Sizing main supply line
I'm converting a detached garage into a living space and I'm trying to calculate if I need to increase the main water supply and sewer lines. My city follows the Uniform Plumbing Code (2009).
The main house has 11 Dwelling Fixture Units and the garage conversion will also have 11 DFU's. I have a 4" main and the UPC (2009) allows 161 DFU's so I think I'm fine in regards to the existing 4" main waste size. Does that sound right?
The main house has 17.5 Water Supply Fixture Units and the garage conversion will have 11. That's 28.5 WSFU's for the property. My main line from the city is 3/4" and it runs about 100 feet to the furthest fixture. The house was re-piped to PEX in a combination of 1/2" and 3/4" (no manifold). I have yet to test the pressure but will tomorrow.
I'm confused about sizing the main water line. If I have 40psi and my main line runs 100 feet to the furthest fixture and my branch lines are all 1/2", then the total WSFU is only 4. Am I reading that correctly? I would think most homes have that type of setup and that means they could only have two sinks and nothing else.
Thanks for helping me out on this.