According to NFPA 13, how many residential heads should be calculated?

The correct choice reflects the guidelines established in NFPA 13, which pertains to the design and installation of sprinkler systems. Specifically, for residential occupancies, NFPA 13 stipulates that the flow must be calculated based on the assumption of four heads being in operation simultaneously. This approach ensures adequate coverage and pressure for effective fire suppression.

The reasoning behind this stipulation often hinges on the concept of ensuring that the system can handle the worst-case scenario during a fire event. When calculating the performance of a sprinkler system, it's crucial to assume that multiple heads may activate at once, particularly in a residential setting where fires may spread quickly. By calculating for four continuous heads, designers account for various factors, including the potential spread of a fire and the varying response times of different sprinkler heads.

This standard also supports the overall safety framework set by the National Fire Protection Association (NFPA), ensuring that properties are equipped with sprinkler systems capable of effectively mitigating fire risks. Thus, the figure provided helps in establishing a robust and reliable system, critical for protecting lives and property in residential environments.