EN 12845 Pre-Calculated LH Design

What pre-calculated design really means for light hazard: the range pipe table defines only the last three or four sprinklers, and the rest is verified by hydraulic calculation.

On a school project the mechanical contractor brought the drawings: pre-calculated LH throughout the classroom corridor, four sprinklers on each 25 mm range pipe, and a single 32 mm line from the design point to the control valve set. "We don't want a hydraulic calculation, we took it from the table," they said. The problem is that the LH table gives rows only for one and three sprinklers — and more importantly, pre-calculated LH does not mean copying from a table; the heavy lifting is still a hydraulic calculation. This article covers where the pre-calculated approach begins and ends for light hazard, the four-sprinkler rule, how the pipe size and friction tables read together, and what breaks that logic on site.

The skeleton of pre-calculated LH

Pre-calculated LH is a combination:

• Sprinkler type: K57 standard spray (the light hazard default).
• Area of operation: 84 m².
• Density: 2.25 mm/min minimum.
• Sprinklers assumed open simultaneously: 4.
• Flow per sprinkler: about 35 L/min at the 0.7 bar minimum operating pressure (from Q = K√P for K57). Four sprinklers at 35 L/min gives 140 L/min of hydraulic demand, but the pump duty point is set well above it.
• System type: wet or pre-action only. Dry and alternate systems are prohibited in light hazard.

The standard limits a light hazard installation to 500 sprinklers. In larger buildings the system must be divided; one control valve set cannot carry a whole school campus.

Range pipe sizes

The LH pipe size table is very short, defining only the last segment below the design point:

Pipe type	Size (mm)	Max. sprinklers
All range pipes and terminal distribution pipes	20	1
All range pipes and terminal distribution pipes	25	3

So one sprinkler on a 20 mm pipe, at most three on a 25 mm pipe. There is one further concession: running 25 mm pipe between the design point and the control valve set is permitted if proven by hydraulic calculation. But where two sprinklers form the design point, 25 mm cannot be used between the third and fourth sprinkler; the size must increase there.

Above the design point

This is where the real work happens: all pipework between the control valve set and each design point is sized by hydraulic calculation. The permitted total friction loss:

Sprinklers on the range or over the room	Maximum friction loss (including changes of direction)
≤ 3	0.9 bar
≥ 4	0.7 bar
≥ 3 on a single line, narrow room or roof ridge	0.7 bar

The friction table then gives unit loss by pipe size and flow:

Size (mm)	Loss at 100 L/min (mbar/m)	Loss at 225 L/min (mbar/m)
25	44	198
32	12	52
40	5.5	25
50	1.7	7.8
65	0.44	2.0

Translated to site: a 32 mm distribution pipe flowing 225 L/min loses 52 mbar per metre. Over a 12 m run to the control valve set, friction alone is 12 × 52 = 624 mbar, about 0.62 bar. The budget is 0.7 bar, leaving 0.08 bar of margin. Add one elbow and one tee and the budget is gone, forcing either a 40 mm pipe or a shorter route.

Pump characteristics

The pump for pre-calculated LH comes from the pump table, split into three height bands by the sprinkler level above the control valve set:

Height h (m)	Nominal: pressure / flow	Characteristic point: pressure / flow
h ≤ 15	1.5 bar / 300 L/min	3.7 bar / 225 L/min
15 < h ≤ 30	1.8 bar / 340 L/min	5.2 bar / 225 L/min
30 < h ≤ 45	2.3 bar / 375 L/min	6.7 bar / 225 L/min

The characteristic point stays at 225 L/min in every band. That is not four sprinklers at 56 L/min; it is the realistic, safety-factored design flow EN loads onto the pump. So even though the sprinkler hydraulic demand is 140 L/min, the pump is selected at 225 L/min plus the hose demand. Where the hose line shares the supply, add 100 L/min for internal hose reels, giving a total pump duty of about 325 L/min. With 30 minutes of duration, the tank is at least 6.75 m³, plus the foot valve clearance and anti-vortex plate details from the water supply clauses.

