cable size

[rayertz]

If I have an adjustable trip 480 3 phase breaker with an engineered calculated load and qualified maintenance personnel. Can I size my cable to the adjusted trip setting or does it have to be sized for the full rating of the breaker.

example 1
400 amp CB rated 80%
320 amp trip adjustment
400 Kcmil cu 75c in conduit

example 2
400 amp CB rated 100%
320 amp trip adjustment
400 Kcmil cu 75c in conduit

Or do I need to pull 500's?


Thanks in advance for you expertise.

 

[charlie b]

You don't have to pull 500's. But you are approaching the situation backwards. First you calculate the load. Let's say it is under 320 amps. Next you select a conductor that can handle that amount of current. I see that a 400 MCM cu can handle 335 amps, so we select that size. Finally you set the overcurrent trip to protect that conductor size. A 400 amp frame breaker with an adjustable setpoint of 320 amps would successfully protect the conductor, so you are good to go. Please note that the next higher standard OCPD rating from the 335 amp ampacity of the conductor is 350 amps. So you could set the adjustable setpoint as high as 350 if you wished.

 

[iceworm]

If I have an adjustable trip 480 3 phase breaker with an engineered calculated load and qualified maintenance personnel. Can I size my cable to the adjusted trip setting or does it have to be sized for the full rating of the breaker. ....

Applicable code section is 240.6.C. You have to decide if the access is restricted.

ice

 

[MasterTheNEC]

You don't have to pull 500's. But you are approaching the situation backwards. First you calculate the load. Let's say it is under 320 amps. Next you select a conductor that can handle that amount of current. I see that a 400 MCM cu can handle 335 amps, so we select that size. Finally you set the overcurrent trip to protect that conductor size. A 400 amp frame breaker with an adjustable setpoint of 320 amps would successfully protect the conductor, so you are good to go. Please note that the next higher standard OCPD rating from the 335 amp ampacity of the conductor is 350 amps. So you could set the adjustable setpoint as high as 350 if you wished.

What....Charlie you said you were not old. But you are still using MCM....now thats old fella:angel:

 

[fmtjfw]

What....Charlie you said you were not old. But you are still using MCM....now thats old fella:angel:

MCM wire works nicely with 60 cycle circuits.