Normally when working with higher voltages you always use heat shrinking tube to make waterproof joints.
Tube usually have a glue inside of it, glue makes it fully waterproof.
For an example if you have to make a cable connection (joint two cables) under the ground you always use shrinking tube.
And electrical connections will always be done with crimp connectors.
ok, aways is too much to say but in most of the cases.
The self fusing butyl tape (
http://tuotteet.etra.fi/en/g14855312/ni ... rical-tape) is usually used in places where it is impossible to use tube.
I have used shrinking tube with all connectors and self fusing tape in places where it's difficult to put a tube, like if I have a multi wire cable and I want to take one wire out of it. like Y connection.
Normal "PVC electric tape" is quite useless.