I have an 800 amp 277/480, three phase, 4 wire service. I want to calculate the available amperage for 120 & 208 single phase assuming a PF of 1. Would this be 1.73 times the 3 phase amperage rating (800)? If not, how do I calculate this. Sorry, but I am an ME, not an EE. Thanks.

You do realize that you will need a transformer to convert the 277/480 down to 120/208 volt?

To calculate the 120/208 volt amperage, first you must calculate the KVA capacity of the 800 amp 277/480 volt service.

800 amp x (480 x 1.73 = 830 Phase to Phase voltage) / 1000 = 664.32 KVA is the minimum size transfomer

664.32 KVA / (208 x 1.73 = 360 Phase to Phase voltage) = 1845 amps

This is less any applicable deratings.

Not sure why you would want to convert the entire 480 volt service to 120/208, usually most is used for 480 volt equipment, and a smaller transformer sized for the minimal 120/208 volt load is installed.