Can a UL-Listed K5.6 Fall Under UNI EN K57 Class?

8/14/2025

Hi everyone, I’m looking into the compatibility between UL-listed K-factors and European classification under UNI EN 12259-1.

Specifically, I’m reviewing a Viking upright sprinkler (presumed marked 5.6 gpm/√psi), with the typical UL listing and a 1/2” NPT thread. The orifice appears to be in the 10 mm nominal range.

According to Table 3 of UNI EN 12259-1, a 10 mm nominal orifice corresponds to a K-factor of 57 ±3 l/min/√bar.

However, UL 199 allows sprinklers to be classified as 5.6 even with some variation in orifice geometry, and the unit conversion from 5.6 US gpm/√psi gives approximately 80 l/min/√bar, which seems inconsistent with the European 10 mm / K57 classification.

Have you encountered UL-listed 5.6 sprinklers that, due to actual geometry, fall within the K57 class under UNI EN?

Are there known cases where K-factors "overlap" across standards?

An excerpt from UNI EN 12259-1 Annex C below.

Thanks for your insights — I'm trying to understand how this is approached in practice during technical evaluations or retrofitting in Europe.

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7 Comments

Pete H

8/14/2025 06:09:41 am

https://www.refiregroup.com.au/product/sprinkler-k-factor-conversion-table/

This link has a metric to imperial conversion table for K Factors.

It doesn't seem to equate 57 or a 10 mm outlet. Plus I think they're using CM and not MM.

Franck

8/14/2025 09:45:24 am

mm is the right measurement unit in Europe for thread.
But I know that in the US you have many problems of conversion between imperial and metric system. And the chosen unit is not always the one that is commonly used.
NFPA and FM even make many mistakes in conversion or round up values, ending with some differences in the standard.

As an example, one inch is 25.4 mm, not 25 mm. The difference is not huge for 1 or 2 in., but if you take the obstruction table in NFPA 13, you end up with huge differences when we start speaking about 18 in. or 36 in.

8/14/2025 09:51:30 am

I had a look at https://www.refiregroup.com.au/product/sprinkler-k-factor-conversion-table/
As you mentioned, the value for the metric system in the table are in an unknown value.

The K-factor is in gpm/(sq ft)1/2 in the imperial system.
It is in l/min/(bar)1/2. This means that the value indicated in the table is in some sort of tenth of liter/min/(bar)1/2. This is definetely not correct as it means nothing to the world outside the imperial unit system (i.e.: the entire World except the US)

Glenn Berger

8/14/2025 08:19:59 am

There are sprinkler manufacturers who have published data for the European (and other non-US) Market and the respective listings.

Recommend that you contact the respective company and use products that are approved in those jurisdictions.

8/14/2025 09:40:57 am

The European standard is using what we could call an "abuse of language" and are mixing threads (the 10, 15 or 20 mm of your table) and orifice size, or making a direct relationship when this relationship is not always verified.

A k5.6 is always a K80 in the metric system, and this whatever the thread size (you can have K80 with 10 mm thread size, 15 mm thread size and 20 mm thread size).
The 15 mm thread size for K80 is the common one. 10 mm and 20 mm are generally used for system retrofit.

The same way, you can have a k8.0 (K115 in metric system) with either a 15 mm or 20 mm thread size. Again, the usual ones have a 20 mm thread and 15 mm thread are generally used for system retrofit.
Because of the orifice size, I do not have any example of K115 on a 10 mm thread.

As a summary, the European correspondence that you have in the table is for the most commonly used sprinklers.
Generally, K57 have a 10 mm thread.
K80 have a 15 mm thread
K115 have a 20 mm thread.

If you have a new installation I strongly suggest you to use 15 mm K80 and not 10 mm K80 to avoid possible mistake in the future.
But both are K_à sprinklers for your hydraulic claculations (no worries).

Note: when the orifice size is not with the common thread size, the sprinkler defecltor generally have a pintle as a visual indication.
Very helpful when making evaluation from the ground floor.

For ours US cisitzens:
10 mm thread is 3/8 in. (generally used with K4.0 sprinklers)
15 mm thread is 1/2 in. (generally used with K5.6 sprinklers)
20 mm thread is 17/32 in. (generally used with K8.0 and K11.2 sprinklers)
25 mm thread is 1 in. (generally used with ESFR and CMSA)

8/14/2025 01:18:59 pm

Thank you Franck, cause I was not going to get there from maths alone.

10 = 0.375
15 = 0.5
20 = 0.53

5 mm went from being worth over a 10th of an inch to being worth less than 4 hundredths of an inch.

James Art FPE

8/20/2025 12:35:17 pm

One sad thing was when the NFPA foolishly deleted the pintles required on fire sprinklers. Prior to that, if the orifice was other than the usual for the thread,
bigger, or smaller, then a pintle was required.

So you could tell right away, often from the floor something was different.
Now the only way is to get close, and hope you can read the SIN, or other markings.

When Smaller Orifices were new, one very tall residential project had been calc'ed for small orifices. It made sense for light hazard.
But when I did the inspection, I could not see the pintle, and asked the C-16 whether they were small orifice.
He assured me they were, but when I got closer I saw they were K-5.6. Never trusted that contractor again.

Q. Where can we find specs on sprinklers if the mfr has gone out of business, or been bought?

Can a UL-Listed K5.6 Fall Under UNI EN K57 Class?

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