MCB

How would that be?

I think you mean kA rating of breakers....Then it is wrong. kA rating of breakers shall be based on the short curcuit current levels at each levels such as MDB, SMDB and DBs...however it might be Main DB at 10 kA and sub DB at 6 kA...calculate the fault current and find out the exact range.

Okay! What about that? What help do you need?

This kind of post is exactly what scares me about unqualified personnel working with electricity! If they were qualified, they wouldn't need to ask the question! People can get killed! Re kaic: what units are those?

Of course the first thing to do is to calculate the prospective SC rating needed at the incomer level .

If it is 6 KA or below, using a 6 KA incomer and 10KA feeders won't harm . A 10 KA works well on a 6 KA short circuit ! It is not economically optimized, but technically OK.

Hi,

Of course MCB short circuit rating has to select based on the load short circuit current, Usually Main DB should be higher KA rating and downstream SDB's designed with lesser than Main DB KA rating, otherwise if any short circuit happens and it is less than SBD short circuit level and more than main DB short circuit level, then main DB will trip first, so the SDB also disconnected. Hence choosing right co-ordination is very important. Second thing is MCB Class of operation, there is Class A, B, C type MCB's available. Mainly these classes represents the tripping time. You have to careful on this otherwise unnecessary tripping will happen.