Why use a Thinner Water Pipe to Branch off From a Tee

Question

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.

Answer 1

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