We are in the process of getting the civil drawings done for DC Water, my local utility. We are in the process of renovating my house. We also decided to build a detached accessory dwelling unit (ADU) in my rear yard. It is understood the ADU is a new construction and requires fire sprinklers. ADU (2 story - 950 sq ft) will create a two family dwelling zoning classification which will require a NFPA 13D system. Based on DC code (See below) the 13D system can use the domestic water service (combined service). Based on the AWWA guidelines, it is allowable to use a multipurpose configuration where a single line is tapped into the water main, and this line enters the building and extends to provide water to both domestic water fixtures and to sprinkler heads. NFPA Standard 13D allows a tree System whereby a domestic supply lines branch from a single primary supply line from water main (see below).
My local water company default position is a new dedicated 1” line to feed the sprinkler system. The meter is located outside at the curb and the connection has to be before the meter. To avoid the expense of a dedicated line, I am thinking of installing a 13D system pump and tank. What size tank will be need to supply 10 minutes of water to the ADU which is 930 sq ft.
903.3.5.1.1 Limited area sprinkler systems.
Limited area sprinkler systems serving fewer than 20 sprinklers on any single connection are permitted to be connected to the domestic service where a wet automatic standpipe is not available. Limited area sprinkler systems connected to domestic water supplies shall comply with each of the following
Requirements: 1. Valves shall not be installed between the domestic water riser control valve and the sprinklers. Exception: An approved indicating control valve supervised in the open position in accordance with Section 903.4. 2. The domestic service shall be capable of supplying the simultaneous domestic demand and the sprinkler demand required to be hydraulically calculated by NFPA 13, NFPA 13D or NFPA 13R.
- Multipurpose configurations. In these configurations (see Figures 2.A—2.C), a single line is tapped into the water main, and this line enters the building and extends to provide water to both domestic water fixtures and to sprinkler heads. NFPA Standard 13D includes three different multipurpose configurations: a. Tree System—domestic supply lines branch from a single primary supply line from water main. The primary supply extends into the building to supply sprinkler heads from dead-ended branch piping off the primary supply pipe (see Figure 2.A).
NFPA 13D Standard—Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes. This is a minimum requirements standard that is intended to provide life safety protection and is not intended for property protection. Certain building spaces don’t have to be sprinklered, and the water discharge requirements are the least of the three standards (13 gpm per sprinkler head). Typically, this flow should be available for a minimum of 7–10 minutes at a minimum of 7 pounds per square inch (psi). The 13D Standard exists only for life safety protection at the most reasonable cost to the construction of the building and upkeep of the sprinkler system.
https://www.tyco-fire.com/TD_TFP/TFP/TFP980_07_2016.pdf - page 2 of typical installation with sprinkler NFPA 13D