EN 12845 Minimum Pipe Diameters

Until you separate what the minimum diameter table actually says from the site habit of "DN50 main", your pipe schedule will always come out wrong.

On a logistics warehouse project we found the contractor running DN20 ranges on an OH3 system. "The standard minimum is DN20", he said — correct, but incomplete. The minimum diameter table in EN 12845 sets the floor below which a fully calculated system may not go; the diameters in the pre-calculated tables are binding values driven by sprinkler count. Confuse the two and either the hydraulic calculation collapses or the material schedule does not match reality. This article separates the two regimes and the way the field reads pipe sizes.

What the standard says

EN 12845:2015+A1:2019 collects the minimum pipe sizes for fully calculated systems in one short table:

Hazard class	Minimum pipe size
LH (light hazard)	DN20
OH and HH — horizontal or upright connection to a single K ≤ 80 sprinkler	DN20
All other pipework (OH and HH ranges and distribution)	DN25

Nowhere else in the standard is there a sentence saying "ranges must be DN32, distribution DN50, mains DN65". Those numbers come from the pre-calculated tables, driven by sprinkler count. So in a fully calculated project, if friction allows, sizing an OH1 distribution pipe at DN40 is entirely legitimate — provided the minimum sprinkler pressure and the density requirement are both met.

The habitual values from the pre-calculated tables

The site language of "DN50 distribution, DN65 main" comes from the pre-calculated tables. Summarised for OH:

Pipe	Configuration	DN	Max. sprinklers
Range (last two at the remote end)	Two-end-side	25 / 32	1 / 2
Other ranges	All arrangements	25 / 32 / 40 / 50	3 / 4 / 6 / 9
Distribution (at the end)	Two-end-side	32 / 40 / 50 / 65	2 / 4 / 8 / 16
Distribution (other)	—	32 / 40 / 50 / 65	3 / 6 / 9 / 18
Design point to control valve	All	Fully calculated — no more than 0.5 bar loss at 1000 L/min

On the high hazard side, a pre-calculated K80 system allows DN25 to DN50 for ranges, but with a maximum of four sprinklers per range, and a range may not connect to a distribution pipe larger than 150 mm. With K115 sprinklers, up to six sprinklers per range are permitted in an end-side arrangement. So there is no flat rule that "HH ranges are minimum DN32"; the pre-calculated table starts at DN25 even for a single sprinkler — though in practice most HH ranges settle at DN32–DN40.

Pre-calculated versus fully calculated: who decides?

The decision matrix is simple:

• Pre-calculated design — the tabulated values are binding and cannot be reduced. DN50 distribution for nine sprinklers is a requirement, not a starting point.
• Fully calculated design — the minimum table (DN20/DN25) binds, and the upper size is left to the hydraulic calculation. If a range needs DN32, the calculation forced it, not the table.
• Mixed system — pre-calculated on LH and OH, fully calculated on the riser and main. This is the most common case, and the design point where the table meets the calculation must be marked clearly; the run from design point to control valve set is left fully calculated.

The practical advantage of going fully calculated: an OH3 line run pre-calculated forces DN65 at the end distribution pipe, while the same geometry fully calculated sometimes fits DN50. But realising that saving means entering the correct Hazen-Williams C value and the true internal diameter — which brings in the steel grade.

Medium versus heavy steel: same DN, different bore

EN 12845 gives two references for pipe material: ISO 65 for minimum wall thickness on threaded, cut-grooved or machined steel pipe, and ISO 4200 range D for roll-grooved or weld-prepared ends where wall thickness is not meaningfully reduced.

ISO 65 recognises three series: light, medium and heavy. The Turkish site vocabulary of "black steel B" and "black steel C" maps onto medium and heavy. Where threaded joints are used — mostly DN50 and below — medium is adequate; on higher-pressure pre-action or high-hazard storage lines, heavy is preferred. Two DN50 pipes that look identical on site have different bores:

Nominal	OD (mm)	Medium wall (mm)	Medium bore (mm)	Heavy wall (mm)	Heavy bore (mm)
DN25	33.7	3.2	27.3	4.0	25.7
DN32	42.4	3.2	36.0	4.0	34.4
DN40	48.3	3.2	41.9	4.0	40.3
DN50	60.3	3.6	53.1	4.5	51.3
DN65	76.1	3.6	68.9	4.5	67.1
DN80	88.9	4.0	80.9	5.0	78.9

At DN50 the difference is 1.8 mm of bore. In Hazen-Williams, friction loss varies inversely with the bore to the power 4.87, so at the same flow (350 L/min, say) the heavy pipe loses roughly 18 % more per metre than the medium. Over a 60–80 m distribution run that becomes 0.15–0.2 bar at the design point. Leaving wall thickness at "default" in the calculation means the design will not match the site.

