EN 12845 Pre-Calculated versus Fully Calculated Design

Table-based pipe sizing against full hydraulic calculation: which to use when, compared on site practice, inspection and cost.

On a logistics warehouse project the architect asked for two proposals: one "quick, no trouble at inspection", the other "optimise the metal, cut the budget". The same building produced two sprinkler designs — one pre-calculated from the tables, one fully calculated hydraulically. The difference in pipe weight was 22 %. This article is about how that decision is made.

What the standard says

EN 12845 defines two methods of determining pipe sizes:

• Pre-calculated: sizes come partly from tables and partly from calculation.
• Fully calculated: all sizes are determined hydraulically.

The standard says the designer may choose, with two firm exceptions. Fully calculated design is mandatory:

• In arrangements with intermediate in-rack high-hazard storage sprinklers;
• In gridded or looped pipe networks.

Outside those limits, for LH, OH, HHP and HHS without in-rack heads, the choice is yours.

How pre-calculated design works

The name misleads: "pre-calculated" does not mean "without calculation". The standard states clearly that pipe sizes come partly from tables and partly from hydraulic calculation. The logic:

1. Range pipes and end distribution pipes come from tables — a short table for LH covering only the last three or four sprinklers, and fuller tables for OH and high hazard. They say "this size feeds this many sprinklers"; the designer simply reads.
2. The design point is established — after the third or fourth sprinkler in LH, at the seventeenth or nineteenth sprinkler junction in OH, and at the forty-ninth in high hazard.
3. The pipework between the design point and the control valve set is sized by calculation: a 0.7 bar friction limit for LH with four or more sprinklers, and a 0.5 bar limit at 1000 L/min for OH.

So a small hydraulic calculation is always present — but it is small and its formula is fixed. It can be done in a spreadsheet and does not confuse an inspector.

An example from the OH range pipe table

Position	Layout	Size (mm)	Max. sprinklers
Last two ranges at the end	Two-end-side	25 / 32	1 / 2
Other layouts	All	25 / 32 / 40 / 50	2 / 3 / 4 / 9
Other range pipes	All	25 / 32 / 40 / 50	3 / 4 / 6 / 9

If an OH3 supermarket needs six sprinklers on a range, the table says 40 mm. No argument, and no need to open the hydraulic software.

How fully calculated design works

Fully calculated design bypasses the tables entirely. Every pipe size is derived from Hazen-Williams:

p = 6.05 × 10⁵ × Q¹․⁸⁵ / (C¹․⁸⁵ × d⁴․⁸⁷) × L

with p in bar, Q in L/min, d the internal diameter in mm and L the equivalent length in metres. The designer:

1. Achieves the design density from the four nearest sprinklers in the area of operation.
2. Finds the hydraulically most and least favourable area positions. In a gridded system, that means iterating by shifting the area one sprinkler pitch at a time.
3. Sets the area shape — rectangular for the unfavourable case, near-square for the favourable.
4. Reduces pipe sizes iteratively, re-solving the network after every change.

The acceptance criteria:

• Algebraic sum of pressure losses around a loop: 0 ± 1 mbar.
• Flow balance at a junction: 0 ± 0.1 L/min.
• Velocity: 6 m/s through valves and flow monitors, 10 m/s elsewhere.

Those cannot be closed by hand; recognised hydraulic software is essential.

Which method when: a practical decision table

Situation	Recommended method	Reason
Hotel or office (LH)	Pre-calculated	Ranges come straight from the table; the metal saving is small
OH1 workshop, OH2 retail	Pre-calculated	72–144 m² areas of operation; the table suffices
OH3 supermarket, OH4 workshop	Usually fully calculated	15–25 % pipe saving affects the budget
HHP1, HHP2 process	Either	Pre-calculated where pump margin is generous; otherwise fully calculated
HHS storage without in-rack	Fully calculated	The pre-calculated K80 assumption is very conservative against K115/K160
HHS with in-rack	Fully calculated (mandatory)	Required by the standard
Gridded or looped network	Fully calculated (mandatory)	Two-way flow
ESFR protection	Fully calculated	No tables exist under EN 12845-2

Pre-calculated: pros and cons

Advantages:

• Speed: pipe sizing for an OH2 office project takes half a day in a spreadsheet.
• Ease of inspection: an independent inspector reads the table and approves, without having to trust a software output.
• Site flexibility: adding a sprinkler on site means checking the table again, not reopening the software.
• Error tolerance: because the tables are conservative, small layout errors do not break the system.

