I have found a sealed enclosure to present a special case for condensation.
A change in the atmospheric pressure or possibly a change in temperature can force moist air or surface moisture past many seals.
Once moisture enters or or condenses inside the enclosure, it is not easily removed by atmospheric "pumping".
Hence the drain holes.
Anti-condensation heaters are most effective when there is a thermal mass involved, such as the iron core of a motor or transformer.
Some motors have the leads well sealed where they enter the peckerhead.
In some motors, the leads enter the peckerhead through a large, unsealed opening in the motor frame.
If there is any chance that moisture in the peckerhead can migrate to the windings, you should definitely have anti-condensation heaters near or on the motor core.
A 13.8 kV motor that is in an area where moisture accumulates in a peckerhead should have anti-condensation heaters in the motor.
Worth repeating: