Hi, r.yu,
I'm Judy from Revit Technical Support. I saw your forum post and will be happy to give you the information we have about voltage drop for non-US wires.
Presently, the voltage drop calculation works only in the US Imperial and US Metric templates. This is because the US wire sizes and impedance information for them are hard-coded into Revit, and there's no means for users to add this information for non-US wire sizes.
There's another forum discussion on this topic: Electrical Distribution: Circuit Voltage Drop with some additional information.
If you'd like to suggest that calculations for non-US wire sizes be included in a future release of Revit, I encourage you to post your idea on the Revit Ideas page, or vote for it if someone has already posted it. The more votes an idea gets, the more likely it is to be given consideration by our development team, so it is certainly worth doing.
I hope this answers your question, though I'm sure you were hoping for a different answer! Let me know if you have any other questions or comments.
Best,
Judy_S