I've always done the taping with 3 layers. First the varnished cambric tape, then rubber, then plastic. When I first started in the trade we used friction tape instead of the plastic on the top layers.
2005 NEC 110.14 (B) "...shall be covered with insulation equivalent to that of the conductors or with an insulating device identified for the purpose."
I usually use a few layers of glass cloth tape (varnished cambric is good too) which helps prevent sharp edges from poking through the tape, next a layer or two of self fusing rubber, followed by at least one layer of vinyl electricians tape. The glass cloth or cambric will also make it easier to remove all the tape should the connection ever need to be taken apart.
Few things are harder to put up with than the annoyance of a good example.
FIRST, duct tape ain't what it used to be...if you tried wrapping some splices with "real" duct tape you would get a small fireworks show...but regardless forget about using something non-electrical to cover your splices.
SECOND, where are these splices that you can just leave them uninsulated ?
and C, if you don't have either glass cloth or cambric , try wrappng them first with vinyl but wrap it with the sticky side out, then rubber or 130C , then more vinyl, but this time wrap it the regular way...and don't forget to twist the tail so the next gal, or guy doesn't have to use an electron microsocope to find the end....
BTW ....when you are assembling your splice, do you whack the bolt and re-tighten... after... you have snugged it up ????
The method, as taught by the Union - so it's likely buried in the NECA standards somewhere - is:
1) 'round out' projections with electrical insulating putty; 2) Wrap several times with self-fusing rubber tape; and, 3) Finish with a two layer wrap of ordinary electrical tape.
The inner tape, and putty, fuse together into a solid rubber block, so you're not going to ever be able to unwrap this assembly; it's time for a sharp knife - and patience.
Some will start off with a layer of 'glue side up' electrical tape, with the idea that this will make taking the thing apart easier .... but I worry that this practice will result in the final assembly not being as waterproof as it should be.