I have a whirlpool electric water heater, I smelled something burning but it was for a couple of days , checked wire connection on heater and the red wire on heater was melting the wire nut, it did this when I first hooked it up about 2.5 3.5 yrs ago , I changed the wire nuts and no problems but now in did it again. Am I using correct connectors or is it the heater?

Well after further searching , I found my problem, I didn't pay any attention to the type of wire that is in the house. The wire from breaker to the water heater is aluminum. I read one site that said do not connect copper to aluminum.