I would like to get some clarification on whether to use phase-to-phase voltage or phase-to-ground voltage when calculating clearances described in Table 235-6 of the NESC Code. The table clearly states circuit phase-to-phase voltage in the column headers but the title references Rule 235A which states using phase-to-ground of the higher voltage circuit or the phasor difference between the conductors involved. Other clearance calculations in Rule 235 are using phase-to-ground voltage, as well as the example calculations. Where this clearance is to a grounded part of the structure I would guess (fyi, I'm a civil engineer) that the phase-to-ground voltage would be more appropriate. Can anyone offer some clarification and maybe an explanation?
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