EN 12845 Light Hazard (LH) Design

The practical limits of the light hazard class, pre-calculated pipe sizes, K57 sprinklers and the wet-pipe-only rule — from a site perspective.

On a private school extension project last year, the mechanical consultant said "it's a school, let's make it LH and keep the system small" and sized a sprinkler run covering several classrooms at 2.25 mm/min. The problem was in the basement: a 180 m² archive room with pallet racking. The EN 12845 hazard classification clause requires light hazard compartments not to exceed 126 m² and to have at least 30 minutes of fire resistance. That archive room fell outside LH on its own — at least OH1, and probably OH2 given its contents. Because the whole project was assumed to be LH, the pump, tank and pipework were all inadequate.

LH is the lightest hazard class in EN 12845, and attractive from a site engineer's point of view: less water, smaller pipes, a smaller pump. But because the standard keeps its limits so tight, "what I thought was low hazard" and "what EN 12845 calls LH" often fail to coincide. This article brings together the numerical targets, the pre-calculated rules, a simple hydraulic example and the NFPA 13 comparison.

What LH is, and which spaces qualify

EN 12845 describes light hazard as spaces with a low fire load and low combustibility, with no single compartment exceeding 126 m² and at least 30 minutes of fire resistance. Fail either condition and the space is not LH.

The classification annex list is shorter than you might expect:

• Schools and educational establishments, certain areas
• Offices, certain areas
• Prisons

The common misconception is that hospitals, hotels, libraries and museums are also LH. The standard does not say so. Looking at the ordinary hazard table, hospitals, hotels, libraries (excluding book stores), museums, restaurants, small broadcast studios and car parks are all listed as OH1. So even the bedroom wing of a hotel defaults to OH1; only small, separated office areas whose contents you are certain of can be LH. That detail shows how narrow a door LH really is.

Design density and area of operation

The LH row of the density-area table is short and clear:

Hazard class	Design density (mm/min)	Area — wet or pre-action (m²)	Area — dry or alternate (m²)
LH	2.25	84	Not allowed — use OH1
OH1 (for reference)	5.0	72	90

That "not allowed — use OH1" entry is the most critical restriction in LH. EN 12845 prohibits dry and alternate systems in light hazard. Where freezing is a risk, you either heat the space — so in practice an unheated roof void, ceiling void or open car park entrance cannot be LH — or you raise the class to OH1. Catching that early matters; issuing a project as LH when a pipe upstairs is at risk of freezing, then reversing at permit stage, is expensive.

Total design flow from density times area:

• 2.25 mm/min × 84 m² = 189 L/min for the sprinklers alone

The standard requires a minimum of 225 L/min at the control valve set for pre-calculated LH; the difference covers friction and displacement allowances. That 225 L/min is the key number to remember.

Pre-calculated LH: K57 sprinklers and pipe sizes

EN 12845 offers two approaches for LH: fully hydraulically calculated or pre-calculated. On small office and school floors, pre-calculated is generally preferred because the standard gives pipe sizes and sprinkler counts directly from a table, cutting the calculation load.

LH range pipes:

Pipe size	Max. sprinklers on a range or terminal distribution pipe
20 mm	1
25 mm	3

So with four sprinklers on a branch in LH, you cannot finish that branch in 25 mm; either step up to 32 mm or split the range. The standard permits 25 mm between the design point and the control valve set where verified by hydraulic calculation; but where two sprinklers form the design point, 25 mm may not be used between the third and fourth sprinkler — a small note frequently caught at inspection.

The standard head for LH is K57; no larger K-factor is needed, because the design density is low. The minimum operating pressure with K57 is 0.7 bar, but the 2.2 bar plus static requirement at the control valve set governs the total pressure. Even where 0.7 bar looks sufficient at the head, static gains along the run pull the total required pressure upward.

A short hydraulic example

Take a small LH office with three sprinklers on a branch and K57 heads, with the design point being the two end sprinklers — the most unfavourable hydraulic point within the 84 m² area.

With flow in L/min and pressure in bar:

Q = K · √P

• End sprinkler at 0.7 bar → Q₁ = 57 × √0.7 ≈ 47.7 L/min
• With friction, the next sprinkler sees 0.9 bar → Q₂ = 57 × √0.9 ≈ 54.1 L/min
• Total at the two-sprinkler design point: 47.7 + 54.1 ≈ 101.8 L/min

That matches the 100 L/min assumption in the friction table (44 mbar/m in 25 mm pipe). In the distribution pipe from the design point to the control valve set, 225 L/min is assumed, with friction losses kept within the permitted maxima: 0.9 bar up to three sprinklers, and 0.7 bar for four or more.

