Have a restaurant with 35 year old 500 mcm al for the main panel, 230v 1 phase. In the summer pulling about 320 amps continuous. Wires get pretty warm to the touch but I did't measure temperature. 180' underground run to the entrance disconnect in 3" conduit. Would I benefit enough in Electric savings (resistance) to justify replacing with 500 mcm copper? Do you think I'm wasting $2-300 month in wire heat resistance?

Using copper wire to reduce I2R losses is a popular reason for incurring the higher cost of using copper, consider it a capital investment, but, the savings of reducing the heat you detected can be significant over the years.

I would need some time to do the calculations to arrive at ROI return on Investment.

Problem is copper wire is so costly nowadays. Unfortunate you did not think of this a few years ago.