How to Choose the Main Breaker

What voltage?

The decision has a lot to do with regulations in the country where it is going to be installed. Most likely since you mentioned 60Hz you are somewhere in North America. Speaking for the USA, the main breaker is to be sized for at least the aggregate total load amps plus 25%. It can be larger, as long as conductor sizing is accounted for.

Sub breakers are the same basic rule.

Motor starters are another issue altogether. It depends upon the type of starter you have. In general, if the DOL starter will consist of a circuit breaker, a contactor and an overload relay, the OLR is considered to be the thermal )long time) current protection for the entire circuit, so the breaker is only required to serve as the Short Circuit Protection Device (SCPD). There are several sizing methods that can be used depending on the type of breaker it is. But in general a Thermal-Mag breaker can be up to 250% of the motor FLA, unless that can be shown not to work in which case it can be increased to up to 400% (for motors 100A or less). All of this is covered in the National Electric Code, section 430.52.

The Canadian Electric Code is very similar.

motol dol starters can be selected based on the type-2 cordination. which has all the details pertaining to selection fuse, contactor,relay, mpcb,mccb etc.

the basic thumb rule is to select the breaker 1.5 times the full load current, contractor are one size higher in case of dol starter, mmc are 2 times higher for better life .

why did the contractor oversized on the starters??

Has the contractor been asked and, if so, what was the reply?

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

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