Just attempting to complete my self build for xmas and find I need to run a sub mains from meter position to new mains location more central in the building. The distance is approx 21m and the SWA will travel through a single storey roofspace before entering under the first floor of the main building for some 5m to new consumer unit position. Having checked the various SWA size calc sites I find they are not consistant with their findings as they varey between 10mm and 16mm, an opinion would be of assistance please re cable size. I am unsure also if 2core cable would be sufficient while using the outer sheath for the earth conductor as no part of the cable run is in danger of damage. Terminating the SWA at the C unit is not a problem but I presume some kind of metalclad switched fused isolator is required at the start end to terminate the cable and the feeding meter tails. Assistance greatly appreciated please.
What size is incoming fuse. If 100A then volt drop around 5.88v which is close to 6.9v maximum and clipped direct it would need to be thermosetting 90 deg cable for 70 deg only rated at 89A clipped direct and method D is more likely so 78A max.
This is not a DIY job and unless you understand how to calculate cable sizes then you should not be doing it.
I would expect you will need 25mm cable.
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