Properly dimension the main pipe of your home

Sizing of the main water supply pipe of a dwelling

The dimensioning of the supply network

The design of the supply network obeys precise rules in terms of flow rates, pressure and according to the nature and number of plumbing fixtures on the same pipe, flow rates and diameters.

1 Design rules

The flow velocities to be respected are:
- 2 m/s for underground pipelines, crawl spaces or technical rooms;
- 1.5 m/s for risers.
- The velocity obtained must be less than 2 m/s, increased by 10 %.

The static pressure must be less than 4 bar at the point of drawdown. If necessary, a pressure reducer must be provided. Note that some equipment requires a lower or higher supply pressure than required: individual gas boilers for hot water production or thermostatic mixing valves, for example. Please refer to the manufacturers' recommendations.
The diameters hot and cold water supply pipes for sanitary hot and cold water can be selected according to the requirements:
- the flow rate delivered to the points of use ;
- of the length of these pipes;
- of the distribution height ;
- of the minimum ground pressure available.

Namely there is a pressure drop on the vertical columns. The larger the cross-section of the column, the greater the pressure drop. It is therefore a question of finding the right balance between the simultaneous use of the devices and the diameter to be used.

2 Statutory flow rates and diameters

The following are the minimum flow rates and minimum pipe inside diameters per unit.


Devices	Débits	Ø intérieurs mini
Sink	0,20 l/s	12 mm
Washbasin	0,20 l/s	10 mm
Bidet	0,20 l/s	10 mm
Bathtub	0,33 l/s	13 mm
Shower	0,20 l/s	12 mm
Tap water station 1/2	0,33 l/s	12 mm
Tap water station 3/4	0,42 l/s	13 mm
Toilet with cistern	0,12 l/s	10 mm
Toilet with flush valve	1,50 l/s	Au moins le Ø du robinet
Urinal with individual faucet	0,15 l/s	10 mm
Urinal with siphonic action	0,50 l/s	Au moins le Ø du robinet
hand washing	0,10 l/s	10 mm
washer	0,33 l/s	13 mm
Washing machine	0,20 l/s	10 mm
Dishwasher	0,10 l/s	10 mm
Industrial machine or other apparatus	Se conformer à l'instruction du fabricant
Multi-jet cabins and stirring apparatus	Comply with the manufacturer's instructions

In the case of several units, each individual unit is assigned a coefficient according to the table below. The sum of the coefficients (see graph below) is used to determine the minimum feed diameter of the device group, based on two devices.


Devices	Coefficients
Toilet, washbasin, urinals	0,5
Bidet, collective W.C., washing machine, dishwasher	1
Washbasin or bathroom furniture, single washbasin	1,5
Shower or water station	2
Sink	2,5
bathtub < 150 liters	3
bathtub > 150 liters	3 + 0.1 for each additional 10 litres

Calculation table minimum inside diameter of feed

Calculation example

For a detached house with the following sanitary appliances (the coefficients are in brackets) :

Cold water: 1 sink (2.5) + 1 dishwasher (1) + 1 washbasin (1.5) + 1 bathtub 150 litres (3) + 1 toilet. (0.5) + 1 washing machine (1) = 9.5 coefficient. The graph above indicates that the required internal diameter for the general supply is 18 mm. Then, the internal diameters are reduced by sections.

Hot water: 1 sink (2.5) + 1 washbasin (1.5) + 1 bathtub 150 litres (3) = 7 coefficient. The graph above shows us that the inside diameter required for the general supply is 16.5 mm, i.e. 16 mm. In the same way, the internal diameters are reduced in sections.

3 Pipe diameters, instructions for use

To ensure a good flow of water, a network is calculated with the internal dimensions of the pipes.
The pipes have different tube thicknesses depending on the materials (copper, multilayer, PER). The inside diameters are even-numbered.

A multilayer, PER or copper tube is defined by its outside diameter and thickness. Example: 16-1.5 corresponds to an inner diameter of 13 mm (16-1.5 x 2 = 13). For the same outside diameter, the thickness - and therefore the inside diameter - can vary from one manufacturer to another. It is advisable to buy from the same supplier to avoid incompatibilities between pipes and fittings.

Namely The old denomination for copper is 10/12, for example. The number 10 corresponds to the inner diameter; the number 12 to the outer diameter.