3-350 MCM AL Good For 800 amp Service

[hbendillo]

Looking over a service riser. They used 3 sets of 350 MCM AL for a service with 800 amp breaker. I know you can use the next breaker size up for breaker of 800 amp or less but what is the limit? 350 MCM AL is good for 250 amps for a total of 750 amps.

 

[Dennis Alwon]

You are good to go as long as there is no standard size breaker at 750amps. Art. 240.6(A) tells us the standard sizes-- it goes from 700-800 so at 750 amps you can use a 800 amp breaker

 

[don_resqcapt19]

Assuming the load calculation shows a load of 750 amps or less you are fine with the 800 amp main.

 

[mayanees]

.. and then there's 240.91(B)

.. and then there's 240.91(B)

I'm writing this down to get it to sink further into my brain, and so I can find it again in the NEC...... that 240.91(B) allows a 95% cable ampacity for supervised industrial installations on conductors over 800 amps. Not applicable here, but pointing out another application where the 800-amp limit is not the only case. It's perfect for the very common - 4 sets of 500 kCM CU with an ampacity of 4*380=1520 amps on a 1600-amp breaker.

 

[MasterTheNEC]

I'm writing this down to get it to sink further into my brain, and so I can find it again in the NEC...... that 240.91(B) allows a 95% cable ampacity for supervised industrial installations on conductors over 800 amps. Not applicable here, but pointing out another application where the 800-amp limit is not the only case. It's perfect for the very common - 4 sets of 500 kCM CU with an ampacity of 4*380=1520 amps on a 1600-amp breaker.

I would say the application you are speaking of is rather rare. The importance of also meeting both requirements in 240.91(B)(1) and (2) would tend to put additional requirements on the manufacturers of the equipment being terminated too. That tends to be where the hangup gets in the way if using 240.91(B).