From my experience, motor contribution to faults is typically not significant and can be ignored in arc flash analysis without compromising accuracy of the study. Every rule has an exception though. I would use minimum available short circuit fault current or the amount of short circuit current coming from the utility only (such as indicated in 'Min Isc3, KA' column below) to calculate predicted arcing current and to determine arc duration based on upstream protection device characteristic. I would use maximum available short circuit fault current including contribution from motors ('Max Isc3, KA' column) for the IE and AFB calculations. I would evaluate motor contribution based on the motor subtransient reactance. It is normally 16.7% for induction motors but varies slightly depending on the motor type and size. You may also apply ANSI interrupting duty 1.5 to 4 cycles multipliers (1.5 to 3 times the motor subtransient reactance) to the part of SC contributed by motors. That should provide for the worst case scenario associated with the longest arc duration and the highest current values.
Sample screenshot of Short Circuit Analytic V2.0 software Calculation Results window.