Tubular Bus Bar Correction or Derating Factor Table

Take a look at IEEE Std. 605-1998, Annex B and C. Annex B gives ampacity tables for various bus conductors (Al and Cu, bar and tubular), which you can use. Annex C provides calculation methods that were used to develop the tables. Annex B will probably be enough, from the size of your bus tubing and what you assume to be your maximum operating temperature (temp rise over ambient). Although the tables start with ambient of 40C, you can just assume that your extra 10C of ambient is part of the temp rise.

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