What do you do when your design area is less than the minimum specified?

Many times during the design of a project, you will have a small area that is required to be calculated, but the total area does not meet the minimum area required per NFPA 13. In the past, we would just calculate all sprinklers in the area and call it a day. For example, you have a small paint booth with 6 sprinklers. The paint booth is typically calculated as Extra Hazard Group 2. So, you calculate your 6 sprinklers at 0.4 gpm / sq ft and size the piping to deliver the required water. That would be the end of it.

However, things are different today. In 2013, additional criteria were added in Chap 23. In the 2013 edition, clarifies how to deal with these areas. The same wording is located at in the 2016 edition. In this text, one is permitted to calculate all the sprinklers in the area. However, if the minimum discharge from the sprinklers in question is less than the minimum density X area required for the hazard, you must add an additional flow at the cross main to make up the difference.

In our paint booth example above, the area is calculated as Extra Hazard Group 2. If we use high temp and/or ≥11.2k sprinklers, the design area is reduced to 2000 sq ft. This gives a minimum criterion of 0.4 gpm / sq ft over the most demanding 2000 sq ft, which gives a minimum demand of 800 gpm. Let’s assume in our paint booth example that the 6 sprinklers require a minimum of 250 gpm at the point of connection to our overhead main. (This assumes the paint booth is supplied from the over head system and is provided with a separate control assembly per NFPA 13 criteria.) (2016 edition) requires that we compare the calculated flow of 250 gpm to the minimum demand of 800 gpm. Since the calculated demand is less, an additional 550 gpm (800-250) is required to be added to the cross main.
This can lead to a significant increase in the pipe sizing required for your overhead system. Let’s assume that our paint booth is located in an auto repair garage. Per NFPA 13, the design criteria for the overhead system would be Ordinary Hazard Group 2. This will have a demand of ±360 gpm (assuming 20% over discharge). This could likely be supplied by a 3” or 4” main provided the water supply is decent. However, when we add this paint booth demand to our system, these 6 sprinklers will have a demand of 800 gpm (see explanation above). This could require 6” or possibly 8” feed/cross mains to supply this area. That will be a significant impact to the material and labor costs to a system.

I will say in most of the systems that I review for other contractors, a conservative estimate is that about 90% of the system designers do not take these criteria into account. It has caused for much grief when I must reject the plans and the designer has to go back to the salesman / project manager to let them know the pipe sizing is going to be considerably larger than anticipated. Be sure to inform your sales and project management teams about this section in NFPA 13 so that projects can be bid with appropriate cost factors.