16mm cable to fuse board isolator size

Hi. The 16mm cable should be protected at the origin by a 63amp RCD unit. At the distribution board at the other end each lighting circuit should be protected with a 6 or 10 amp circuit breaker and the power circuits (if wired as a 'ring circuit') should be protected with a 32amp circuit breaker. Should the power circuits be wired as a radial circuit (Where 1 cable is run from the DB to the sockets) then these circuits should be protected with a 16amp circuit breaker. I hope this has been of some assistance to you. Kind regards John Jackson.

Not a direct answer to your question but worth noting that if you are intending on creating a new circuit (and it appears you are planning on creating a few) then it is notifiable work and (unless you are registered with a scheme such as NICEIC, NAPIT, STROMA etc) you will need to get in touch with your local building control.They will then ask you to pay a fee (depending upon where you are usually somewhere between £150 and £500) and will send an inspector round to check and test your work at various stages and sign it off assuming it tests ok and has been done in line with the Wiring Regulations and Building Regulations.

16mm?? What are you running off of it? Why so big? I would be more concerned about the size of the MCB backing it up, along with Rcd protection rather than what size the isolator should be.

To design an electrical installation it needs different details to know. Which type of earthing arrangement. Leght of the cable, from house to outhouse. Buried or not Total load (kilowatts) Needs future circuits. Outside equipment. Then different calculations are done to establish the size of main protective device (using diversity) and the size of the supply cable (voltage drop) A competent electrician will be capable to the job.

I think this is a job that is best left to a professional electrician. There are too many techincal considerations involved for this type of job to be done on a DIY basis. To begin with, 16mm is a somewhat excessive size of cable to select for the purpose of supplying two lighting circuits and two power circuits when 10mm, or even 6mm would usually be sufficient. Standard twin and earth cable is unsuitable here, and a steel wire armourerd cable will almost certainly be required. RCD protection will be required upstream, but not at the point of supply if SWA is used. Your earthing system must be determined in advance, and if it's a TN-C-S system, then in all probability, an earth rod will need to be installed in the soil adjacent to the outbuilding. Protective bonding of any water pipes and extraneous metal parts in the outbuilding will also be required. The installation will then need to undego a series of tests before it is energised. An Electrical Installation Certificate complete with a schedule of test results will also be required. As I say, this is NOT a job that can be done on a DIY basis.