"Our plants 600v system is at 640v and we have never had any problems.We did call the POCO and found they had run out of taps when we opened the kiosk"
Here's a possible problem.
As you know, most devices can "tolerate" plus or minus 10%. And the 460 to 480 is within that (approx. 4%).
But as you noted, what voltage are you actually getting?
We supply nominal 480 volt systems too. But if you are close to one of our substations, where we are allowed to boost the voltage by 5% to compensate for line loss, we are actually supplying closer to 504 volts.
If your utility is giving you 640 instead of 600 volts, you already have about a 6% higher voltage than nominal. Add that to the 4% that your transformer is off and you are pushing your luck.
If nothing else, when you buy your 600/480 volt transformer, see if you can get one with taps so you have a few options.