CB Sizing

Under British Standard 7671 it is based upon the rating of the downstream cables.

You can undersize the breaker if you want. Just make sure the cable is adequate.

circuit breaker sizing is strictly based on the fault current interruption requirements.

What do you mean by sizing,is it only Amp rating?.If fed by transformer or genset you should consider fault level(kA) at the breaker terminals too in addition to load current and cable size.If load is small compared to genset capacity better have a MCCB with adjustable OL.

by sizing I meant Ampere rating.

Mainly making current rating [highest fault current that the breaker should withstand]

and breaking current [highest current that the breaker should be able to break]

These definitions are used in IEC 60909.

For breakers, close to generator and motors, both are different. The making current is higher than the breaking current, and decrements to steady state fault value depending on X/R ratio and changes in source impedances [generators, motors].

For some system conditions, these are relatively constant and same[no significant change in source impedance]. Consequently difference between making and breaking current is restricted to dc offset component only.

A few corrections, if I can Doctor!

i) Making current is NOT the highest fault current that the breaker should WITHSTAND; it is the highest current that the breaker can safely MAKE. By the by, the highest current that a breaker can withstand is called its SHORT TIME CURRENT.

ii) "The difference between making and breaking current is restricted to DC Offset component only" - Nope! Do you mean to say that if a system has no reactances, then the making and breaking currents will be same? Please note that even within the AC component, breaking current is RMS and making current is PEAK.