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Protection Riddle No.116 - MCCB size calculation for motor branch
how to calculate MCCB size  for motor load.if there is thumb rule please advice me
Author : tajuddeen - From: India
Thu, May 28th, 2015 - 07:47
It is possible to use an MCCB as a branch protective device for a motor load, however there are other circuit breakers, such as the MCP and MPCB, that are better suited for this application. After the calculation for sizing is completed, it should be apparent that even though this is technically correct, in some cases, the motor protection and wire protection can be less than ideal.
Per NEC Article 430.52(B), the motor branch circuit short-circuit and ground fault device shall be capable of carrying the starting current of the motor. Further, 430.52(C) indicates that the protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. In the case of an inverse time MCCB, such as the 140G, the calculation for the maximum setting or rating of the protective device is 250% of the motor being protected. A point to consider is that generally MCCBs have a magnetic trip of approximately 10X the rating of the MCCB. When starting, motors usually exhibit an inrush characteristic of 6 to 10 times the full load rating of the motor, depending on the type of motor being used. 
Author : Hamid - From: Iran
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