The use of medical high voltage cable mainly depends on the KV of the x ray tube, which has nothing to do with the length of the medical high voltage cable.

The medical high voltage is connected to the x ray tube and generator. There are two types of high voltage cables, 75KV and 90KV. The length of the medical high voltage cable can be chosen from 2m to 15m. The length of the medical high voltage cable is decided by the user and does not affect the mechanical operation and the matching of the machine.
The choice of 75KV or 90KV medical high voltage cable depends on whether the tube used in the X-ray machine is 125KV or 150KV.
Generally 300 mA X-ray machine uses a spherical tube 125 KV, so you can choose 75 KV medical power cable used in 300 mA X-ray machine.