210.19 and 240.4(B)

[TM1238]

A co-worker and myself have a difference of opinion on conductor sizing for a large RTU. The unit in question has a MOCP of 400A and an MCA of 366A. The unit is served by a 400A breaker and it will be fed with a set of parallel aluminum conductions. My assertion is that a parallel set of 250 MCM AL must be utilized to satisfy 210.19 (the conductor current carrying capacity must meet or exceed the rating of the load) 410A exceeds the MCA of 366A. My co-worker argues that 4/0 AL can be utilized because 240.4B allows "rounding up" to the next sized overcurrent protection. While I agree 240.4B allows this, I disagree that parallel 4/0 AL are code compliant since a parallel set is only rated to carry 360A, below the MCA of 366A. So 250's or 4/0's?

 

[LarryFine]

Welcome to the forum.

You're correct. The conductors must be fully sized for the MCA.

A breaker may sometimes be rounded up, but the conductors must always be sized for the load.

 

[infinity]

The conductors, no matter what you choose, must have a minimum ampacity of 366 amps. There is no rounding up rule that would apply.

 

[Dennis Alwon]

Tell your friend he owes you lunch....lol

 

[TM1238]

Thanks everyone, I appreciate the responses! Case closed, 250's it is!

 

[kwired]

A co-worker and myself have a difference of opinion on conductor sizing for a large RTU. The unit in question has a MOCP of 400A and an MCA of 366A. The unit is served by a 400A breaker and it will be fed with a set of parallel aluminum conductions. My assertion is that a parallel set of 250 MCM AL must be utilized to satisfy 210.19 (the conductor current carrying capacity must meet or exceed the rating of the load) 410A exceeds the MCA of 366A. My co-worker argues that 4/0 AL can be utilized because 240.4B allows "rounding up" to the next sized overcurrent protection. While I agree 240.4B allows this, I disagree that parallel 4/0 AL are code compliant since a parallel set is only rated to carry 360A, below the MCA of 366A. So 250's or 4/0's?

Since this supplies a RTU, likely with hermetic compressor in it, 240.3 sends you to art 440 for overcurrent protection requirements for this application.

440 will eventually end up telling you that you must have supply conductor with an ampacity of no less than 366 amps and overcurrent protection of no more than 400 amps for this unit.