EN 12845 Area of Operation Shape and Location (Clause 13.4.3)

On a fully calculated sprinkler system, the result of the calculation is governed less by the pipe diameters than by where the area of operation is placed and what shape it has. TS EN 12845+A2:2026 answers those two questions separately, in Clause 13.4.2 and Clause 13.4.3. Source: TS EN 12845+A2:2026, Clauses 3.9-3.11, Clause 13.4.1, Clause 13.4.2 and Clause 13.4.3.

Three definitions

• Clause 3.9 (TS p.19 / EN p.16), area of operation: the maximum area over which it is assumed, for design purposes, that the sprinklers will operate in a fire.
• Clause 3.10, hydraulically most favourable area of operation: the position of the area of operation, of the specified shape, in a sprinkler arrangement where the water flow rate is at a maximum for a given pressure measured at the control valve set.
• Clause 3.11, hydraulically most unfavourable area of operation: the position of the area of operation, of the specified shape, in a sprinkler arrangement where the water supply pressure measured at the control valve set has to be at a maximum in order to deliver the specified design density.

The two positions are calculated together: the unfavourable area determines the pressure of the pump and the favourable area its flow (Qmax). Clause 3.42 defines the maximum demand flow rate as the flow at the point of intersection of the pressure-flow demand curve of the most favourable area of operation of the system with the pressure-flow curve of the water supply.

Where does the size of the area come from?

Clause 7.1 (TS p.38 / EN p.35): the design density shall not be less than the appropriate value in that clause with all the ceiling/roof sprinklers in the relevant room or those in the area of operation, whichever is the fewer, plus any in-rack and supplementary sprinklers operating. The minimum design density and area of operation requirements are in Table 3 for LH, OH and HHP, and in Clause 7.2 for HHS.

The wet/pre-action areas from Table 3 (TS p.39 / EN p.36): LH 84 m²; OH1 72 m²; OH2 144 m²; OH3 216 m²; OH4 360 m²; HHP1-HHP3 260 m².

The location of the most unfavourable area: Clause 13.4.2.1

Clause 13.4.2.1 (TS p.102 / EN p.99): in determining the hydraulically most unfavourable position of the area of operation, account shall be taken of the sprinkler spacing, the layout, the level, the range centres, the sprinkler orifice size and changes in pipe diameter, and of all possible positions on distribution pipes or between distribution pipes where they are connected by range pipes (see Figures 23, 25 and 26).

The method of proof is also written down:

• On gridded installations, the correct location of the most unfavourable area of operation shall be proved by moving the area one sprinkler pitch in each direction along the range pipes, until the area with the highest pressure demand has been determined.
• On looped installations, the same shift shall be made along the distribution pipe.

Clause 13.4.2.2 likewise requires account to be taken, for the most favourable position, of all possible positions on distribution pipes or between distribution pipes where they are connected by range pipes.

The shape of the area: Clause 13.4.3.1

Clause 13.4.3.1 (TS p.102-103 / EN p.99-100), for the most unfavourable position: the area of operation shall be as nearly rectangular as possible and symmetrical with respect to the sprinkler arrangement (see Figure 23), and shall be as follows:

• a) On terminal and looped configurations, the remote side of the area shall be defined by a range, or by a pair of ranges in an end-centre layout. Sprinklers that do not form a complete range or pair of ranges shall be grouped in the rectangular area on the next range up, as close as possible to the distribution pipe (see Figures 23 and 25).
• b) On gridded configurations where the ranges run parallel to the ridge of a roof sloping more than 6°, or along bays formed by beams deeper than 1.0 m, the remote side of the area shall have a length L parallel to the ranges, and L shall be greater than or equal to twice the square root of the area of operation.
• c) On all other gridded configurations, the remote side of the area shall have a length L parallel to the ranges, and L shall be greater than or equal to 1.2 times the square root of the area of operation.

Clause 13.4.3.2, for the most favourable position: the area of operation shall be as nearly square as possible. On terminal and looped configurations the area shall, where possible, contain sprinklers on one distribution pipe only; the number of sprinklers calculated to operate shall be placed in the hydraulically most favourable position on each range, or on each pair of ranges on end-centre installations. Sprinklers that do not form a complete range or pair of ranges are placed in the hydraulically nearest positions on the next range (see Figures 24 and 26).

