So we have a sprinkler and or standpipe system designed around a water supply that fluctuates perhaps a lot. They design a fire protection system on this water supply. Which is based on someone holding a pitot blade in a stream of water flowing, hopefully using a calibrated gauge, that is rock steady when they take the reading. They then use this number to calculate the flow, hopefully they use the correct "C" for the hydrant they tested and did the math correctly. They then put this information into a computer that calculates the pressure demand to 56.90 psi. Do you see where I am going with this??
Taking this into consideration this is what NFPA 13, 2010 ED. says about all this. So 10% safety cushion OK???
A.23.2.1 Care should be taken in making water tests to be
used in designing or evaluating the capability of sprinkler systems.
The water supply tested should be representative of the
supply that might be available at the time of a fire. For example,
testing of public water supplies should be done at times
of normal demand on the system. Public water supplies are
likely to fluctuate widely from season to season and even
within a 24-hour period. Allowance should be made for seasonal
or daily fluctuations, for drought conditions, for possibility
of interruption by flood, or for ice conditions in winter.
Testing of water supplies also normally used for industrial use
should be done while water is being drawn for industrial use.
The range of industrial-use demand should be taken into account.
In special situations where the domestic water demand
could significantly reduce the sprinkler water supply, an increase
in the size of the pipe supplying both the domestic and
sprinkler water can be justified.
Future changes in water supplies should be considered. For
example, a large, established, urban supply is not likely to
change greatly within a few years. However, the supply in a
growing suburban industrial park might deteriorate quite rapidly
as greater numbers of plants draw more water.
Dead-end mains should be avoided, if possible, by arranging
for mains supplied from both directions. When private fire
service mains are connected to dead-end public mains, each
situation should be examined to determine if it is practical to
request the water utility to loop the mains in order to obtain a
more reliable supply.