Plumbing and water supply

Water supply system

The water supply system follows the same general pattern in almost all localities. The source is a well or public water main. Under direct pressure at the main or by pressure supplied by a pump, water is forced through the service pipe to the building and from the service pipe through branches and risers to the fixtures. If the service pipe runs from a public water main, there will be a curb cock or valve where the service pipe joins the main, a stop valve just inside the building, and a water meter. Once past the meter, the water runs through a continuation of the service pipe, through water heaters and water softeners, through branches and risers, including turns, T's, couplings, and so on, until it reaches the various outlets.

Lengths of pipe for this system can be plotted. The kind of material wether iron, plastics, brass, or copper will be determined by the hardness of the water, local conditions, comparative costs, and other consideratios.The size of pipe is another matter.

Size of pipe

The correct size, or diameter, of pipe is the smallest size that will do its maximum job adequately. What this correct size happens to be in any individual instance depends on the pressure at the start of the particular run, the flow required at its end, and whether the pressure is sufficient to overcome the obstacles in its path friction in the pipe, turns and fittings, any rise in height - and still deliver the required flow at the desired point. Loss by friction is a major factor in reducing pressure, and it varies greatly with the diameter and length of the pipe and the amount of pressure at the source, not to mention such possible developments as corrosion and rust.

Briefly then, the question is: Can the pressure overcome the obstacles and deliver the desired flow. And the answer in general is: Yes, if the pipe is large enough. The following information will be satisfactory for choosing pipe sizes for the average home, without your having to calculate all the factors involved. After this, there is an explanation of bow to do your own figuring if you find it necessary.

The pressure at the street main can be found by calling the water company.

Total the fixtures and then total the rate of discharge, and you will have the maximum demand for water that your entire system can make. The maximum demand, however, is usually a great deal larger than the peak demand at any one moment, because, of course, you will not be likely to use all the fixtures at once. The general average for homes is 25% of the maximum demand.
Minimum plumbing pipe sizes

Minimum sizes

The left table shows the minimum recommended sizes of iron pipe for the service supply and fixture branches. Usually copper and brass piping one size smaller than iron pipe can be used safely. This is because greater allowance must be made in the case of iron pipe for eventual corrosion and rust.

In arriving at the averages and minimum sizes above, which will suffice in most circumstances, the following assumptions have been made:

  1. That there are no very great runs or lengths of pipe.
  2. That there are no very unusual rises in height.
  3. That there is no great number of turns and fittings.
  4. That there will be no unusual demand on the supply at any one time.

If your system exceeds the average by a considerable amount in any of these factors, you will have to consider carefully whether a larger size of pipe is needed.

Other factors

Other factors that may affect the size of pipe are local codes, the kind of water you have, and the height above sea level. Local codes do not usually place restrictions on the size of water pipes, but it is always wise to check and make sure.

The kind of water you have will be a factor because of its hardness or softness. The corrosive and clogging effects of water vary greatly and result from many complicated factors, but in general it can be said that soft water is usually more corrosive than hard water, and that the more corrosive the water, the larger the size of the pipe should be. Thus, if 1/2" pipe will do for quite hard water, 3/4" should be used for medium hard or soft water, and 1" for very soft or corrosive water. Check local experience in this matter; it will be more valuable than any book or table.

The height above sea level, when it gets to be considerable, must be taken into account, because the pressure supplied by the volume of water itself decreases as you go up (the atmospheric pressure on the water decreases). Compensation must be made insofar as the pressure, of the volume of water itself is used or must be overcome. This is a particularly important factor when calculating pump lifts at high altitudes.

How to size pipe

The method outlined below can be used to find the correct pipe size between any two points in a system, provided that no water is drawn off between the two points. Sizing of pipe, even by experts, is not an exact science. Many factors enter in, and the tables for such quantities as friction loss are at best only averages of many experiments. While it is possible to arrive at a reasonable answer in most cases, if your situation is quite complicated or if a very precise answer is necessary, it is far better to get professional advice, than to try, to work out the problem yourself.

In figuring pipe size, our basic: problem remains the same as stated above: To find the most economical size that will deliver the required flow. To do this, we must know the pressure at the source and find out the resistance it has to overcome.

First of all, we must understand that the pressure of water can be expressed in two different terms: pounds of pressure per square inch and feet of head. The "pounds of pressure" represents the weight of the water itself; if any extra, mechanical pressure is added, it is also included in "pounds of pressure." For our purposes, we may say likewise that "feet of head" represents the pounds of pressure that so many vertical feet of water will exert perpendicularly upon a given point.

This is equal to 0.433 pounds per square inch for every foot of head, if no extra, mechanical pressure is added. To convert feet of head into pounds per square inch, multiply by .433; to convert pounds per square inch into feet of head, multiply by 2.3; we may use whichever term is more convenient, since they express the same thing.

Finding out the resistance that the pressure must overcome is complicated by the fact that part of the resistance is made by the pipe itself and varies with the size and length of pipe and yet it is this size that you want to find out. This means that at one point in your calculations you must assume you are using a certain size and then see if it will work. Let us outline the method and then give a concrete example using the method.

Step 1 : Factors involved

Step 1: To find the correct pipe size in any particular instance, you must first determine the following factors:

a) The length or run of the pipe;
b) The total vertical rise in the run, less any drop below the starting point;
c) The number of various, turns, fittings, etc.;
d) The pressure at the start of the run;
e ) The required rate of flow at the end of the run.

