EN 12845 Pipe Joining: Steel Welding, Flexible Connectors and Copper

Wall thickness, C values and certification rules for welded, grooved, threaded, flexible and copper connections under Clause 17.1.

At the final installation inspection of a logistics warehouse we opened one joint of a DN100 distribution pipe connected with grooved couplings in the OH3 area. The pipe end had not been roll-grooved but cut-grooved on site, and the wall thickness at the groove root was down to 2.8 mm. EN 12845 Clause 17.1.2 is unambiguous here: the grooving method and the required wall thickness are linked. After opening that one joint we had to replace the whole riser. The joining method is not merely an installation detail; it decides whether the hydraulic calculation and compliance with the standard hold together.

What the standard says

EN 12845:2015+A2:2026 Clause 17.1.2 governs above-ground pipework. Downstream of the control valves, pipe shall be steel, copper (see Clause 17.1.9) or other material in accordance with the specifications valid at the place of use. For dry, alternate and pre-action installations galvanised steel should preferably be used. For underground pipework, the note to Clause 17.1.1 recommends cast iron, ductile iron, spun cement, reinforced glass fibre and high-density polyethylene.

The backbone of Clause 17 is the relationship between wall thickness and joining method. Confuse the two and you produce a non-compliant installation.

Steel pipe: joining method and wall thickness

The key paragraph of Clause 17.1.2, summarised:

• Steel pipe with a nominal diameter up to 150 mm that is threaded, cut-grooved or otherwise machined shall have a minimum wall thickness in accordance with ISO 65.
• Where pipe ends are formed without significantly reducing the wall thickness (e.g. roll grooving or weld end preparation), a minimum wall thickness in accordance with ISO 4200 range D is sufficient. This is a thinner series; roll grooving forms the material rather than cutting it.
• Where mechanical joints (grooved couplings etc.) are used, the minimum wall thickness shall also follow the manufacturer's recommendations.

The most common site error is buying thin-series pipe and cut-grooving it. That meets neither the ISO 65 wall thickness nor the coupling manufacturer's recommendations. Cut-groove pipe supplied as "ISO 4200 range D" and the installation is outside the standard.

Welding: who, how and where

Clause 17.1.3 draws firm lines:

• Pipes and fittings less than 50 mm in diameter shall not be welded on site unless the installer uses an automatic welding machine. Manually welding DN25 to DN40 pipe on site with stick electrodes or TIG is therefore outside the standard.
• In no case shall welding, flame cutting, soldering or any other hot work be carried out in situ, i.e. on the installed pipework. Welded fabrication therefore belongs in the workshop or a separate fabrication area on site.
• All joints shall be welded continuously, the inside of the weld shall not interfere with the flow of water, and the piping shall be deburred and the slag removed.
• Welders shall be approved in accordance with EN ISO 9606-1. Each welder's certificate and its validity date should be in the project file.

Field practice: risers and distribution pipework are welded in the workshop and joined on site with flanges or grooved couplings, while sprinkler outlets are almost always threaded (DN15 to DN25, R1/2" to R1"). That combination satisfies the standard and keeps installation quick.

Grooved couplings

Grooved couplings are a common joining method on sprinkler installations. EN 12845 does not define a separate certification class for them; Clause 17.1.2 constrains mechanical joints through wall thickness and the manufacturer's recommendations, and Clause 17.2.2 through support positions. In practice the accepted approvals are UL Listed, FM Approved or LPCB certified couplings and fittings.

Clause 17.2.2 adds two support rules for mechanically jointed pipework:

• At least one support within 1 m of each joint.
• At least one support on each pipe section.

Field error: coupling bolt torque below the manufacturer's figure leaks at the gasket; above it, the lugs can crack. Each manufacturer gives its own torque by size, and it is worth re-checking every coupling bolt with a torque wrench before the pressure test.

