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Sizing main supply line

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Posted by fromhollywood (My Page) on
Wed, Jul 20, 11 at 22:17

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).

Sewer:
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?

Water Supply:
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.

Follow-Up Postings:

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RE: Sizing main supply line

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I forgot to include how I arrived at my calculations:

DWELLING FIXTURE UNITS
House
Kitchen: 2.0
Bathtub/Shower 2.0
Lavatory: 1.0
Toilet: 3.0
Clothes water 3.0
HOUSE TOTAL: 11 DFU

Garage
Kitchen: 2.0
Shower: 2.0
Lavatory: 1.0
Toilet 3.0
Clothes washer: 3.0
GARAGE TOTAL: 11 DFU

TOTAL DFU = 22
--------------------------
WATER SUPPLY FIXTURE UNITS
House
Bathtub 4
Showerhead 2
Lavatory 1
Toilet 2.5
Kitchen sink 1.5
Laundry sink 1.5
Two hose bibs 5.0
HOUSE TOTAL = 17.5 WSFU

Garage
Showerhead 2
Lavatory 1
Toilet 2.5
Laundry sink 1.5
Kitchen sink 1.5
Hose bibb 2.5
GARAGE TOTAL = 11 WSFU

TOTAL WSFU = 28.5

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RE: Sizing main supply line

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I always oversize the water service line. One inch line to the house and a 3/4 to the detached unit.

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RE: Sizing main supply line

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You cannot run the drain line from the detached garage to the house. Once the drain line leaves the garage it is defined as sewer line and it must run to the house sewer, septic tank or municipal sewer. It may not be brought into the house and connected to the house main drain.

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