The wire gauge recommendations in our "Wire Gauge for Connection" specifications are based on relatively short lengths of wire up to about 25 feet. If you have to run wire longer than that you need to take into account voltage drop and upsize the wire accordingly. Voltage drop can significantly affect the performance of a kiln by making less power available to the kiln. This can result in Error 1 issues, slow heating and shorter element life.
It never hurts to upsize the wire gauge if you want to err on the side of caution. It is a lot cheaper in the long run to do this while you are running the wire.
Here is a voltage drop calculator that can also help you figure out the voltage drop with precision. Click on this link.
Here are some actual formulas for voltage drop.
A suggested way to approach this problem is to see what the voltage drop will be for various gauge wires and balance the least amount of voltage drop with the cost of the run. A voltage drop of up to 5 volts is probably OK - but the less the better.