0 for a two core 25.0 cable.

oscar21 · 2024-09-20T10:15:58+0100

Rockomuir said:
Sorry I might being completely daft. The swa is going to need a Cpc from the supply end but disconnexted at the outbuilding end as this will become TT. Just wanted to work out if 25mm armoured is big enough to use for cpc

You mean for if the cable is damaged by say a spade or something and you want to ensure the MCB/fuse blows in the required time? Well I've not done any calculations or anything but I would imagine that they wouldn't make the SWA in long lengths if that was a possibility, that's just common sense to me.

Also I thought problems around the size of the cable armouring only came in to play if 10mm bonding was required. If you are that concerned why not put a time delayed RCD upfront, I'd do that as opposed to up the cable size purely due to cost.

westward10 · 2024-09-20T10:47:48+0100

The size of the armour comes into play as it must be able to withstand fault current without suffering thermal damage the bonding issue is a separate situation.