How do you calculate 3 phase kWh to amps when 8000 kWh monthly average is used?

It depends on the voltage. Please restate the question and provide the voltage.

In general, however, simply divide total power by KV to get KA. Remember the KWH is an integral, so you need to back calculate KW. If the month is a 30 day month, then KW is KWH / 30 / 1440. Then, if the load is star, simply divide by three; if the load is delta, divide by three and multiply by 1.732, the square root of 3.

Example: 480 three phase running star. 8000 / 30 / 1440 is 185 amperes. 185 divided by 3 is 62 amperes per phase. For delta, that becomes 107 amperes.