Answer Posted / marios

According to the 16th edition Wiring Regulations(British
standards) regulation 433-02-01 states that:
In>=Ib
In<=Iz
I2<=1.45Iz

Note:
"Iz" is the current capacity of a cable
"In" is the nominal current (current of the load)
"I2" is the current causing effective operation of the
protective device

So to calculate the size you do the following.
First you must find the In. Then you go to Appendix 4 and
you can find the size for the conductor by checking Iz to be
grater or equal to In.

e.g. Calculating Iz for a single core PVC insulated cables,
Wiring method enclosed in conduit, System of supply 240Vrms
50Hz single phase TNC-S
SOLUTION
In=20A, Iz must be > or = to In.Go to table 4D1A. When
Iz=20A Cross Sectional Area = 2.5mm^2 (Iz=In is what we
want). So the size of a conductor for 20A must be 2.5mm^2.

If you take into consideration the Temperature (T) then you
must follow the following equation: Iz>=In/(Ca*Cg*Ci) where
Ca,Cg,Ci are constants and you must find them at Appendix 4
always according to the Temperature given and also the type
of calbe been used.

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