I want to know what size MDPE pipe to put in from the street water main along a 15 metre run to where it will be divided up into 7 individual dwellings, each with a bathroom. Static pressure is around 4 bar. Is there a standard rule of thumb for this? What would the water board normally do?
Yes, the standard rule of thumb is to ask the water undertaker. If they don't like what you've put in they won't connect it 15mm dia water capacity would be more than enough per dwelling esp as you have the diversity factor across the 7 homes. If 15mm pipe is 150mm2, and multiply by 7 = 1050, then a 40mm pipe would be 1200mm2 give or take. Actually is better than that due to reduced friction on pipe skin etc. But, heed my first point. It matters not what you or I think!