LV Cable Sizing.

The calculated feeder rating is done by knowing the electrical wiring standard for your this location. Since you are neither certified to perform this calculation nor do you know enough to even tell us where you or this wiring are you are not capable of calculating this.

Why don't you try going to a medical forum and ask them how to safely remove a damaged appendix. This will only put one person at risk.

http://www.openelectrical.org/wiki/index.php?title=Cable_Sizing_Calculation

Simples: using British Standard 7671 in this location.

Cable sizing is one of the Electrical section's most popular topics. It is inconceivable that entering the words "cable sizing" into the box on the right and searching will fail to turn up useful information, information that has been posted dozens of times before.

Does British Standard 7671 also apply to high temperature superconductors immersed in a LN2 bath?

What about pulse circuit operation like a beam kicker magnet?

One of the options is to obtain a copy, and actually read it.

If you have to ask, you don't need to know.

See-for instance NEC Article 215, 215.2 Minimum Rating and Size. (A) Feeders Not More Than 600 Volts.

See-for instance- IEC 60364-5-52 Annex A Current-carrying capacities.

One could calculate the necessary cable conductor cross section for 100% load factor following IEC 60287 or Neher and McGrath method for different load factor. In this case you have to know if the cable runs: in open air-with or without sun exposure-in underground-how deep, what the RHO is, directly or in conduit or duct- and other important data. And, of course, what kind of cable will be employed-copper, pvc/xlpe/epr insulation, jacketed or not, serving or not and other.