Rbradshaw

[Rbradshaw]

Is it true that when using a 400amp 480vac 3p breaker from a incoming source to a different control panel you have to also supply a 400a 480vac 3p breaker there with the wire sized for 400a. The actual load is 300a

 

[augie47]

2nd breaker

2nd breaker

I'm not sure if I understand the question, however, if you have 400 amp conductors protected by a 400 amp breaker, those conductors can feed as many "panels" as you wish. The second panel would not need a breaker as long as the conductors were 400 amp. This is not ment to address "disconnect" issues in regard to the 2nd panel, but merely addressing overcurrent protection.
Does that answer your question ?

 

[Rbradshaw]

My issue is. I have a source of power for a motor control panel that is a 400a breaker. The motor panel has a 300a breaker installed. Do I upsize the 300a breaker to a 400a and run wire big enough for 400a

 

[don_resqcapt19]

The conductors between the 400 amp supply breaker and the 300 amp breaker at the load end must be protected. We do not have enough information to tell you what size conductors are required. They could be 400 amp conductors, 300 amp conductors or even smaller depending upon the detail of the installation.
Don

 

[rootboy]

The short answer is no.

Basically you can oversize the conductors from the buss plug to the panel and then drop down in size to what is necessary to run the equipment.

We do this all of the time since replacing the fuses in the buss plug is a whole lot more painful than replacing them in the panel on the ground.


There's an excellent example of this in the examples section of the NEC handbook.