This article claims that Apollo ground support equipment used 65 V for electrical power, and may have been involved with the cause of the Apollo 13 explosion:
The oxygen tanks had originally been designed to run off the 28 volt DC power of the Command and Service modules. However, in 1965 the tanks were ordered to be refitted to also run off the 65 volt DC ground power at Kennedy Space Center. All components were upgraded to accept 65 volts except the heater thermostatic switches, which were overlooked.
Why was 65 V used in general for ground support equipment? Not only is it different from the voltage of the spacecraft, it also seems like quite an arbitrary number.
This question is part of a series honoring the 50th anniversary of Apollo 13, "NASA's finest hour".