Plant Feeder circuit breaker sizing

[Ashu]

Hello,
This question is regarding the sizing of the plant feeder circuit breaker.
I design machines which have a main power cabinet consisting of the main circuit breaker (800Amp) supplying the power to circuit breakers ranging from 40Amp to 125Amp) for individual machine sections. The individual machine sections have the motor loads (Induction/Servos etc.) and controls and no heaters.
The Main circuit breaker's size is calculated based on 125% times the largest circuit breaker+100% of the loads of other motors' FLC according to NEC 430.62.
Now we need to build a new machine testing bay to test the physical machine. What should be the size of the Plant Feeder Circuit breaker (Shown with red color in the attached diagram) that will feed the main circuit breaker of the machine? I am thinking it should be the same size as the Main circuit breaker of our main power cabinet. However, some folks say that plant feeder sizing is calculated based on the actual running kW of the machine and that we need the measurement or estimation of actual running power of the machine to do that. Any help will be appreciated. Thank you!

 

[d0nut]

Since you have overcurrent protection in your main power cabinet to protect the equipment, your feeder breaker and conductors can be whatever size you need to operate your load. That could be a number smaller than your 800A, but I would probably just match your 800A breaker size. It could also be 5000A if you wanted, and your 800A breaker on your machine would protect the equipment. I would make sure your breaker and feeder are large enough to test the largest machine you plan to build.

 

[Ashu]

Since you have overcurrent protection in your main power cabinet to protect the equipment, your feeder breaker and conductors can be whatever size you need to operate your load. That could be a number smaller than your 800A, but I would probably just match your 800A breaker size. It could also be 5000A if you wanted, and your 800A breaker on your machine would protect the equipment. I would make sure your breaker and feeder are large enough to test the largest machine you plan to build.

Thanks for confirming my understanding. I appreciate it.