MCA and MOCP for chiller

[Adulrhman]

I have a chiller with MCA 250A and MOCP 350A.
I am planning to power the chiller but I have a little confused about cable and CB size coming from the distribution panel.
would you please explain what the cable and CB size to be installed.

 

[GoldDigger]

The CB can be anywhere you choose up to 350A. It can even be less than 250A, but that would not be wise. To minimize nuisance trips use 350A.

The cable must be rated, after any applicable derating, at least 250A. The cable does NOT have to be large enough to be protected by the 350A breaker, since the motor overload is expected to provide the overload protection, as opposed to short circuit and ground fault protection provided by the CB.

 

[LarryFine]

In other words, the conductors must qualify for the MCA, and the breaker must not exceed the MOCP.

This allows for starting current.

 

[Adulrhman]

In other words, the conductors must qualify for the MCA, and the breaker must not exceed the MOCP.

This allows for starting current.

The CB can be anywhere you choose up to 350A. It can even be less than 250A, but that would not be wise. To minimize nuisance trips use 350A.

The cable must be rated, after any applicable derating, at least 250A. The cable does NOT have to be large enough to be protected by the 350A breaker, since the motor overload is expected to provide the overload protection, as opposed to short circuit and ground fault protection provided by the CB.

If CB carrying 350A and cable 250A it will not burn the cable?
I get confused .

 

[GoldDigger]

If the cable is rated for 250A and it short circuits, the 350A CB will protect it just as well as a 250A CB.
If the motor gets overloaded and draws more than 250 in a constant way, the motor overload will open and also protect the cable.

The code assumes that there will not be a fault anywhere along the circuit but in the motor(s) that can steadily draw more than 250A but less than 4705A (to trip the 350A breaker.)

 

[Adulrhman]

Thank you