How to Calculate Cable Size as per Current Load

There are tons of formula information. Search for it. Or search here

In principle, you can calculate this. But in practice, you would typically look it up in a table in the NEC (U.S. National Electrical Code), or your local equivalent. The ampacity of a conductor depends primarily on its cross-sectional area, but other factors are also involved: type of insulation, location (in air, buried in earth, in conduit or not), presence of adjacent conductors, etc.

The formulas for this have been worked out by wire manufacturers and code agencies, and published in catalogs and regulations. You need to be sure to use the correct regulation that applies to your particular situation.

If you are using the metric size, you can check here.

That was a nicely organized table, and a good illustration of looking things up rather than calculating from scratch. If this is acceptable in the OP's jurisdiction, it seems like a perfect answer.

Many previous threads will aim you at British Standard 7671, Boss.

There are too many variables for this forum, but im sure you'll like this. Try this calculator.http://www.electrician2.com/calculators/elcal.html