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Thread: Supply sizing

  1. #1

    Default Supply sizing

    I'm adding a bathroom (tub/shower, sink, toilet) in the basement. Our supply pipes are 3/4" to the main floor bathroom, after which they drop to 1/2" to supply the kitchen. Can I branch off the 1/2" (for a total of about 20' of 1/2" leading to the bathroom) to supply the bathroom, or do I have to replace it with 3/4"?

  2. #2
    DIY Senior Member e-plumber's Avatar
    Join Date
    Sep 2004
    New York

    Lightbulb Supply sizing

    I would suggest bringing 3/4" copper lines to the new bathroom. I always pipe fixtures 3/4" to the second to last fixture, then reduce to 1/2".
    I believe recent code changes here permit (2) fixtures on one half inch line but I have not changed the way I've been doing it for years.

  3. #3
    Plumber Deb's Avatar
    Join Date
    Sep 2004

    Default Deb

    No, 1/2" is meant for 2 fixtures, IMHO. Regardless, you are way over fixture units for a 1/2" line, no matter what code you are under and how much they have lowered it for water saver things. You are over with both hot and cold. I think that a 3/4" hot should be run too and then branch off the 3/4 with a 1/2 for the bathroom and a 1/2 for the kitchen sink and DW. Buy a good quality pressure balance valve for the tub/shower.
    The Pipewench

  4. #4


    That's what I was afraid of. I guess I'll go pick up some 3/4"!

    Thanks for the input...


  5. #5

    Default 1/2" vs. 3/4" for bathrooms

    Isn't 1/2" better for the hot water line to the bathroom if there are only a tub and lavatory on the line? With 3/4" the waiting time for hot water to reach the bathroom can be substantially increased. Or is 1/2" too small to provide adequate flow? I know the UPC allows 4 fixture units on a 1/2" line, which is more than a tub (2fu) and lavatory (1fu) but then, code is a minimum, not a maximum.

    I've seen this done both ways on the various houses I've owned and was wondering if there was a consensus amoungst our pros here.

  6. #6
    Plumber, Contractor, Attorney LonnythePlumber's Avatar
    Join Date
    Sep 2004
    Wichita, Kansas

    Default 1/2" Hot

    I'm one of those that runs 1/2" hot to supply a tub and a lavatory faucet. I do this so that the water does not have to run so long before it gets hot. However I know I am in a minority view and most run hot like they do cold which is 3/4" to the last two fixtures.

  7. #7
    In the Trades Gary Swart's Avatar
    Join Date
    Sep 2004
    Yakima WA

    Default Agree with Lonnie

    I tried a 3/4" supply line for my hot water and found it took forever to purge the cold water from the line. Pulled the 3/4" and replaced with 1/2". Much improved. No big difference in flow, very adaquate.


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