I might also suggest you have something else that is causing your wires to get green besides heat. Green is usually a result of corrosion. It can be from dissimilar metals and/or infusion of outside chemicals. I would look at replacing the three wires, installing paste designed to minimize corrosion and improve connectivity like carbon conductive grease, and fire it up, pun intended. If you have an inferred voltage gauge, after it runs several hours, check the temperature from the heating element up the wires. If everything is functioning properly, it should be hottest at the heating element and then quickly drop as you progress up the wires. Keep in mind you only want a thin coat of grease to prevent it from making a mess. The grease is rated at 200 degrees Celsius, but it does get softer as it gets hot.
Lastly, I would look for any source of liquid on the wires? Is there coolant/water wicking through them or water getting into the box?
If not it may just be old wires in a harsh environment.
Mine failed about a year ago. I had a junction box right above the Aqua-Hot and replaced the two feet of cable to the box. Ran the same test repair and everything checked OK. One year of use and no more corrosion and wires stay cool to the touch.
Hopefully this helps.
