There are two main reasons within NFPA 20 where “alternate power” is required: 1) “…when the height of the structure is beyond the pumping capacity of the fire department apparatus.” (9.3.1); or 2) “…where the normal source is not reliable.” (9.3.2)
There is enough interpretation there to drive a Firefighting Apparatus through. What is “not reliable”? It usually comes down to “reliable enough”. For a mid-rise building, where the fire pump is only needed to provide 100 psi for standpipes, but the sprinkler system is designed to work with the city pressure, then utility power is probably “reliable enough”, particularly since the guys bringing the hose drove up on a truck with the biggest, most reliable fire pump in town. However, if the city pressure is so low that a fire pump is needed to make the sprinklers work throughout the building, then standard utility power is probably NOT reliable enough, since seconds count when applying sprinkler water to a fire.
The nature of the building also should enter into the equation of “reliable enough”. Overhead lines vs underground lines should also be part of the equation.
One further thing, “alternate power” may be either a generator or a second power feed where the second source of power “…is provided independent of the normal source of power.” But more interpretation. Are two overhead lines, subject to the same storm “independent? Are two underground feeders, but served from the same substation in an urban setting “independent”. Are two feeders fed from a single small town power plant independent?