Factors (a), (b), and (c) are determined by plotting your system.

Factor (d) may be determined: if a street main, from the water company; if a well, from rate of flow or by pressure gauge; if at some later point in the system than the main or well, by using table below (as explained later). Always use the minimum pressure that the source is going to provide.

The pressure at the source is almost always the maximum pressure for the entire system. The maximum will be greater than at the source, however, if there is a booster such as a pump, in the system. And it will also be greater at any point that is lower than the source because of the additional weight of water at this point, provided an equal amount of pressure has not been lost before this point is reached.

Factor (e) can be determined from your plot, Table 1 (above), and your estimate of the peak momentary demand, that is, the sum of the flow of all fixtures that may be in use at one time.

Steps 2 to 4

We must now find out, given the pressure at the start and the rate of flow at the end, what size of pipe most economically overcomes the loss from friction, turns, fittings, and vertical rise. Briefly, this is done by making allowances for the turns and fittings and the rise, and then going to tables below to see what is the smallest size that will do the job.

Step 2: To compensate for the rise in height, consult tables below. This table gives the pressure in pounds per square inch for different heights of water and thus tells you the amount of pressure needed merely to overcome the vertical rise in the system. Find the box for the appropriate number of feet, take the pressure given and subtract this from the pressure at the start of the system. Use the result of this subtraction as the pressure for later figuring.

Step 3: To compensate for turns, fittings, and so on, go to tables below and total up the sum of the various items. Loss in pressure from a turn or fitting may be considered as a loss due to friction. Table below gives the equivalent in length of pipe for a 90' turn, a 45' turn, and so on. In this way, you will get all the loss from friction in one term - the length of the pipe and then the tables can tell you what you will lose in that length. When you go to tables below, you must assume a certain size of pipe because, as the table shows, the friction or length varies with the size. So assume a reasonable size and add up the number of feet given in the table for each elbow, turn, valve, and the like in your problem. Add this sum to the length of pipe already measured out (Step 1). The result is called the developed length. Now you can work with one figure in calculating friction loss.

Step 4: Now also, you have made allowances for all the various factors except the friction loss in the pipe, which is taken care of in tables below. Using the last pressure figure you obtained (Step 2), and the developed lenght of pipe (Step 3), find in table below the most economical size that will give you the required discharge (Factor (e) ), which is the answer you want.

A practical problem

Now let us take a concrete problem and follow through the steps outlined above. Let us say we want to find out what size of pipe to use for a run from the service pipe to a bathtub. We will assume that we have a pressure at the start of the run of 50 lbs. per sq. in., a run of 110 feet, a vertical rise of 25 feet, two 90' elbows in the run, and a required rate at the end of 10 gallons per minute.

The pressure at the source is 50 lbs. The vertical rise is 25 feet. From table below we find that 25 feet is approximately equal to 11 lbs pressure. Taking 11 from 50 leaves us 39, lbs pressure for the run.

The length of run is 110 feet. Assuming for the moment that we will use 3/4" pipe, we find from table below that the two elbows total 10 feet, which, added to the 110 gives us a developed length of 120 feet.

Going to table below, we now find that 120 feet of 3/4" iron pipe with 40 lbs pressure at the source will deliver 17 gal. per min., which is more than needed.

Trying 1/2" pipe, we get a developed length of 118 feet. Returning to table below, we find that 120 feet (the nearest to 118 ft) of 1/2" iron pipe with 40 lbs pressure at the source will deliver 8 gal. per min., which is a little under demand.

In determining the correct size for his instance, you must now decide wether 8 gal. per min. will be satisfactory, in which case 1/2" pipe will be all right, or whether, to be on the safe side, you had better use 3/4" pipe, in which case the flow will be ore than sufficient.


The correct pipe size may be easily determined in theory, as shown above. Whether the theoretical size will be all right in practice depends on the consideration discussed above: local codes, height above sea level, softness of water, and whether iron, copper, or brass is to be used. These considerations are briefly reviewed in the paragraphs immediately following.

Our figuring has been based on using clean iron pipe at sea level. If your system is going to be at a considerable height above sea level, you may have to make allowances for the difference in static water pressure. Also, there may be special circumstances in your locality or local building codes that will dictate a different size than that obtained by theory and averaged tables.

In any case, don't use less than the minimum recommended sizes, and, insofar as possible, keep the pressure drop in your system within 10 lbs per 100 feet, which will keep the noise down.

Determining pressure

You may also run into the problem of determining the pressure at some point in the system other than the source - the street main or pump. To find this out, determine by the method given above the rate of flow at the desired point and also the developed length of pipe to that point. Table below gives the loss in pressure in pounds per sq. in. for each 100 feet of iron pipe discharging at various rates.

Subtract the appropriate figure in table below from the pressure at the source. Subtract also the loss in pressure for the vertical rise, if any. The result is the pressure at the desired point. You must, however, know or find out to begin with what the pressure at the source is.

One section at a time

To work out the sizes for the entire system for the house, you will have to proceed section by section, for instance, from the street main to the point where the service main branches, from the branch to the next branch or the fixture, and so on. Obviously, as you proceed from the source nearer and nearer to the final points of the system, the required pipe sizes will diminish. Table below will show you the relationship between mains and branches, that is, the number of branches that a main can supply without falling short of the rate of flow demanded.

In most cases, a consideration of tables used on this page and a check on the common practice in your neighborhood will tell you all you need to know without any calculations.

Plumbing basics

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