Circuit breaker selection factors.

 Circuit Breaker selection factors as follows:

01.  Voltage Rating.

02.  Frequency : Circuit breakers up to 600 amps can be applied to frequencies of 50-120 Hz. Higher than 120 Hz frequencies will end up with the breaker having to derate. During higher frequency projects, the eddy currents and iron losses causes greater heating within the thermal trip components thus requiring the breaker to be derated or specifically calibrated. 

03.  Maximum Interrupting Capacity or breaking capacity: The interrupting rating is generally accepted as the highest amount of fault current the breaker can interrupt without causing system failure to itself. Determining the maximum amount of fault current supplied by a system can be calculated at any given time. 

04.  Continuous Current Rating: This ampere rating is the continuous current of the breaker will carry in the ambient temperature(40 deg.) where it was calibrated.

05.  Operating Environment : High ambient temperature and altitude(>4000 Ft) caused derated circuit breaker. At altitude, the thinner air does not conduct heat away from the current carrying components as well as denser air found in lower altitudes. In addition to overheating, the thinner air also prevents the of building a dielectric charge fast enough to withstand the same voltage levels that occur at normal atmospheric pressure.

 The National Electric Code (NEC) determines that circuit breakers should handle 80% of their rated capacity for continuous loads (those being on for three or more hours) and 100% for intermittent loads. For a total current of 15 A, the calculation is as follows: 15 A * 1.25 = 18.75 A. The installer should use a 20 A CB because the immediately lower CB rating, 15 A, would not be enough for this load.

So,a thumb rule is first of all calculate total max. wattage value, then divide by system voltage to get maximum ampere rating. Now just multiply the ampere value by 1.25 to get circuit breaker rating.