10.4 Multipurpose Piping Systems & NFPA 13D Residential Sprinklers

What an MPP system is

A multipurpose piping system is a water distribution system that serves both plumbing fixtures and residential fire sprinklers (SPS 381.01 (156)). Water flows through the sprinkler piping every time a fixture is used, so no stagnant line needs backflow protection, and the occupants notice right away if the water is shut off. NFPA 13D (2019) and the Multi-Purpose Piping Systems Design Manual are both on DSPS's list of permitted exam references.

Code requirements (SPS 382.40 (3) (e))

1. Design and install per SPS 382.40 and NFPA 13D; materials must be acceptable under NFPA 13D and SPS 384.30, specifically 384.30 (4) (e) and (5).
2. A partial or single sprinkler may be installed in a dwelling unit not required to be sprinklered under NFPA 13D. Limited-purpose or limited-area sprinklers may be installed in areas not required to be sprinklered.
3. Fire department connections are prohibited.
4. Add 5 gpm to the multipurpose calculation for each dwelling connected to a common water supply system.
5. Perform a flow test at the controlling sprinkler before the system is put into operation.

A code note points to the Uniform Dwelling Code (SPS 321.095) and the Commercial Building Code (SPS 362.0903 (10)) for when sprinklers are required.

How MPP fits the rest of the plumbing code

NFPA 13D essentials (2019 edition)

NFPA 13D, Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes, is a life-safety standard. It aims to prevent flashover in the room of origin long enough for occupants to escape. It doesn't require sprinklers in every space.

• Water supply duration: the sprinkler demand for 10 minutes. Where stored water is the sole source, 7 minutes of the two-sprinkler demand is allowed for dwellings that are one story and less than 2,000 sq ft.
• Design: typically up to two operating sprinklers in the most demanding compartment, at the listed flow for each sprinkler.
• Multipurpose systems: acceptable when the 13D conditions are met. In common supply connections serving more than one dwelling, 5 gpm is added to the sprinkler demand when sizing common piping. This matches the Wisconsin rule above.

The MPP manual provides the tables and worksheets for sizing an MPP system: listed sprinkler flow and pressure, pipe friction, meter and elevation losses, and available supply pressure. The approach mirrors the empirical method in SPS 382.40 (7) (section 5.2), except that the governing demand is the sprinkler flow plus domestic allowances rather than fixture units alone.

Worked example

A duplex shares one water service. Each unit's controlling sprinkler design needs 26 gpm (two sprinklers at 13 gpm each). Wisconsin and NFPA 13D both add 5 gpm per dwelling connected to the common supply. The common water service must deliver the most demanding unit's 26 gpm plus 2 x 5 = 36 gpm at the required pressure, so the service, meter and any treatment equipment must be checked at 36 gpm.

The installer then:

1. Uses piping listed for both potable water and sprinkler service.
2. Omits any fire department connection.
3. Performs and documents the flow test at the controlling sprinkler before occupancy.

Common MPP inspection problems

• Unlisted fittings or pipe for sprinkler service, or a sprinkler branch built in a material acceptable for water distribution but not listed under NFPA 13D.
• Undersized meters or treatment devices that pass domestic flow but starve the sprinkler demand; the pressure loss must be in the calculation.
• Stagnant sprinkler legs. The appeal of an MPP system is that water moves through it, so dead-end sprinkler piping defeats the purpose.
• A fire department connection added by habit from commercial work; it is prohibited.
• No documented flow test at the controlling sprinkler before occupancy.

Related references

• Combination water services that also feed fire protection: test and disinfect per NFPA 24 (section 5.5).
• Mixing valves and treatment devices on the MPP supply must not starve sprinkler flow, so check their pressure losses in the hydraulic calculation.
• Larger residential and commercial sprinkler systems (NFPA 13 and 13R) are installed by licensed sprinkler contractors under ch. 145 and SPS 305 Subchapter V, not as plumbing MPP systems.