Required Duration for Secondary Water Supply?

1/26/2023

What is the required duration for a Secondary Water Supply within Seismic Zone?

IBC Section 403.3.3:

"The secondary water supply shall have a duration of not less than 30 minutes as determined by the occupancy hazard classification in accordance with NFPA 13."

Does this mean minimum 30 minutes, with the gpm/demand determined by NFPA 13?

Or the minimum duration is also determined by NFPA 13, such that the secondary water supply could require an even longer duration, like 60 minutes?

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8 Comments

Anthony

1/26/2023 08:11:47 am

Per NFPA 13-2016:

Table 11.2.3.1.2 Hose Stream Allowance and Water Supply
Duration Requirements for Hydraulically Calculated Systems

Go check out that table there are waterflow minimums required for the secondary water supply. This is usually used for sizing you tank. The two times listed there differentiate between monitored systems and non-monitored systems see: NFPA 13-16 11.2.3.1.3

Eg for light hazard 30 min x ((1500sqft x 0.1 gpm/sqft) + 100gpm) = 7500 gallons required for a tank in this example

Glenn Berger

1/26/2023 08:28:59 am

As I re-read and re-read your question and I am understanding why this question was asked to the group.

I would recommend using NFPA 13 to determine the required water flow and the required supply duration.

Todd E Wyatt

1/26/2023 09:03:50 am

IBC 403.3.3 (2021) is the subsection for “Section 403 High-Rise Building” (HRBs) so the 30 minute duration of the secondary water supply is only required for HRBs :

Section 403 High-Rise Buildings
403.3 Automatic Sprinkler System
Buildings and structures shall be equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1 and a secondary water supply where required by Section 403.3.3.
403.3.3 Secondary Water Supply
An automatic secondary on-site water supply having a capacity not less than the hydraulically calculated sprinkler demand, including the hose stream requirement, shall be provided for high-rise buildings assigned to Seismic Design Category C, D, E or F as determined by Section 1613. An additional fire pump shall not be required for the secondary water supply unless needed to provide the minimum design intake pressure at the suction side of the fire pump supplying the automatic sprinkler system. The secondary water supply shall have a duration of not less than 30 minutes as determined by the occupancy hazard classification in accordance with NFPA 13.

Otherwise, the applicable chapter for automatic sprinkler systems (ASPS) is ...

Chapter 9 Fire Protection and Life Safety Systems
Section 903 Automatic Sprinkler Systems
903.1 General
Automatic sprinkler systems shall comply with this section.
903.2 Where Required
903.2 Where Required
Approved automatic sprinkler systems in new buildings and structures shall be provided in the locations described in Sections 903.2.1 through 903.2.12.

... which references NFPA 13 as the standard for ASPS installation, classifying the “Hazard”, and development of “hydraulic calculations” :

Chapter 9 Fire Protection and Life Safety Systems
Section 903 Automatic Sprinkler Systems
903.3.1 Standards
Sprinkler systems shall be designed and installed in accordance with Section 903.3.1.1 unless otherwise permitted by Sections 903.3.1.2 and 903.3.1.3 and other chapters of this code, as applicable.
903.3.1.1 NFPA 13 Sprinkler Systems
Where the provisions of this code require that a building or portion thereof be equipped throughout with an automatic sprinkler system in accordance with this section, sprinklers shall be installed throughout in accordance with NFPA 13 except as provided in Sections 903.3.1.1.1 and 903.3.1.1.2.

The only reference to “Secondary Water Supply“ in “Chapter 9 Fire Protection and Life Safety Systems” is as follows “

Chapter 9 Fire Protection and Life Safety Systems
Section 904 Alternative Automatic Fire-Extinguishing Systems
904.11 Automatic Water Mist Systems
904.11.1.4 Secondary Water Supply
Where a secondary water supply is required for an automatic sprinkler system, an automatic water mist system shall be provided with an approved secondary water supply.

The applicable edition of NFPA 13 per 2021 IBC is the 2019 edition. The 2019 NFPA 13 includes an entire chapter (Chapter 5) to “water supply” but this chapter does not reference “secondary water supply”.

5.1 General.
5.1.1 Number of Supplies.
Every automatic sprinkler system shall have at least one automatic water supply.

ENHANCED CONTENT
It is not necessary for a single water supply source to be sized to account for the simultaneous operation of all systems. The water supply must be capable of meeting the most hydraulically demanding area of the systems. If a building’s size requires that it have three separate sprinkler systems, each system is not required to be connected to its own water supply. Instead, the water supply must be capable of meeting the demands of the hydraulically most demanding areas of the three systems. NFPA 13 assumes that only one fire will occur in a building at any given time and that the sprinkler system will control the fire before it spreads to adjacent portions of the building.

Chapter 5 includes (13) references to “water supply” but (0) referencing secondary water supply”.

Dan Wilder

1/26/2023 09:18:02 am

If the building needs a secondary water supply, I recommend looking at the volume from the standpipe calculations in addition to the sprinkler.

Durations would be per
NFPA 13 - 19'
Durations as shown in the storage tables (4.3.1.7.1 goes up to 120 minutes in some cases for example)
8.7.9.5 - for exposure protection
9.3.15(7)/19.4.4 - for window sprinklers based on the wall rating
19.3.3.1.2 - for Light, OH, EH
20.12.2.6 - for storage*

*There are several storage subsections that have individual duration requirements.

NFPA 14 - 19' - 9.1.4.1 - 30 minute duration

COLIN LUSHER

2/1/2023 12:04:57 pm

I just dealt with this in a long battle with our AHJ, and we won the dispute, see my other comment. IBC 403.3.3 specifically states that the secondary supply is only for the SPRINKLER system. In fact, section 403.3 only relates to "Automatic Sprinkler System". Standpipe systems do not need to be accounted for in this calculation.

Colin Lusher

2/1/2023 12:25:06 pm

I just dealt with this issue in an ongoing battle with our AHJ, and we ended up winning the dispute and convincing them that our interpretation was correct.

Your second statement is correct, the duration needs to be per the occupancy classification of NFPA 13, with a minimum duration of 30-minutes (which coincides with NFPA 13 light hazard min duration).

One thing we did discover by conferring with the AFSA experts was that the "hose stream" is NOT referring to outside hose stream, but rather to the sprinkler system small hose connections covered in chapter 8.17.5 of NFPA 13-2016, used for mop up operation, with demand covered by 11.1.6.3 (50gpm or 100gpm). Since most sprinkler systems do not utilize these small hose connections, the secondary supply size on most projects can be calculated with 0 gpm hose demand. If the outside hose is supplied by the fire pump (rare in my experience), then that is a different story, and it would then be practical to include this in the tank calculations where that is the case.

NFPA 14 standpipe system demand need NOT be included either, as section 403.3 of IBC only relates to "Automatic Sprinkler System" per the title, and 403.3.3 only states that the supply should be for the sprinkler system.

Matt Kindall

5/18/2023 12:39:39 pm

I am struggling with a review for this very situation, and hoping someone can help me understand. I work for a small city in Oregon and we have a new high-rise courthouse being built (the first in our town, and the first I've ever reviewed). The building will be six stories and a basement, with the 30,000 gallon auxilliary water tank installed in the basement. I am having a really hard time understanding the tank sizing requirement for this. Is the tank based on the most demanding remote area gpm requirement, the combined remote areas, or just off of (NFPA 13 2016) Table 11.2.3.1.2? So far they have only submitted the basement for this phased submittal, which includes four separate remote areas, and I'm sure there will be others in the floors above. I'm a bit out of my element here and would love any help y'all can provide.

Colin Lusher

5/21/2023 05:04:42 pm

Matt, assuming the auxiliary supply is to satisfy IBC, the required duration is based on most demanding remote area flow requirements (based on hydraulic calculations or a conservative estimate), and the duration for the supply is per NFPA 13 for the hazard being protected (e.g. 30 minutes for Light Hazard, 60-90 minutes for Ordinary Hazard, etc.). The 30k gallon tank is fairly typical for Ordinary Hazard Group 2 with no area reduction used. The tank size can be reduced down if Quick Response area reduction or other flow reduction methods are used, but this size sounds pretty safe for your application.

Required Duration for Secondary Water Supply?

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