Sizing SWA Armoured for CPC

[claret73]

Can I ask why you suggest a 32A? The install in the outbuilding will have 51A Total demand so MCB at Head end I've selected at 63A. The figures you recalculated were not my R1+R2 but the resistivity figures of the cable size & Armour & I did not apply the 1.28 factor to the SWA as this isn't grouped.

Might be overcomplicating my Maths?

 

[ackbarthestar]

I applied your figures. I assumed that you had used a multiplier of 1.28 to make adjustments for ambient and operating temperature of the SWA CPC.

However, that's easily checked.

Now an earth fault at the intake of the new building, albeit slight, will mean that your cable is unprotected with a B63A breaker. since you will need a resistance of less than 0.73 ohms.

So if you use a Ze of 0.8 ohms plus 0.55 ohms for the resistance of your cable as your starting figure you will have 1.35 ohms.
From app4 fig 3A4 your 63A breaker will take roughly 40secs for a disconnection.

I think placing a B63A RCBO at the front end would be your only option using your method of design, but as others have stated its not what I would regard as slick.