Juan,

There are a lot of rules used for storage tank sizing and many
interpretations of those rules. In general they can be summarized as: A
storage tank should have volume equal to the sum of:

1. equalization storage (depends on how you operate system),
2. fire flow (depends on supply capacity compared with max fire demand
plus max day demand),
3. emergency storage (depends on reliability of source of supply).

Tom Walski