Threaded joints

Clause 17.1.2 ties threaded steel pipe up to 150 mm nominal diameter to ISO 65 wall thickness. In practice threaded joints are used at DN50 and below; because cutting a thread removes wall thickness, thin-series pipe cannot be threaded. DN65 and above normally uses grooved or flanged joints.

Sealing is by PTFE tape or fibrous sealant. Clause 17.1.7 requires galvanised piping to be painted wherever the coating has been damaged, e.g. by threading; otherwise corrosion starts at the thread roots and turns into leaks over the years.

Flexible pipes and joints

Under Clause 17.1.4, if relative movement is likely between different sections of the pipework (e.g. at expansion joints or with certain types of racking), a flexible section or joint shall be fitted at the point of connection to the distribution main, and it shall meet the following requirements:

• Before installation it shall withstand a test pressure of four times the maximum working pressure or 40 bar, whichever is greater.
• It shall not include parts which, when subject to fire, might impair the integrity or performance of the sprinkler system.
• Flexible pipes shall contain a continuous pressure-retaining stainless steel or non-ferrous metal inner tube.
• They shall not be fitted in the fully extended position.
• They shall not be used to take up misalignment between a distribution main and the feed pipes to intermediate sprinklers.

Clause 17.1.4 does not separately address flexible drops feeding sprinklers in suspended ceilings. The error we see most often is stretching a flexible drop to seat a sprinkler in a ceiling tile; that conflicts with the "not fully extended" principle of Clause 17.1.4 and with the product installation instructions. A flexible pipe should be installed with some slack and keep its bend.

Copper pipe: limited but useful

Clause 17.1.9 sets specific rules for copper:

• Wet pipe systems only.
• Only LH, OH1, OH2 and OH3; therefore not in OH4, HHP or HHS.
• Only downstream of any steel piping.
• Pipe to EN 1057 (Clause 17.1.2).
• Joints by mechanical joints or hard soldering, with fittings to EN 1254; hard soldering to EN ISO 3677 and only by properly trained personnel.
• Copper-to-steel joints flanged, with stainless steel bolts.
• No bending on site; precautions against galvanic corrosion.
• Clause 17.2.1: supports for copper pipe need a lining with sufficient electrical resistance to prevent contact corrosion.
• Clause 17.2.2: supports no more than 4 m apart on steel and 2 m on copper, increasable by 50 % for pipes over 50 mm when the conditions are met.

The C value difference

Clause 13.2.1 gives friction loss by the Hazen-Williams formula; Table 22 in summary:

Pipe type	C value
Cast iron	100
Ductile iron	110
Mild steel / galvanised steel	120
Spun cement / cement lined cast iron	130
Stainless steel / copper / reinforced glass fibre	140

Because friction loss varies inversely with C^1.85, moving from C = 120 to C = 140 multiplies friction loss by (120/140)^1.85 ≈ 0.75 at the same diameter and flow, i.e. a reduction of about 25 %. That is copper's hydraulic advantage; the drawbacks are cost and a limited scope of use. Calculating for steel and then substituting copper in part of the system is wrong; the calculation report should state the C value used for every pipe section.

CPVC and other plastic pipe

EN 12845 does not name plastic pipe; the wording "other material in accordance with the specifications valid at the place of use" in Clause 17.1.2 leaves the door open only indirectly. European practice:

• CPVC is generally used in LH installations, in runs concealed above suspended ceilings.
• Its temperature resistance is limited, so it is not used in hot areas such as kitchens or boiler rooms.
• The product must carry UL, FM or LPCB certification and be calculated to the manufacturer's design guide. EN 12845 Table 22 gives no C value for plastic; NFPA 13-2025 Table 28.2.4.8.1 uses C = 150 for listed plastic pipe.

Using CPVC in OH and above, or in storage areas, does not fit the approach of EN 12845 and is usually not accepted at inspection.