L ≥ 1.2√A: in figures

The values below are calculated directly from the areas in Table 3:

The other side of the rectangle is W = A / L. For the 216 m² area in OH3, taking L = 17.6 m gives W = 216 / 17.6 = 12.3 m; the area is therefore a rectangle that is long in the direction parallel to the ranges and short in the direction of the range rows. That is why the rule prevents the calculation from being eased by "squaring off" the area on gridded systems.

Into how many sprinklers is the area divided?

The number of sprinklers depends on the coverage limit in Table 19 (Clause 12.2, TS p.73 / EN p.70):

Worked example (arithmetic using the values of the standard): OH3, gridded layout, sprinkler spacing along the ranges 3.0 m, ranges 4.0 m apart.

• Area per sprinkler: 3.0 x 4.0 = 12.0 m². That fits exactly the 12.0 m² OH limit in Table 19; since the maximum S and D in a standard layout is 4.0 m, the spacings are acceptable too.
• Area of operation: 216 m² from Table 3. Number of sprinklers: 216 / 12.0 = 18 sprinklers.
• Clause 13.4.3.1 c): L ≥ 1.2 x √216 = 17.6 m. With a pitch of 3.0 m along the range, at least 17.6 / 3.0 = 5.87, that is 6 sprinklers, are required per range.
• 18 / 6 = 3 ranges. The area is set up as 3 ranges x 6 sprinklers; the requirement 6 x 3.0 = 18.0 m ≥ 17.6 m is satisfied.

Had a sprinkler spacing of 3.2 m x 4.0 m been chosen in the same example, the area per sprinkler would be 12.8 m² and the 12.0 m² limit of Table 19 would be exceeded; that is why the coverage limit is checked before moving on to the area calculation.

At which point is the density checked?

Clause 13.4.1 (TS p.101 / EN p.98): the discharge density is taken as the total flow from the four closest sprinklers divided by the area covered by those four sprinklers. The density obtained from every area of operation containing the relevant group of four sprinklers shall not be less than the design density in Clause 7. The area covered by each sprinkler is defined by centre lines drawn exactly midway between the adjacent sprinklers, perpendicular to the line joining the sprinklers, and by the boundary of the covered area or half the distance to the nearest sprinklers, whichever is the greater (Figure 22).

Clause 4.4.3.3 d) 8) (TS p.29-30 / EN p.26-27) ties this to the documentation: for fully calculated pipework, the following shall be given for each design area of operation: the area identity, the hazard class, the design density in mm/min, the maximum area of operation assumed, the number of sprinklers in the area, the sprinkler nominal orifice size, the maximum area covered per sprinkler, and, on the drawings, the location of the most unfavourable area, the location of the most favourable area and the four sprinklers on which the design density is based.

Clause 13.4.4 (TS p.105 / EN p.102) then ties the result to a lower limit: the pressure at the most unfavourable sprinkler shall not be less than that required to provide the density, or the greater of LH 0.70 / OH 0.35 / HHP-HHS 0.50 / K115 in-rack 1.00 / K80 in-rack 2.00 bar.

The area on pre-calculated systems: Clause 7.3.2

Clause 13.4 governs fully calculated systems. On pre-calculated HHP and HHS systems, the proportioning of the area is governed by a separate set of rules:

• Clause 7.3.2.2 (TS p.44 / EN p.41): where the area of an HHP or HHS part of an occupancy is smaller than the area of operation, the flow in Table 7 may be reduced pro rata; however, the pressure at the highest design point of the area shall be equal to that in the table or shall be determined by hydraulic calculation.
• Clause 7.3.2.3: where the HHP or HHS part contains fewer than 48 sprinklers, the flow in Table 7 and the appropriate pressure shall be available at the level of the highest sprinklers at the point where the sprinklers enter the HHP or HHS area.
• Clause 7.3.2.4: where the area of operation is larger than the HHP or HHS protected area and that area adjoins OH protection, the total flow is calculated as the sum of the pro rata reduced HHP/HHS part plus the OH part calculated at a design density of 5 mm/min.
• Clause 7.3.2.5: where the area of operation is fed by more than one distribution pipe, the pressure at the level of the highest sprinklers at the design points shall either be as shown in Table 7 for the appropriate design density or be determined by hydraulic calculation. The flow for each distribution pipe shall be determined pro rata.
• Clauses 7.3.2.6 and 7.3.2.7: where the area is increased or reduced, the flow is increased or reduced pro rata, but the pressure at the design point does not change. Equation (1) gives the proportion as linear:

Q₂ = Q₁ x a₂ / a₁

where Q₂ is the required flow (or the flow in each distribution pipe), Q₁ the flow given in Table 7, a₁ the area of operation for the design density (see Table 4), and a₂ the required area of operation (or the area fed by each distribution pipe), in l/min and m². There is no square root in the relationship.