What breaks pre-calculated LH on site

Error 1 — ceiling obstructions breaking the four-sprinkler assumption. A typical school corridor with a 250 mm downstand beam or HVAC duct at ceiling level. The obstruction rules do not permit the spray to strike an object beneath, so an additional sprinkler goes under the beam. Pre-calculated LH rests on the assumption of four sprinklers operating; if extra heads take that to five or six, the tables no longer apply and fully calculated design is required. This is what we see most often: the designer leaves obstructions to the structural team, extra sprinklers are added when the beam appears, and the hydraulic calculation is never updated.

Error 2 — suspended services creating a concealed ceiling void. Where the gap between the ceiling and suspended pipework or cable tray exceeds the threshold, separate protection is required. If those unseen voids in an LH corridor need sprinklers, the line exceeds its four-sprinkler budget.

Error 3 — forgetting that the three-sprinkler limit on 25 mm is cumulative. Three sprinklers are permitted on 25 mm pipe, but the count is a total; the fourth sprinkler cannot sit on the same 25 mm pipe where it continues into the distribution segment beyond the room.

Error 4 — reading only the nominal column of the pump table. Some projects select a pump on "1.5 bar / 300 L/min is enough" and skip the characteristic point. What EN expects is that flow is still delivered at the characteristic point (3.7 bar / 225 L/min); the nominal point is only the full-flow value.

Comparison with the NFPA 13 light hazard pipe schedule

Size	Max. sprinklers per branch
1 in (25 mm)	2
1¼ in (32 mm)	3
1½ in (40 mm)	5
2 in (50 mm)	10
2½ in (65 mm)	30

NFPA allows only two sprinklers on 25 mm where EN allows three. The reason is the difference in area of operation: NFPA light hazard is built on 139 m² (1500 ft²) at 4.1 mm/min, while EN LH is built on 84 m² at 2.25 mm/min. Total flow is lower on the EN side, so more sprinklers fit on a thin pipe. Recent NFPA 13 editions have largely retired the pipe schedule method; in practice every project runs a hydraulic calculation. On the EN side, pre-calculated LH remains a practical shortcut for small office and school projects — provided the limits of the pipe size table are properly understood.

Turkish practice

BYKHY treats NFPA 13 as the primary sprinkler standard with EN 12845 accepted as an alternative. A pre-calculated EN design in an LH corridor matches the regulation's low-hazard definition. Two points to watch in practice: BYKHY's 30 minute duration requirement applies to light hazard (as does EN 12845), and the pump room must meet the regulation's requirements — which do not conflict with EN 12845. The real conflict arises not in sprinkler counting but in sharing pipework with the hose system; sharing a pump between internal hose reels and sprinklers is common in Turkey, while EN prefers them separated.

Frequently asked questions

What flow and pressure per sprinkler applies in pre-calculated LH?

A minimum density of 2.25 mm/min, which for a K57 sprinkler corresponds to about 35 L/min at the 0.7 bar minimum operating pressure. The pump characteristic flow is 225 L/min.

Why does the range pipe table only cover the last three or four sprinklers?

Because in light hazard it defines only the segment below the design point — one sprinkler on 20 mm, three on 25 mm. Everything above the design point is hydraulically calculated. "Pre-calculated" means only that the final segment comes from a table.

Why does a downstand beam increase the sprinkler count?

Because the obstruction rules require an additional sprinkler beneath it. That breaks the four-sprinkler assumption, invalidates the pre-calculated method, and forces fully calculated design.

How does the EN table differ from the NFPA light hazard pipe schedule?

NFPA allows two sprinklers on 25 mm, EN allows three, because of the difference in area of operation (139 m² against 84 m²). NFPA's pipe schedule is largely retired; EN pre-calculated LH remains practical.

How does hose demand affect pump selection in LH?

The pump table gives only the sprinkler demand (225 L/min at 3.7 bar). Where the hose line shares the supply, add 100 L/min, giving a total of about 325 L/min, with a tank sized for 30 minutes at that flow.

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