What Obstruction Rules Apply to Cable Tray?

10/4/2021

Looking at installing a cable tray that runs the length of the room in an Ordinary Hazard Occupancy.

The cable tray is about 2-feet wide and the sprinklers are standard uprights. The cable tray is less than 18-inches below the sprinkler.

However, the cable tray may be centered directly below some sprinklers, but off to the side for other sprinklers.

What obstruction criteria from NFPA 13 (2016 Edition) would apply?

Three Times Rule (Section 8.6.5.2.1.3)?

If Three Time Rule applies, is that measured vertically, horizontally, diagonally, or all?

Thanks in advance.

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5 Comments

Alex

10/4/2021 06:49:33 am

Hi Anonymous,

Since the cable tray is less than 2 FT in length BUT less than 18 inches below the sprinklers, you will need to tailor your sprinkler design around the cable trays. This will be done using NFPA 13 (2013) 8.6.5.1.2(a). Since the cable tray is less than 18 inches, it is assumed that the discharge pattern will not fully develop, so you can treat the cable tray as it goes all the way to the deck (similar to a beam).

For Example: If we assume that the top of the deflector, to the bottom of the cable tray is 16 inches, Table 8.6.5.1.2 states that the sprinklers need to be at a minimum of 4.5 ft to 5 ft from the sides of the cable tray.

You would not use the Three Times Rule. NFPA 13 (2013) 8.6.5.2.1 states that “structural members only shall be considered when apply the requirements of 8.6.5.2.1.3” (code section that calls out the three times rule). There are additional explanations of the rule in the appendix.

To answer the sub question, even though it does not apply to your situation, you measure the width and height of the obstruction, not the overall length for the Three Times Rule. Refer to Figure 8.6.5.2.1.3(b) for a detail showing the rule.

Best of luck with your design!
Alex

Franck

10/4/2021 08:31:40 am

Alex cover it all.

If you need to apply the 3 times rule, it is the shortest distance where it applies, i;e. vertically when the obstruiction is directly below, and diagonally if off the side. Normally you should not have a horizontal distance as in that case the obstruction is not an obstruction anymore to your pattern discharge.

Not that this would be the same with groupes pipes, or even one large pipe (which is covered by the 3 times rule for sprinkler branch lines larger than 3").

10/4/2021 08:47:06 am

When using the Three Times Rule for an Ordinary Hazard Occupancy you only have to consider structural members when using it as per 8.6.5.2.1.4 NFPA 13 2016. Since the cable tray is not a structural element you do not use the three times rule here. When doing further reading in the annex you may exempt the cable tray as an obstruction assuming the sprinkler is not too close to the obstruction. The annex states the beam rule applies when water cannot spray on both sides of an obstruction due to how close the sprinkler is.

Michael Lantaigne

10/5/2021 09:12:10 am

I agree with JI - the 3x rule only has to be applied to structural elements in light and ordinary hazard occupancies and the beam rule only applies when the top of the obstruction is level with or above the plane of the deflector.

If the sprinkler deflectors are higher than the top of the cable tray, then it is not an obstruction in light and ordinary hazard occupancies.

Jesse

10/4/2021 11:02:25 am

Agree with Alex.

While its not a structural member, the proximity (<18") to the sprinkler deflector is our concern. 2016 NFPA 13 Table 8.6.5.1.2 governs.

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What Obstruction Rules Apply to Cable Tray?

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