X-ray machines vary WIDELY from one manufacturer to the next, as do all imaging systems. Don't get caught guessing. Insist that the owner provide you with specific equipment specifications or if that is absolutely not possible, I would put in an empty conduit, a junction box and leave space in the panelboard for a large breaker and let him know that the breaker, conductors, outlet or disconnect switch will be specified when the equipment is submitted on or purchased. Put in an allowance for the cost and pay the contractor the difference if it is not sufficient.
X-ray loads are truly intermittant loads. It is a low level load until the image is taken at which time the load spikes for a few cycles or seconds. Usually not enough time to heat up the conductors unless seriously undersized, but enough to trip instantaneous settings on breakers if too closely sized.
Good luck,
EEJaime