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Sorry if this question sound naive. Normally in water pipes, when we see another pipe branch off from a T-junction, that pipe is a thinner one. Say, a 3/4" pipe will branch off only a 1/2" pipe in a T. What is the reason behind using a thinner one?

If water resistance is the concern, isn't that a thicker pipe will always have a lower resistance? Using a thinner pipe only increases the resistance (am I right about resistance Vs size?) If we want higher pressure at the outlet, isn't that only the outlet need to have a smaller hole?

Borrowing the same concept from electricity, there is really no need to reduce the size of the electric wire (except to save cost) if we expect the end point can only consume lower power. We can always use a wire as thick as the main wire anywhere.

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You are correct about the electricity analogy, the reasons are normally cost and ease of installation its easier, quicker and cheaper to install a 1/2" pipe than a 3/4" pipe. The 3/4 pipe will give less frictional loss than the 1/2" pipe. The sizing of water through a house is of worked of

  1. Main connection: The total simultaneous demand in the house ( calculation given in a table for domestic houses which gives the nominal amount of flow to normal fixtures, or this can be worked out manually for non standard installation eg. a mansion)
  2. The Main index run:The index length to the furthest (or most disadvantaged ( which in domestic is usually the furthest) fixture and the total loading units for all fixtures.
  3. Branch Pipes: sized using the index length still and the loading units of fixtures on each branch
  4. Branch from Branch: As above

    Some things that affect the calculations are the available pressure at the main ( for main fed houses), the height of the house above or below main height (higher means less pressure, lower means more), the product used eg, materials have different frictional loss and i\d sizes.

With the pressure increase it is good to keep in mind (which it seems you already understand), increasing the pipe size will not increase the pressure just decrease the frictional loss providing more available pressure and flow at the end destination.

If your interested in the calculations or regs have a look at this link for an example

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That means if I want to install a new gas heater, and dont care about the little extra cost, I can always install 3/4" pipes without considering all the things you mentioned(because all the pipes to my bathroom are 3/4" pipes and I don't know what pipes to choose and just want to follow whatever already installed). Assuming all copper pipes available in the market give similar frictional loss –  JavaMan May 6 '12 at 9:32
    
3/4" is the recomended min input size to a HWS anyway and its recommend 3/4" outlet to the first branch then 1/2" after that for hotwater –  UNECS May 6 '12 at 10:09
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Oh yeah and the reason for the 1/2" hot lines is so that you don't have a huge dead leg or a large amount of water that is cold before the hot water gets to the tap –  UNECS May 6 '12 at 10:33
    
I saw a comment elsewhere that some plumbers now run individual 1/2" lines (rather than a trunk/branch setup) for hot water exactly for that reason -- you get the hot water faster if you don't have to fill up a 3/4" run before it gets to your tap. –  TomG May 7 '12 at 2:19
    
Yeah the only reason your supposed to run 3/4" to the first branch( one closest to HWS) is to allow for demand from different fixtures running at the same time. eg. if its all 1/2" and the last fixture is on and someone turns on the fixture at the branch closest to the HWS it will rob all the water from the last fixture. Having said that I would just run 1/2" all the way unless you have multiple showers and K/sinks. –  UNECS May 7 '12 at 2:26
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