Relationship of MCB size to cable size

Note - this considers overload protection only. Other factors such as voltage drop and disconnection times need to be considered when designing a circuit. Also, it only quotes the example tripping/non-tripping currents for Type B MCBs, not cartridge or rewirable fuses, which are not the same .
 

Protective devices have 3 values of interest here:

I_n: its nominal rating (e.g. 32A)
 
I₁: its non-fusing/non-tripping current. For a Type B MCB this is 1.13 I_n 
 
I₂: its fusing/tripping current. For a Type B MCB this is 1.45 I_n, and it must trip within 1 hour  


So if we have a circuit whose design current (i.e. the current that it is expected to carry in normal use) is I_b, then this should not be greater than the rating of the protective device:

I_b ≤ I_n

If Iz is the current-carrying capacity of the cable for continuous service allowing for the way that it has been installed, then this must be no less than the rating of the device:

I_n ≤ I_z

Finally, 1.45 times the current carrying capacity I_z of the cable must be no less than I₂, the current causing effective operation of the device:

1.45 I_z ≥ I₂

So I_b ≤ I_n ≤ I_z

and

I₂ ≤ 1.45 I_z.


Or if you prefer pictures:

Running an MCB at or near its limit for long periods will cause it to overheat, and eventually weaken. But you shouldn’t do that - look at the diagram, I_b is lower than all the rest.