Have a look at Table 41.3 in BS761, not for the maximum Zs values for these circuits but for the size of circuit breakers commonly available. Taking the scenic route and avoiding too much thermal insulation we’ll settle for reference method 100 and this will give us 45A current carrying capacity for a 10mm sq twin and earth. However, it’s not a simple as that and it’s also highly unlikely that it’s going to be clipped directly throughout the building and we must allow for that. Looking at Table 4D5 BS 7671 we see that if we have the cable clipped direct (ref method C) we could use a 6mm sq twin and earth as it will carry up to 47A So, we’ll need a cable that can carry 44A or more. More than likely it will be a 10kW shower these days and it will use 10000/230 = 44A. We are going to assume that we are dealing with a domestic installation and as such Volt Drop will not be an issue, unless the shower is on the 3rd floor over on the east wing. Although it’s just a simple radial circuit there are a few pit holes you’ll need to avoid. Perhaps one of the easiest circuits to install is the one that feeds a shower.
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