Miniature Circuit Breaker

1. What kind of a load? Motor, transformer, capacitor, resistance furnace, induction furnace, incandescent lights, flourescent lights, .....?? Each type of load has its own characteristic, and requires carefully coordinated protection.

2. Overload protection - selected depending on the type of load as stated above.

3. Short-circuit protection - depends on your supply system.

4. Earth-fault protection - depends on your overall system layout.

This is a complicated process, you would need to educate yourself on what is an MCB and an MCCB, how are they different, what types are there...so on. Plenty of stuff on the web.

http://www.electrical-installation.org/wiki/Selection_of_a_circuit-breaker

http://www.electrical-installation.org/wiki/Circuit-breaker

http://cr4.globalspec.com/thread/36311/Sizing-Circuit-Breakers

http://cr4.globalspec.com/thread/56416#comment586936

a small presentation on schneider electric MCCB

http://www2.schneider-electric.com/documents/contractors/en/shared/training/discovercompactnsx1epart-en-1108.exe

while going through the process described by KVShridhar you will also do well to remember . . .

that the source and devices upstream of the MCB or MCCB in question should also be studied to know the system fault level and whether the upstream devices are correctly selected for the fault current . . . if this care is not taken then possibly the protection may be inadequate.

Hi

As you say the rating of the circuit breaker should be higher than the load, but lower than the cable maximum current for the installation method used. In general terms, you would select the next rating up from your load, so in this case a 40A.

But as others have said you need to consider the type of load, from which you would select the tripping curve of the MCB (B/C/D) and you have to ensure it is capable of safely clearing the fault during a short circuit, that is its fault rating is higher than the circuit prospective fault current.