800Hp Motor on Compressor

[Sparky4074]

I was wondering if I can get some input on a install I have going on. A new 800HP 480V 3PH compressor is being installed and doing the calculation (Name plate is 850A X 1.25 for CD = 1062.5. The issue I have run into is the pecker head has 2 - 4'' threaded hubs for conduit entry. Considering I only have 2 entries it would require me to run 2 parallel runs of 1000 kcmil and good luck terminating that in the pecker head. I was looking into DLO/RHH listed cables but that is getting hard to figure as most specs list the ampacities based on free air. The motor leads in the pecker head consist of 4 sets of 2/0 DLO. Any help and expertise would be greatly appreciated.

 

[augie47]

How about parallel 350s -- 2 sets in each 4"

 

[Sparky4074]

That would work. I would like to believe that I checked out that option but was probably so focused on trying to utilize the flexibility of DLO due to the size of the pecker head. Thank you.

 

[don_resqcapt19]

For standard size conductors you just use the ampacities direct from the ampacity table as you would for any other conductor. In this case it is marked as RHH so you use the 90°C column for ampacity adjustment and/or correction, but the final ampacity is limited to the 75°C column. Once you get to the "odd" sizes, such as 262.5 kcmil, the 2020 code added language to permit you to calculate the ampacity based on the ampacities shown in the code.

Under engineering supervision, ampacities of sizes not shown in ampacity tables for conductors meeting the general wiring requirements shall be permitted to be determined by interpolation of the adjacent conductors based on the conductor’s area.

So for 262.5, I would look at the difference between the ampacities for 250 and 300. Finding the difference to be 30 I would divide that by 50kcmil, giving me 0.6 amps per kcmil. I would take 12.5 kcmil time the 0.6 amps and add that number to the ampacity for 250 giving me a 90°C ampacity of 297.5 amps for the 262.5 kcmil DLO/RHH conductor.

You can still use DLO for the parallel runs, just like you would use any other conductor. Note, it is not easy to pull unless you get 100% coverage of the conductor with wire pulling lube. Any dry spot really adds to the friction when pulling the wire. However it is worth it for the ease of termination in the motor junction box.

 

[Sparky4074]

That helps out. Thank you!