If it is roof drains you are talking about, this calculator may help you: http://www.portalsplus.com/drain_calc.htm
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250 mcm
A 200 amp service panel with a 60 amp sub-panel.
250 MCM aluminum minimum
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.
On large service entrance services the neutral can be reduced. The amperage of the service and the voltage of the service and the insulation factor of the 500 MCM conductors need to be stated to answer this question correctly.