Cable sizing depends on the regulations for your location and on four factors 1. Length 2. Cross-sectional area 3. Temperature 4. Conductor material.
The cable must satisfy the following conditions: Ib ≤ In ≤ Iz, Iz = In / (Ca x Cg)
Where Ib = design current. In = protective devise rating. Iz = current carrying capacity of cable under specific conditions.
Ca = cable ambient temperature factor. Cg = cable grouping factor.
The volt drop for the run of cable then must be checked = mV x A x m
Where mV = volt drop per meter (for a perticular size of cable). A = Ib. m = length of run (meters).
The max overload must then be checked = I2 ≤ 1.45 (this is an Irish figure) x Iz
Where I2 = the current that will operate a protective device (obtained from the manufacturer)
The cable withstand energy for short-circuits must also be checked = I^2 x t ≤ K^2 x S^2
where I = fault current. t = time for protective device to operate. K = conductor materrial factor.
S = cable cross sectional area.