In plumbing we need to know the pressure in the city main supply , my question is how can the city give us the minimum pressure of its supply for our building lets say before pluging in our whole plumbing system to the city main line ??? usually the pressure at a node is determined when it is connected to a system , if we add anything to the system lets say a new floor with new fixtures the pressure at the same node will change to balance the whole system , so how the pressure of the city wall supply is determined to the engineer before he design his system ???