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Sprinkler System Design Capacity

When planning an efficient automatic irrigation system, you must first determine the correct Sprinkler System Design Capacity—how much water is available for residential irrigation. If the system will be installed using city water, follow the steps below. If the water will be drawn from a lake or well, your Hunter dealer or the pump installer will have the specifications for pressure and volume.

1. Water pressure (PSI)

To check the PSI or water pressure

"Unit of measure for pressure. Number of pounds of pressure per square inch."
, attach a pressure gauge to the outside faucet closest to the water meter. Make sure that no other water is flowing at the residence. Turn on the faucet and record the number in the area provided to the right. This is the static water pressure in pounds per square inch or PSI.

2. Water volume (GPM)

To determine the volume of water available for the system, you need two pieces of information:

A. What size is the water meter?

Water meters will generally have the size stamped on the meter body. The most common sizes for residential meters are 5/8", 3/4", and 1". In some areas the water supply hooks directly into the city main without the use of the water meter. In these cases, simply enter the size of the service line in the space provided.

B. What size is the service line?

Measure the outside circumference of the pipe that runs from the city main to the house. An easy way to do that is to wrap a piece of string round the pipe, measure the string, and use the table to the right to convert the string length to pipe size.

3. System Design Capacity

Using the System Design Capacity Chart on this page, locate the three numbers you just recorded to determine the Sprinkler System Design Capacity in gallons per minute (GPM). Record this number in the GPM box. Next, locate your system’s static pressure and move down that column and find the system’s working pressure; record it in the PSI box. Working pressure will be used when choosing sprinkler heads and designing the system.

You have now established the maximum GPM and the approximate working pressure available for the sprinkler system. Exceeding these maximums may result in inefficient watering or a condition referred to as water hammer, which could cause serious damage to the system. These two numbers will be used in the design process.

Service lines are based on 100’ of Sch 40 PVC. Deduct 2 GPM for copper pipe. Deduct 5 GPM for new galvanized pipe.

Working pressure is the approximate working pressure at the head, and should be used only as a guide when choosing the proper sprinkler heads and designing the system. The numbers in the Design Capacity Chart are based on generally accepted flow rates (velocity). In some cases, designers increase the velocity in copper pipe only from the accepted 7 ½ feet per second (fps) to 9 feet per second (fps). If you do not deduct the 2 GPM for copper pipe, the rate is approximately 9 feet per second (fps). The friction loss is substantially increased at this speed, and the working pressure will be affected. In order to use the numbers in the chart, the length of copper service line should not exceed 50’ if you decide not to deduct the 2 GPM.

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Courtesy of Hunter Industries Inc.


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