1200amp bus rail

[elvis_931]

I am bidding on a job to install 20' of 480v 3-wire bus rail with a 1200amp tap box. I am using 4" rigid conduit. I am running 3 seperate conduits, each with 3 500MCM THHN's and 1 3/0 THHN ground. According to Table 310-16 500MCM THHN is rated at 430amps if there are not more than 3 current carrying conductors in the conduit. Should I run 4 sets of 500MCM for this 1200amp bus or will the three be enough. I get 1290amps with three runs. The breaker will be 1200 amps. They cannot use 1200amps because they will exceed 80% of the breaker and have tripping problems.

 

[benaround]

Elvis,

500mcm is good for 380amps in the 75deg column, you can't use the 90deg

column unless all terminations are rated for 90deg. , which I'm told are very

rare indeed. You can use the 90deg. column for derating.

3x380=1140amps if the load is 1140a or less 3 runs will work, if the full 1200a

is expected you need to figure another way.