I purchased an older home. Checked with the city and I have 15gpm at water meter (city said it is good). When I turn on any one faucet or appliance I have good pressure (volume), but, when I turn on another faucet or appliance the pressure (volume)drops drastically. The supply line is 5/8 inch galvanized. the previous owner replaced all the lines in the house with copper. My question is what size copper lines should I have at the house, or could there be something else wrong? I do not want to pay for a plumber yet until I can understand the problem. Any comments would be greatly appreciated.

 

I think that you just identified your problem: old galvanized supply line (5/8?). In any case, galvanized lines are notorious for scaling up inside, significantly reducing water pressure, and eventually plugging up altogether. I recommend that you replace your main supply line from the meter to your home with at least a 3/4" pvc or copper line. If the home itself has been replumbed, that should solve your problem permanently. Good Luck!