What equations/formulas should I use to calculate the IE at various distances for the following application (Large conductor gap):

Voltage: 700VAC
Phase: 3phase delta ungrounded
Conductor gap: 3FT
ISC: 65KA
Clearing time: 2.0 sec or less

Thanks

See IEEE Std 1584

It looks like the IEEE 1584 max gap at this voltage level is 40mm (1.6")???
I need equations for a gap of 36 inches.

Thanks

About your only choice is to use the 1584 equations except they are only for smaller gaps based on more typical equipment and that could sustain an arc (208 volt arc sustainability is still under review). Using 36 inches as a gap may give you an answer using the equations but it won't be very realistic. It is very unlikely that an arc can be sustained at 36 inch arc at 700 volts. I did some testing a few months ago and could not sustain a 480V arc at around 18 inches.

The Lee method is used in IEEE 1584 for cases where the voltage is over 15 kV or where the gap is outside the range of the empirical model.