Since there has been no answer of yet I will give a quick summary from an Electrical Engineer's standpoint and the method for an Electrician.
Firstly, in response to post 2, you must understand that Ohm's Law (I=V/R) is strictly for calculations involving purely resistive circuits/loads. You cannot use it for complex AC systems involving impedance via inductive/capacitive loads without first working through other necessary calculations such as Norton/Thevenin's, Superposition Theorum, Kirchoff's Rules etc due to phase degrees and other electro motive forces in effect. That being understood, you need to remember that you can not just apply Ohm's law to any circuit and expect to get an accurate Ib (design current) for a particular circuit, unless it is strictly resistive e.g convection heater. Beyond that, the Ib will change according to harmonic resonance, grouping factors, voltage drop, thermal dissapation, fusing factors etc so these also must be taken into consideration.
Tables from Appendix 4 in the OSG will provide you with the necessary formula and factors to calculate your carrying capacities (Iz) assuming you have the correct circuit design current. These are close estimates but will differ from manufacture and age of the cable. It's always good practice to get a clamp reading of the circuit under normal load in order to cross-reference your calculations.
Iz is calculated by using the formula:
Iz = It x Ca x Cg x Ci x Cc
Where It is the tabulated current for a single circuit at an ambient temperature of 30°C. Ca is a correction factor due to the ambient temperature (values from table 4B1) Cg is a correction value for cables grouped with other circuits (values from table 4C1) Ci is a correction value for cables in insulation (Table 52.2) Cc is a correction factor of 0.725 for BS3036 fuses and 0.9 for cables "in a duct in the ground" or "buried direct". If a buried cable is protected by a BS 3036 fuse then Cc = 0.725 x 0.9 = 0.653
Method
1. Determine the design current (Ib)
2. Select the nominal rating and type of the protective device (In)
3. Divide the protective device rated current(In) by the correction factors (Cg),(Ci),(Ca),(Cr) etc;
4. Select cable from the tables (Iz)
5. Calculate the voltage drop and check for compliance - See the OSG for the basic calculation at 230V: -
VD = mV/A/m × Ib(I "base" design current) × L(metres) .
6. Check circuit complies with shock protection.
7. Check that circuit complies with thermal constraints.
Check - ensure that Ib <= In <= Iz; and I2 <= 1.45 Iz
Hope this helps mate, take care.
