hi i need a power supply to my garage/workshop, i have a spare in my consumer unit in my house type b 32a mcb, and will i need a 4mil swa?? or 6mil swa?? from the cu in the house to the garage cu,(63A. 30MA. RCD., 1 x 6A lights + 1 X 16A scokets. MCB),, i will be useing a welder, grinders with a(240v- 110v transformer) and there will be 2 double scoket outlets in the workshop,,,is this all correct?? thanks,
1) Unless in Scotland then Part P means it's just not worth DIY. 2) Welders do need permission obtaining from the DNO before you can use them. So if you want to follow the rules then not a DIY job and loads of loops to jump through. However the rules don't stop you doing it they just make it hard and expensive.
So you need to work out the running load of welder. Rule of thumb the output voltage starts a 25 volt and increases by 1 volt per 25A so a welder set at 125A will have a voltage of 31 x 125 = 3875W at 230v that's 16.8A which means most welders can't run off a 13A socket and minimum is a 16A socket. Add lights etc then really looking at least a 20A supply.
You then need to look at the existing loop impedance and work out what the loop impedance will be at the garage both to ensure the MCB will trip under fault conditions and the volt drop is within the 3% for lights and 5% for power. Again the welder is a major factor as type B MCB may not allow you to use a welder it may need to be type C or even D. Welding transformers are very different from welding inverters as far as supply goes and you have not said which will be used.
You will need to submit plans to the LABC which should have all the calculations. Although they may allow you to do the work without detailed plans they may also reject the completed work. Detailed plans mean once the give go ahead they can't then say "This does not comply re-do it" as it would have been up to them to reject at planning stage.
I have found the price of 4mm and 6mm SWA is not that much different and both fit into 20mm glands so normally I would use 6mm 3 core which could be bought by the role and used on many jobs so I would in the main use 6mm for a garage feed. Mainly as it would be in stock. Earthing arrangements on out buildings is not easy to select as there are so many factors. Mainly I would use the TN from the house but sometimes it did require a TT supply.
It will need a lot of reading and work to do correctly. I would strongly advise using a scheme registered electrician to do the work for you. Not really a DIY job.