Converting a section of overhead line construction to underground

[Cal high voltage]

I'm converting a section of 480 volt 3 wire overhead line construction to underground. My question is: Do I have to run a ground wire on the underground section that I'm installing? There is no ground reference on the overhead line construction.

 

[GoldDigger]

I'm converting a section of 480 volt 3 wire overhead line construction to underground. My question is: Do I have to run a ground wire on the underground section that I'm installing? There is no ground reference on the overhead line construction.

If the line is service conductors, you do not need to have an EGC, just a grounded conductor if the service is grounded.
If the line is a feeder, then even if the overhead was constructed at a time when Code did not require an EGC, just a GES at both ends, you will need to provide an EGC for the new installation. It can be either a raceway or a wire EGC.
If there is overhead with no EGC on both ends of the section you are changing, I can't see any use for an EGC, but not running one will make later possible upgrades harder.

 

[Cal high voltage]

Thanks! That's what I was thinking. I'm just going to install a ground rod at each end of the underground run. Run my ground on the pole and bonded to the ground rods.