Equivalent NFPA 13 "Grouped Obstructions" Calc?

7/7/2026

Hello, Following FM DS 2-0 §2.5.4.5.3.3 - if two or more objects are considered a “grouped object”, their overall width is the collective sum of each of the objects that make-up the grouped object; the open spaces between them do not need to be included in the calculation.

Is there an equivalent in NFPA or do I need to take into account the open spaces if my project is in NFPA 13?

Thanks in advance.

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4 Comments

Dan Wilder

7/7/2026 08:05:27 am

13 (25') has things like "Non-Solid Obstruction" which is defined as "An obstruction that consists of openings that constitute at least 30 percent of the footprint of the obstruction."

ESFR Specific - 14.2.10.3.2 * - For pipes, conduits, or groups of pipes and conduit to be considered individual, they shall be separated from the closest adjacent pipe, conduit, cable tray, or similar obstructions by a minimum of three times the width of the adjacent pipe, conduit, cable tray, or similar obstruction.

In this case, you would still use the overall width for both the classification of but also spacing to/from said obstruction. Chapter 9 has general criteria for when the obstruction is less than and greater than 18" below the deflector in addition to the specifics in Chapter 14.

Jocelyn

7/7/2026 08:26:00 am

The NFPA 13 Handbook covers this (or....doesn't) in the Insight for Section 9.5.5.3.2)

Q: When determining if the obstruction is more than 4 ft (1.2 m) wide for 9.5.5.3.2 and for similar requirements throughout Chapter 9 through Chapter 15, what separation is necessary between obstructions to deem the adjacent obstructions to be their own separate obstructions rather than one overall obstruction?

A: There is no guidance for this concern in the standard; therefore, design judgment will be necessary.

Glenn Berger

7/7/2026 01:20:36 pm

I have been surprised that this subject has not been discussed within NFPA 13 commentaries.

Judgement call at a minimum, with the acceptance that grouping of items does constitute an obnstruction.

Jack G

7/7/2026 10:37:28 pm

I ve dealt with fm on chemical plants and 13 does a poor job so it’s a design judgment call.
I buse common sense. If a cable tray protect the full roof and protect under the cable tray.
If individual pipes 3 times rule.
Groups 70 % open protect at roof.,
Hasn’t failed yet.

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Equivalent NFPA 13 "Grouped Obstructions" Calc?

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