A common error: ignoring galvanised heavy versus black medium

The error we see repeated most: the calculation team enters every pipe as Hazen-Williams C = 120 with medium wall — but because the system is dry or alternate, the contractor buys galvanised heavy in line with the standard's recommendation. The consequences:

• The bore is already 1–2 mm smaller because the pipe is heavy.
• Hot-dip galvanising adds 50–80 µm of internal roughness, reducing effective bore by another 0.1–0.15 mm.
• On ageing galvanised lines, C falls toward 100; even on new lines, taking 110 is prudent. A design assuming C = 120 fails after four to eight years.
• Galvanised pipe is not welded — site welding below 50 mm is prohibited in any case — so joints stay threaded and wall thickness is calculated to ISO 65, making the heavy assumption unavoidable.

The correct workflow: fix the material schedule before the calculation, write the steel series and surface treatment into the specification, and run the hydraulics on exactly that bore and C value.

Site check list

• Identify the design regime: is the line pre-calculated or fully calculated? In pre-calculated design, the tabulated values are exact, not minima.
• Never skip the minimum table: even fully calculated, DN20 for LH and ≥ DN25 for everything in OH and HH except a single-sprinkler connection.
• Sprinkler count per range: at most 9 on a DN50 range in OH pre-calculated; 4 per range on HH with K80, and ranges may not connect to distribution pipes above 150 mm.
• Write the steel series into the specification: medium or heavy? Cross-check wall thickness against ISO 65 or ISO 4200 range D.
• Hazen-Williams C: 120 for new black steel and new galvanised, but also run a C = 100 scenario for the position five to ten years out.
• Sprinkler orientation: an upright sprinkler may not connect directly to pipe larger than 65 mm (50 mm if lagged); pendent heads on pipe above 80 mm need an arm pipe.

Brief comparison with NFPA 13

NFPA 13 has a similar minimum pipe size definition: 1 inch (DN25) minimum in a hydraulically calculated system, with 3/4 inch (DN20) permitted on a short branch to a single sprinkler. The logic is identical to the EN 12845 table; the difference is in detail. NFPA gives separate pipe schedule tables, where EN 12845 does the same job through its pre-calculated tables. Wall thickness is discussed as Schedule 10 or 40 on the NFPA side and as ISO 65 light, medium or heavy on the EN side — and a site engineer needs both languages, because imported fittings often arrive as Schedule 10 while local pipe is ISO 65 medium.

Turkish context

BYKHY accepts NFPA 13, FM Global or EN 12845 for sprinkler installations. Working to EN 12845, TS EN 10255 medium and heavy are the most common material choices; insurance auditors often want to see heavy black steel, particularly in high-hazard storage. On the galvanised side, TS EN 10240 specifies internal and external coating thickness, with hot dip requiring at least 55 µm. Writing a specific line such as "TS EN 10255 medium series, TS EN 10240 class A1 galvanising" into the specification ends the ambiguity over bore and C value.

Frequently asked questions

What is the absolute minimum pipe size for a fully calculated system?

DN20 for LH, DN20 for a horizontal or upright connection to a single K ≤ 80 sprinkler in OH and HH, and DN25 for everything else.

Are "DN50 distribution, DN65 main" written into the standard?

Not for fully calculated systems. The OH pre-calculated tables list 32/40/50/65 by sprinkler count; the site habit of "DN65 main" grows out of that.

How much does the medium-versus-heavy difference affect hydraulics?

At DN50 the bore differs by 1.8 mm, giving roughly 18 % more friction per metre at the same flow — 0.15–0.2 bar over a 60–80 m run.

Does galvanising reduce the bore?

Yes. The internal layer adds 50–80 µm of roughness, costing 0.1–0.15 mm of effective bore on small sizes. With age, C falls toward 100 — run that scenario at design stage.

How many sprinklers can a DN25 range serve?

In OH pre-calculated design, three on a general range and one or two on the last range at the remote end. In HH pre-calculated with K80, a single sprinkler. Fully calculated, there is no count limit — the minimum diameter and the minimum 2.0 bar sprinkler pressure bind instead.

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