Pressure of the gravity fed water delivered from the overhead tank will have nothing to do with the pipe size used. The pressure is dependent only on the vertical distance between the top of the water in the tank and the delivery point. 

Where pipe size does come into play is in the flow rate that is needed at the delivery point. If you only have one delivery point being used at a time you can get by with a pipe size that can deliver the desired flow rate all the way back to the tank. 

If you have multiple delivery points in use at one time then the branch lines from those delivery points will have to merge into a larger pipe at some point and then this larger pipe goes the rest of the way back to the tank. You roughly work out the size of this larger pipe needing to have a cross sectional area that is the sum of the cross sectional areas of the multiple use delivery point pipes.

Water Supply pressure
It is well-known that an aqueous ionic solution can conduct electricity. Let's take the electrolyte NaCl for example. In the diagram below, the circuit consists of a voltage source, a light bulb, and a concentrated NaCl solution. 

Initially, the bulb will shine brightly. Assume the voltage source can source energy forever. My question is: will the NaCl solution allow current to flow forever, or will the conductivity of the NaCl solution degrade over time?