Disadvantages:

• Conservative: the high hazard tables assume K80 sprinklers and a particular supply characteristic; a better supply means money spent unnecessarily.
• Limited scope: no gridded layouts, no loops — the inevitable consequence of a one-way, toward-the-end flow assumption.
• Little design flexibility: the K-factor is locked to the table. Moving to K115 or K160 means moving to fully calculated design.
• No optimisation: 100 mm appears where 80 mm would do.

Fully calculated: pros and cons

Advantages:

• Economy: pipe sizes optimised individually, with 15–30 % metal savings possible — substantial money in large warehouses.
• Flexibility: gridded layouts, loops, K-factor changes and different supply characteristics can all be tried in turn.
• Clearer pump design: the pressures and flows at both the most and least favourable areas emerge, so the pump curve is selected precisely.
• Mandatory for ESFR and CMSA: those systems are only approved on a full hydraulic calculation.

Disadvantages:

• Software required: a certified package and a trained engineer.
• Inspection burden: the inspector must read the output node by node, which takes hours.
• Expensive site changes: moving a sprinkler or adding a branch means re-running the calculation, and the as-built must be updated.
• Sensitivity: closing a 1 mbar balance in iteration demands accurate C values, fitting equivalent lengths and static differences.

Common errors

1. Putting the design point in the wrong place in pre-calculated design. In OH it is at the seventeenth or nineteenth sprinkler depending on layout. Get it wrong and the downstream pipes come out undersized.

2. Calculating only the most unfavourable area in fully calculated design. The standard also requires the most favourable area; the pump must be capable there and the velocity limit must not be exceeded.

3. Using C = 100 for standard galvanised pipe. The standard gives C = 120 for mild and galvanised steel. Using 100 raises losses by 44 % and produces unnecessarily large pipe.

4. Starting pre-calculated with a gridded layout. Explicitly prohibited, and returned at inspection.

5. Forgetting velocity in fully calculated design. The software reduces sizes but may reach 12 m/s. The limits are 10 m/s generally and 6 m/s through valves and strainers; exceed them and the calculation is rejected.

Comparison with NFPA 13

NFPA 13 names the same two worlds differently:

• Pipe schedule = EN 12845 pre-calculated, with size-versus-sprinkler tables for light and ordinary hazard.
• Hydraulically calculated = EN 12845 fully calculated, with Hazen-Williams, a design area and a density-area curve.

The important difference: the NFPA pipe schedule is permitted only for LH and OH, with hydraulic calculation mandatory for high hazard. EN 12845 brings HHP and HHS without in-rack heads into pre-calculated scope through their own tables. European practice is therefore somewhat more table-oriented than American.

Turkish practice

BYKHY leaves sprinkler design to "TS EN 12845 or NFPA 13" without prescribing a method. In practice:

• Projects at OH2 and below — hotels, offices, small shops — are 90 % pre-calculated.
• Logistics warehouses, automotive manufacturers and e-commerce centres are 95 % fully calculated. The move to ESFR made it unavoidable.
• Independent certification firms ask for the table number and design point location in pre-calculated design, and for the software output — node list, most unfavourable area map, velocity report — in fully calculated design.
• A 20 % margin on pump selection is common in pre-calculated design; in fully calculated design it drops to about 0.5 bar because the calculation is tight.

The conclusion: the choice of method is commercial as much as technical. On a small project pre-calculated design cuts the design fee quickly; on a large project fully calculated design cuts the material invoice.

Frequently asked questions

Which hazard classes allow pre-calculated design?

LH, OH, and HHP and HHS without in-rack sprinklers. Designs with intermediate high-hazard storage sprinklers and gridded or looped layouts must always be fully calculated.

Is there really no hydraulic calculation in pre-calculated design?

There is. Range pipes and end distribution pipes come from tables, but the distribution pipework between the design point and the control valve set is sized by calculation within a defined friction limit.

How much metal does fully calculated design save?

Typically 15–25 % of pipe weight on OH3 and OH4 projects, and up to 30 % in high-hazard storage, because the tables assume conservative K80 sprinklers and a modest supply.

Is the NFPA pipe schedule the equivalent of pre-calculated design?

In logic, yes — both take pipe size from a table. But the NFPA schedule is limited to light and ordinary hazard, while EN 12845 pre-calculated design also covers HHP and HHS without in-rack sprinklers.

Why must a gridded system be fully calculated?

Because water can reach the same sprinkler from two directions, invalidating the one-way flow assumption behind the tables. Finding the most unfavourable area requires shifting it one sprinkler pitch at a time — something only hydraulic software can do.

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