Is NFPA 20 10-Diameter Req'mt One Straight Run?

2/23/2026

NFPA 20 – Does the 10 pipe diameter suction requirement have to be one continuous straight run?

I’m looking for clarification on the NFPA 20 requirement for 10 pipe diameters of straight pipe on the suction side of a horizontal split‑case fire pump.

NFSA’s Layout Book states: “The length of pipe that needs to be 10 pipe diameters long can be either vertical or horizontal, or some combination of the two, as long as there are no more elbows or tees with a horizontal change in direction in the length of pipe.”

This makes sense—but when I look at NFPA 20’s own figures (specifically the “Right and Wrong Pump Suctions” illustration), one of the “Right” examples shows a 90° elbow dropping directly into a tee located right at the pump suction flange. In that configuration, there is no 10‑diameter straight run at all. My questions: Where in NFPA 20 is it actually stated that the 10 pipe diameters must be a single, continuous straight run of pipe?

NFSA says the 10 diameters can be vertical, horizontal, or a combination, but NFPA 20’s figure seems to show exceptions.

Is NFPA 20’s 10‑diameter rule only triggered by elbows/tees that introduce a horizontal component to the change in direction? (For example, a vertical elbow turning up does not trigger the 10D requirement.)

Is this 10‑diameter rule only applicable to horizontal split‑case pumps?

Or does it apply to all pump types?

I’m trying to reconcile the NFSA explanation with NFPA 20’s figures and determine whether certain configurations truly require the 10D straight run. Thanks in advance for any clarification.

Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe

4 Comments

Dan Wilder

2/23/2026 07:56:39 am

My way of visualizing this is if there is any water flow not perpendicular to the shaft of the engine/motor & impellor in a horizontal plane, that face of fitting for the change in direction is the 10 pipe diameters start point.

In your example, the straight pipe, into an elbow, into the bullhead of a tee, and into the pump...all of that is perpendicular to the shaft if you view it from the top or suction side elevation (this would show towards and away but no left or right direction), regardless of elevation. Or if you were to superimpose the shaft to any point along those fittings, everything is perpendicular.

4.16.6.3.1 and additional sections are specific to "horizontal split-case pumps" and does not apply to other types of pumps.

Maurice

2/23/2026 10:34:36 am

I like how you said "view it from the top" that really helps putting it into perspective. Thank you.

Brett

2/23/2026 08:01:01 am

"Where in NFPA 20 is it actually stated that the 10 pipe diameters must be a single, continuous straight run of pipe?"

It doesn't say that, but that's what it ends up being most of the time. Technically, you just can't have elbows and tees that are parallel to the pump shaft in those 10 pipe diameters. You can use elbows and tees that are perpendicular to the pump shaft.

"Is NFPA 20’s 10‑diameter rule only triggered by elbows/tees that introduce a horizontal component to the change in direction?"

I'm assuming we're talking about section 4.16.6.3 of NFPA 20, 2019. The 10 pipe diameter rule is only triggered by elbows and tees that have a centerline plane that is parallel to a horizontal split-case pump shaft. The "right" configurations shown in those NFPA 20 figures either show the 10 pipe diameters being met or they show elbows and tees that have a centerline plane that is perpendicular to the pump shaft. The elbow and the tee in the figure you mentioned are both perpendicular to the pump's shaft, so they're permitted.

NFSA's explanation simplifies it.

"Is this 10‑diameter rule only applicable to horizontal split‑case pumps?"

Yes, section 4.16.6.3 and its subsections specify horizontal split-case pumps.

Jack G

2/23/2026 11:32:02 am

There are sizes in a pump room that are governed by nfpa 20 and nfoa 13. The suction piping is sized for low flow to stop "bubbles" ( think of them as little explosions ) to prevent cavitation and increased wear and axial wear on the fire pump.
The discharge is based on the design flow and increased pressure for the building requirements. The piping past the pump design, bypass, fdc. ,test header is governed by 13.
Is the 10-diameter straight suction pipe requirement ( nfpa 20) applicable to all fire pumps?
Placing an elbow with the centerline in the horizontal plane within 10 pipe diameters of the suction flange creates an unbalanced flow of water to the impeller . An unbalanced flow causes an axial load to be placed on the pump shaft and bearings ( fire pump handbook) causing excessive wear to the bearings and subsequent damage to the pump over time and/ or severe cavitation if not corrected.
The requirement to place an elbow a minimum of 10 diameters away only applies to horizontal pumps.
In online vertical shaft type pumps, the imbalance already creates an axial load imbalance so the imbalance from an elbow is of less concern .
The 10d rule applies specifically when elbows and /or tees are in a " horizontal plane " parallel to the pump shaft, ( creating a horizontal bend.
An alternative if the straight run is impossible, a suction diffuser or a specifically engineered eccentric reducer may be required , though the 10d is the standard practice.

Is NFPA 20 10-Diameter Req'mt One Straight Run?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES