are easily determined by tracing the layout of the water supply system as drawn in a print or sketch. Determining  the  size  pipe  you  will  require  to  meet the  fixture  demand  of  a  facility  is  more  com- plicated  and  will  be  discussed  in  this  section. Sizing  Cold-Water  Supply  Systems Some factors that affect the size of the water service in a plumbing system are the types of flush device  used  on  the  fixtures,  the  pressure  of  the water supply in pounds per square inch (psi), the length of the pipe in the building, the number and kind of fixtures installed, and the number of fix- tures used at any given time. The stream of water in a pipe is made up of a series of layers moving at different speeds with the center layer moving the fastest. The resistance to flow is called  pipe friction  and  causes  a  drop  in  pressure  of  the  water flowing  through  the  pipe.  Friction  loss  may  be overcome by supplying water at greater pressure than would normally be required or by increasing the  size  of  the  pipe. The  two  most  important  things  to  consider  are the  maximum  fixture  demand  and  the  factor  of simultaneous  fixture  use.  The  maximum  fixture demand  in  gallons  is  the  total  amount  of  water that  would  be  needed  to  supply  all  fixtures  if  they were being used at the same time for 1 minute. Since it is very unlikely that all fixtures would be turned on at the same time, a probable percentage of the fixtures in use at any given time must be found. This is the factor of simultaneous use. The more  fixtures  in  a  building,  the  smaller  the possibility that all will be used at the same time. Therefore,  simultaneous  use  factors  decrease  as the  number  of  fixtures  increase. To estimate the maximum fixture demand in gallons, the number and type of all fixtures in the completed  plumbing  system  must  be  known. Table 7-8 is used to obtain the maximum fixture demand.  For  example,  assume  a  plumbing  system consists  of  three  urinals,  two  water  closets,  one slop sink, two shower stalls, one kitchen sink, one laundry tray, and four lavatories. From table 7-8 a  maximum  fixture  demand  of  321  gallons  per minute  (gpm)  can  be  figured.  Normally  only  a small percentage of fixtures would be used at the same  time,  so  the  maximum  fixture  demand  is reduced  by  applying  the  factor  of  simultaneous u s e. The factor of simultaneous use, also called the probable demand, is only an estimate. Table 7-9 gives  data  for  making  an  estimate  of  probable demand.  When  using  this  table,  take  the  actual number of fixtures installed, not the fixture unit value.  For  example,  five  fixtures  would  have  a probable  demand  of  about  50  percent,  while  45 fixtures would have a probable demand of about 25 percent. When a table showing the factors of simultaneous use is not available, a practical way of figuring the probable demand is 30 percent of the  maximum  fixture  demand  in  gallons. Many factors affect the flow of water through pipes  resulting  in  a  loss  of  water  pressure.  Dif- ficult calculations are required to consider all the factors  involved  that  may  cause  a  loss  of  water pressure.  These  calculations  are  beyond  the  range of this manual. For simple systems, approximate figures   are   acceptable   for   most   plumbing installations. Table  7-10  (for  galvanized  iron  pipe)  and  table 7-11  (for  copper  tubing)  may  be  used  with  the maximum  fixture  demand  and  the  factor  of simultaneous use to find the correct size of pipe for  water-service  lines.  The  minimum  practical size for a water-service line is 3/4 inch. This size should be used even when calculations show that a  smaller  size  could  be  used. To  continue  the  example  above,  the  14  fixtures would have a factor of simultaneous use of about 35  percent.  Since  the  maximum  fixture  demand was 321 gpm, the water-service line must have a capacity of 35 percent of 321, or 112 gpm. Assum- ing a length of pipe 60 feet long and a pressure at  the  main  of  40  psi,  table  7-10  or  7-11  shows that  either  a  1  1/2-inch  galvanized  iron  or  a  1 1/2-inch  copper  tubing  water-service  line  would be large enough for the example fixture demand. Sizing  Hot-Water  Supply  Systems The   hot-water   system   is   that   part   of   the plumbing   installation   that   heats   water   and distributes it to various fixtures. There are many ways of heating the water, but whichever is used must  be  able  to  supply  maximum  demand.  The materials  used  in  hot-water  systems  are  similar  to those used in cold-water supply systems. The use Table  7-9.—Factors  of  Simultaneous  Use 7-11