Is there ever a potential for voltage on a neatral bus bar?

As referenced to what?? if the other meter lead is on the ground bus the answer is no if they are properly bonded at the service equipment, if there is potential between neutural and ground they are not bonded properly.

If you want the right answer ... ask the right question!

Ultimately, there is ALWAYS a voltage potential in any neutral wire or neutral buss. Even if your meter reads "0".

You'll get a measurement greater than zero, representing the voltage drop over the path you are measuring. Thus, a bad connection somewhere will lead to a higher reading.

Secondly ... and this is the tricky part to understand .... "0" volts read does NOT mean that electricity isn't flowing like mad through the buss. Just open that neutral, and you;ll see quite a spark ... and probably damage something. This is because all that power delivered by the 'hot' wires is trying to get back to where it was made ... and the neutral buss is part of that path home.

Slight quibble here. If there is sufficient load on the sub panel(s), you can measure a voltage difference between the neutral and the ground busses. In fact, when in a sub panel, if there is 0 Volts between them, and there is significant loading of the panel, I would check for an improper neutral to ground bond.

Larry C

Reno's point about the "grounded" conductor really gets driven home in a corner grounded delta.