I know in electrical that the wire gage determines the amount of voltage drop per square ft (all materials being equal) I know that house in america run on 120vac or 240vac power company supply voltages What I want to know is in plumbing is their a standard pressure/force that the water companies supply to a house? (Their must be an equivalent to the power companies but for water) Also I am assuming the size of the plumbing pipes (copper pipes have a reason to be a certain diameter ...etc? Is this for how much Pressure drops pure cubic foot of copper pipe...? Thanks for anybody that can shed some light on this. Is their a reason why you should use a 3/8in or 1/2in copper pipe or can you usually use them interchangable? I am assuming their has to be a similar reason to why you use different pipe sizes similar to the smaller gage (AWG) to do longer runs in electrical house circuits... Is their an equivalent to ohms law in plumbing? if so how much pressure do you need to have go to a sink or shower???