Three numbers complete the calculation:

• Minimum 225 L/min at 2.2 + ps bar at the control valve set
• Pump characteristic for h ≤ 15 m: 1.5 bar / 300 L/min nominal plus 3.7 bar / 225 L/min characteristic
• Minimum tank volume for pre-calculated LH at h ≤ 15 m: 9 m³

Common errors

1. Installing a dry system in LH. We see people say "it's a small run, I'll make it dry" for a freeze-prone ceiling void, roof space or unenclosed car park. The table is explicit: dry and alternate are prohibited in LH. With freezing risk you move to OH1 — which takes density to 5.0 mm/min and the area to 90 m², driving flow to 540 L/min and changing the pump, tank and pipe sizes.

2. Forgetting the 126 m² compartment limit. An open-plan office of 200 m² without walls is not LH but OH1. A glass partition is not a fire compartment; separation requires a 30 minute wall.

3. Writing hospital, hotel or library wings as LH automatically. The annex lists them as OH1, and an LH assumption is returned at inspection.

4. Using K80 heads while applying the pre-calculated LH pipe table. That table is derived for K57; with a larger K-factor, a full hydraulic calculation is required.

5. Meeting only the nominal point on the pump characteristic. The table gives two points for LH (1.5 bar / 300 L/min and 3.7 bar / 225 L/min). The pump curve must pass above both; a selection made on one point alone produces low pressure at 225 L/min.

Comparison with NFPA 13 light hazard

NFPA 13's light hazard class sits conceptually in the same place as EN 12845 LH — low fire load, offices, schools and places of worship — but the numerical targets differ:

Parameter	EN 12845 LH	NFPA 13 light hazard
Design density	2.25 mm/min	about 4.1 mm/min (0.10 gpm/ft²)
Area of operation	84 m²	about 139 m² (1500 ft²)
Total sprinkler flow	about 189 L/min (225 L/min at the valve)	about 570 L/min
Hose allowance	Not given directly; fire service connection separate	100–250 gpm via NFPA 13 and 14
Dry or alternate	Prohibited — move to OH1	Permitted, with an area increase
Duration	30 min	30–60 min depending on project and hose conditions

The EN density is roughly half, but the area of operation is also smaller, so total flow comes out at about a third of the NFPA figure. That makes EN 12845 LH look more economical — but hose allowance, tank duration and fire service connection rules sit in separate clauses, and overall the two standards lead to different pumps and different tanks. Do not mix them on one project; stay with whichever standard the tender specifies.

Turkish context

BYKHY references EN 12845 or NFPA 13 for sprinkler design. LH is commonly chosen for schools, administrative buildings and small offices in Turkey; but classifying without regard to the compartment and combustibility limits above causes problems at permit stage. Practical advice: where in doubt, go with OH1. The cost difference between LH and OH1 in small buildings is not as large as assumed, while a design change later is far more expensive.

Frequently asked questions

Which buildings fall into LH?

Spaces with a low fire load and low combustibility, with no single compartment above 126 m² and at least 30 minutes of fire resistance: schools (certain areas), offices (certain areas) and prisons. Hospitals, hotels and libraries are OH1.

Can I use a dry or alternate system for LH?

No. The table says "not allowed — use OH1". Where freezing is a risk, heat the space or re-size the run as OH1 at 5.0 mm/min over 90 m².

What are the LH pipe size and sprinkler count limits?

One sprinkler on 20 mm and three on 25 mm. 25 mm is permitted between the design point and the control valve set where verified hydraulically, but not between the third and fourth sprinkler where two sprinklers form the design point.

What pump characteristic applies?

For h ≤ 15 m: 1.5 bar / 300 L/min nominal plus 3.7 bar / 225 L/min characteristic. For 15–30 m: 1.8 / 340 plus 5.2 / 225. For 30–45 m: 2.3 / 375 plus 6.7 / 225.

Are NFPA 13 light hazard and EN 12845 LH the same?

Close, but not identical. NFPA light hazard is about 4.1 mm/min over 139 m² (roughly 570 L/min plus hose); EN 12845 LH is 2.25 mm/min over 84 m² with 225 L/min at the valve. They are not interchangeable.

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