How do you determine the Arc Gap for overhead power lines? We have always used 6" for 15kV per the IEEE 1584 Table 8. However, OSHA 1929.960 states that for single conductors in air, 15kV and less (and > 15kV) for a three-phase arc the "Phase conductor spacing" should be used. Typical spacing on an overhead power line would be around 36". This would produce values far greater than has been the "industry standard" to date. What are your thoughts on this?

One thing to note is that the NESC Table 410-2 is based off of a 4" gap for 15.1kV to 25kV. So it appears that NESC was not on board with OSHA's recommendation to use the conductor spacing for Arc Gap.

Take a look at the notes for Table 6 at https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.269AppE. I think you'll see similar values to NESC.