Quick electrical chord question

I'm running about 70ft of 6/4 from my main breaker to my gfci 60A spa panel.

I normally only use one element at a time but have begun using 2 - 5500 watt elements ( one in HLT, one in BK) at the same time to heat my water. I think I am good to do this with 60A breaker and 6/4 wire. However, I did notice that the wire running to my spa panel from my main breaker box was a little warm after about 35-40mins of use. Is this normal when drawing that much power or is this an indication that if left alone the wires would heat up and melt the casing?

given the run being 70 ft you would have been better off with #4
but you are within the amp rating of the wire i am not sure what the amp draw of the run is so you might want to look into that