For the line wires, I just check the maximum load current and select a conductor from the manufacturer datasheet that can support that maximum current.
What about the earth continuity conductor though, how is it sized?
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It only takes a minute to sign up.Sign up to join this community
In North America equipment grounding conductors (circuit grounds) can be much smaller than circuit conductors. In fact it's only the first few circuit sizes (up to 30A) that require the EGC to be the same size as the circuit conductors.
Generally speaking, the grounding conductor may be required to carry (at least briefly) a fault current that's large enough to trip the breaker for the circuit(s) that it is protecting. This would imply that it needs to be the same size as the line wires.
Nowadays, most jurisdictions require the use of GFCI/RCD or AFCI breakers, which are much more sensitive to faults, so it could be argued that the current-handling requirements of the grounding conductor can be relaxed for circuits that have them.