The position of the design point in a pre-calculated layout is not independent of the area either. Table 26 (Clause 13.3.2.3, TS p.89 / EN p.86): in OH, where the number of sprinklers on one distribution pipe in a room is more than 16, the design point is at the connection of the range carrying the 17th sprinkler (two end-side layout), and where it is more than 18, at the 19th sprinkler (other layouts); in HHP and HHS, where it is more than 48, at the 49th sprinkler.

Field note: on a gridded installation, moving the area by one sprinkler pitch often produces a difference of several hundred mbar; the shifting requirement of Clause 13.4.2.1 was written to prevent the calculation being based on the first area that happens to be tried.

Frequently Asked Questions

What shape is the area of operation taken to be?

Clause 13.4.3.1 (TS p.102 / EN p.99): in the most unfavourable position, the area of operation shall be as nearly rectangular as possible and symmetrical with respect to the sprinkler arrangement. Clause 13.4.3.2 requires the area in the most favourable position to be as nearly square as possible.

What is the 1.2 square root A rule on gridded systems?

Clause 13.4.3.1 c (TS p.103 / EN p.100): on all other gridded configurations, the remote side of the area shall have a length L parallel to the ranges, and L shall be greater than or equal to 1.2 times the square root of the area of operation. Clause 13.4.3.1 b makes that factor 2 on gridded configurations where the ranges run parallel to the ridge of a roof sloping more than 6° or along bays formed by beams deeper than 1.0 m.

How is the correct location of the most unfavourable area proved?

Clause 13.4.2.1 (TS p.102 / EN p.99): on gridded installations, the correct location of the most unfavourable area of operation shall be proved by moving the area one sprinkler pitch in each direction along the range pipes until the area with the highest pressure demand has been determined. On looped installations the same shift is made along the distribution pipe.

Where are sprinklers that do not form a complete range placed?

Clause 13.4.3.1 a (TS p.102-103 / EN p.99-100): on terminal and looped configurations, the remote side of the area is defined by a range, or by a pair of ranges in an end-centre layout. Sprinklers that do not form a complete range or pair of ranges shall be grouped in the rectangular area on the next range up, as close as possible to the distribution pipe.

What happens on a pre-calculated HHP/HHS system if the area is split between several distribution pipes?

Clause 7.3.2.5 (TS p.44 / EN p.41): where the area of operation is fed by more than one distribution pipe, the pressure at the level of the highest sprinklers at the design points shall either be as shown in Table 7 for the appropriate design density or be determined by hydraulic calculation. The flow for each distribution pipe is determined pro rata. Equation (1) in Clause 7.3.2.7 gives that proportion as Q₂ = Q₁ x a₂/a₁, that is, linearly.

References

• TS EN 12845+A2:2026 Clauses 3.9, 3.10, 3.11 (TS p.19 / EN p.16), Clause 3.42 (TS p.22 / EN p.19)
• TS EN 12845+A2:2026 Clause 4.4.3.3 (TS p.29-30 / EN p.26-27)
• TS EN 12845+A2:2026 Clause 7.1 (TS p.38 / EN p.35), Table 3 (TS p.39 / EN p.36)
• TS EN 12845+A2:2026 Table 7 (TS p.43 / EN p.40), Clauses 7.3.2.2-7.3.2.7, Equation (1) (TS p.44 / EN p.41)
• TS EN 12845+A2:2026 Clause 12.2, Table 19 (TS p.73 / EN p.70)
• TS EN 12845+A2:2026 Clause 13.3.2.3, Table 26 (TS p.89 / EN p.86)
• TS EN 12845+A2:2026 Clause 13.4.1, Figure 22 (TS p.101-102 / EN p.98-99)
• TS EN 12845+A2:2026 Clauses 13.4.2.1, 13.4.2.2, 13.4.3.1 (TS p.102-103 / EN p.99-100), Clause 13.4.3.2, Figures 23-26 (TS p.103-105 / EN p.100-102)
• TS EN 12845+A2:2026 Clause 13.4.4 (TS p.105 / EN p.102)

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