CMSA (Control Mode Specific Application) sprinklers are large-drop sprinklers tested and listed for specific storage arrangements, and they replace the density/area method for many warehouse designs. This part of NFPA 13 answers one question: for this commodity, this storage arrangement and this ceiling height, how many sprinklers must open, and at what pressure?
What this part of the standard covers
With standard spray sprinklers you pick a density (mm/min) and an area of operation (m²). CMSA works differently. Full-scale fire tests establish a number of sprinklers and a minimum operating pressure that control a fire in a given arrangement, and the designer reads those values directly from the tables. You are not computing a density; you are selecting the correct table row and then making sure every condition of that row is actually delivered on site.
The requirements fall into four groups:
- Applicability: which commodity class, storage type (palletised, solid-piled, rack), storage height and ceiling height each CMSA option covers.
- Sprinkler selection: K-factor, orientation (upright or pendent), temperature rating and system type (wet or dry).
- Installation: protection area, spacing, distances below the ceiling and above the storage, and obstruction rules.
- Hydraulic demand: number of design sprinklers, minimum pressure, hose stream allowance and duration.
Key numeric criteria
The values below are the general limits you meet most often. Final values must always be taken from the relevant table row, the product listing and the edition of NFPA 13 in force.
| Parameter | General criterion | Note |
|---|---|---|
| Minimum K-factor | K11.2 (K160 metric) | Common options: K11.2 / K16.8 / K19.6 / K25.2 |
| Protection area per sprinkler | Up to approx. 12.1 m² (130 ft²) | Some construction/storage conditions require less |
| Minimum distance between sprinklers | 2.4 m (8 ft) | Prevents cold soldering of adjacent heads |
| Deflector to top of storage | At least 914 mm (36 in) | Space needed for the spray pattern to develop |
| Design method | Fixed number of sprinklers × minimum pressure | Both values come from the table |
Design decisions: reading the table row correctly
Each row in a CMSA table represents a tested scenario. The variables that define the row are commodity class (Class I–IV, Group A plastics, cartoned or exposed, expanded or unexpanded), storage arrangement, maximum storage height, maximum ceiling height, K-factor, orientation and system type. If one variable is not covered, you do not interpolate from a neighbouring row; you move to another protection option or add in-rack sprinklers.
Commodity class is the critical input
A CMSA selection is only as good as the commodity classification behind it. The step from cartoned to exposed Group A plastics can make a design valid or invalid. See commodity classification in NFPA 13 for the logic.
Ceiling height and slope
The tables are built around the highest point of the ceiling, not the average. On pitched roofs the ridge governs, and the permitted roof slope for CMSA is limited (in general around 2 in 12). If the roof is steeper, the answer is a flat ceiling below it or a different protection approach.
Wet or dry?
An important difference from ESFR is that some CMSA table rows accept dry pipe systems. The number of operating sprinklers usually rises significantly, and water delivery time must be checked as a separate requirement. This matters in applications such as cold storage sprinkler protection.
What "control mode" means
CMSA controls a fire; it does not promise extinguishment. It limits heat release, protects the structure and stops the fire spreading, while final extinguishment is left to the fire service. The hose allowance and duration are therefore part of the demand and must be included in the tank calculation. The fire water tank sizing calculator is a quick way to check storage volume.
CMSA, CMDA and ESFR compared
| Feature | CMDA (density/area) | CMSA | ESFR |
|---|---|---|---|
| Protection mode | Control | Control | Suppression |
| Design input | Density + area | Number of sprinklers + pressure | 12 sprinklers + pressure |
| Dry systems | Possible with area increase | Possible in some rows | Generally wet only |
| Sensitivity to obstructions | Moderate | High | Very high |
For a deeper comparison read ESFR vs CMSA sprinklers and the field view in CMDA vs CMSA sprinklers. To see quickly whether ESFR is even an option for the building, try the ESFR suitability checker.
Common site mistakes
- The commodity changes after handover. The design is based on cartoned Class III goods; a few years later the same racks hold products in exposed plastic packaging, and the CMSA row no longer applies.
- Losing the 914 mm clearance. Storage height creeps up in operation until the top load sits close to the deflectors and the spray pattern is disrupted.
- Obstructions added later. Lighting, cable trays and ductwork cut across the large-drop pattern.
- Mixed-up sprinklers. A standard spray head with the same K-factor is fitted in place of a CMSA head. Always check the sprinkler identification number.
- Wrong ceiling height. Selecting the table row from eaves height instead of the ridge.
- Heads closer than 2.4 m. Water from the first sprinkler can wet and delay its neighbour.
Rule of thumb: a CMSA design is a recipe. If one condition of the recipe is missing on site, getting the others right does not deliver protection. Write the selected table row and every assumption into the design report so that operational changes can be checked against it.
Design checklist
- Is the commodity class and packaging type recorded in writing?
- Is the storage arrangement (palletised, solid-piled, single/double/multiple-row rack) and maximum storage height defined?
- Have the highest ceiling point and roof slope been measured?
- Does the listing of the selected sprinkler cover this row?
- Are protection area and maximum/minimum spacing met?
- Is there at least 914 mm clear between deflector and storage?
- Have obstruction rules been checked for every ceiling element?
- Are the hose allowance and duration included in the water supply calculation?
- Does the temperature rating match the table? (temperature rating selector)
Frequently Asked Questions
What is the main difference between CMSA and standard spray sprinklers?
Standard spray design is based on density and area. CMSA takes a tested number of sprinklers and a minimum pressure straight from a table for a specific storage scenario, and uses large-K sprinklers that produce larger drops.
Can CMSA sprinklers be used on dry pipe systems?
Some table rows permit dry systems, but the number of operating sprinklers usually increases and water delivery time is checked separately. Always confirm against the product listing and the relevant row in the edition in force.
Do CMSA designs need in-rack sprinklers?
Many rack scenarios can be protected by CMSA ceiling sprinklers alone. When storage height, commodity or rack arrangement exceeds the table limits, in-rack sprinklers are added or another protection option is used.
Why is the number of CMSA sprinklers fixed?
Because it comes from full-scale fire tests in which that number of sprinklers controlled the fire. The hydraulic calculation must prove that the most demanding group of that many sprinklers reaches the minimum pressure.
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App StoreNFPA 13 (current edition) – storage protection and CMSA sprinkler requirements; product listings (UL/FM). Numeric values are general criteria; confirm against the edition in force and the authority having jurisdiction for each project. The findings here are typical defect patterns, not an account of events at any particular site.