Three typical field cases

1. Grooved coupling torque error: hand-tightened bolts leak during the pressure test. The fix is a second pass with a torque wrench to the manufacturer's figure.
2. Manual site welding of pipe under 50 mm: a breach of Clause 17.1.3. The only remedy is to cut out the welded part and change to threaded joints.
3. Direct threaded copper-to-steel joints: galvanic corrosion soon produces leaks. Clause 17.1.9 requires a flanged joint with stainless steel bolts.

Comparison with NFPA 13

NFPA 13-2025 lists pipe materials and dimensions in Table 7.3.1.1. The differences that matter:

• NFPA 13 (7.5.2.5) requires welding procedures and welders to be qualified to AWS B2.1 or ASME Section IX, or another qualification standard required by the authority having jurisdiction; EN 12845 requires welders approved to EN ISO 9606-1. On an international project, settle at the outset which applies.
• NFPA 13 explicitly recognises listed CPVC pipe under defined conditions; EN 12845 does not name plastic and defers to the specifications valid at the place of use.
• NFPA 13 Table 28.2.4.8.1 distinguishes C by system type: black and galvanised steel 120 in wet systems and 100 in dry and pre-action systems (120 where nitrogen, vacuum or a vapour corrosion inhibitor is used); copper and stainless steel 150; listed plastic 150. EN 12845 Table 22 distinguishes by material only.
• Grooved couplings are accepted under NFPA 13 and in EN 12845 practice alike; both expect a listed or approved product.

Turkish context

Article 96(5) of the Turkish Regulation on Fire Protection of Buildings (BYKHY) requires sprinkler system design to TS EN 12845. Common site practice in Turkey is:

• Roll-grooved galvanised steel with UL/FM couplings on risers and distribution pipework of DN65 and above,
• Threaded galvanised steel on distribution and sprinkler connections of DN50 and below,
• Listed flexible pipes at expansion joints and in demountable ceilings.

Copper is rare on site because of cost and the prohibition on site bending. At acceptance inspection the documents usually requested for the jointing are the welders' EN ISO 9606-1 certificates, the UL/FM documents for the couplings and the listing documents for the flexible pipes; having them ready in the file makes acceptance easier.

Summary

EN 12845 ties three decisions together in pipe jointing: material, wall thickness and joining method. Tie your wall thickness choice to the grooving method, and the grooving method to the coupling manufacturer's recommendation, and calculation and site will agree. Copper is hydraulically advantageous but confined to wet systems in LH and OH1 to OH3. The flexible joint of Clause 17.1.4 exists for expected relative movement, not to take up misalignment. Welding belongs in the workshop as far as possible: pipe under 50 mm may be welded on site only with an automatic welding machine, no hot work at all is allowed on installed pipework, and every weld needs a welder approved to EN ISO 9606-1.

Frequently Asked Questions

Can pipe smaller than 50 mm be welded on site?

Only if the installer uses an automatic welding machine (Clause 17.1.3). In no case may welding, flame cutting, soldering or any other hot work be carried out in situ, i.e. on the installed pipework.

What wall thickness applies to grooved pipe?

ISO 65 for steel pipe up to 150 mm nominal diameter that is threaded, cut-grooved or otherwise machined; at least ISO 4200 range D where the pipe end is formed without significantly reducing the wall thickness, e.g. roll grooving or weld end preparation (Clause 17.1.2). With mechanical joints the manufacturer's recommendations also apply.

Where can copper pipe be used?

Wet systems only, in LH, OH1, OH2 and OH3, always downstream of steel, with flanged transitions and stainless bolts. Prohibited in OH4, HHP and HHS.

What can a flexible connector not be used for?

Taking up misalignment between a distribution main and the feed pipes to intermediate sprinklers. Clause 17.1.4 provides flexible sections for expected relative movement at the connection to the distribution main; they must not be fitted fully extended and need a continuous pressure-retaining stainless steel or non-ferrous metal inner tube.

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