Spacing for ceiling mtd FA strobes

[greenspark1]

Hi. I'm accustomed to breaking large rooms into smaller squares and using wall mounted fire alarm strobes to get my coverage(NFPA 72-18.5.5.4.5).
I have a large room (200x200) without many walls so want to use ceiling mounted devices. However there's a part of NFPA 72 that talks about only using the ceiling-mounted device table if the device is in the middle of the room (attached). This seems to prohibit using ceiling strobes in large spaces. Can someone point out the flaw in this thinking? Can I use the standard technique of breaking the space into smaller squares?

 

[jeremy.zinkofsky]

You use the table only if you choose to place the ceiling mounted strobe in the center of the room. If you choose not, then you are not allowed to use the table. You will then use the double-distance method detailed in that code excerpt.

 

[ron]

The spacing from that section is counting on reflection off of the walls.

There is good narration in Annex A Section A.18.5.4 and the few sections following covering your issue.

 

[greenspark1]

Good points, thank you. The annex was helpful and confirms that overhead notification devices are preferred in large open spaces such as warehouses and produce better alerting results. However, the prescriptive tables don't apply to large warehouse spaces. And at say 100' from the nearest wall my device needs to be sized for a 200' room which isn't in the table.

I don't see any prescriptive spacing or layout for large open spaces. So I'm left scratching my head on how to get good visible coverage in this space...

 

[greenspark1]

Hang on, 18.5.5.4.5 allows you to subdivide a room into smaller square rooms. So if I want to cover a 100x100 room I can subdivide it into 25x25 rooms and use 4x95 cd devices (20' ceiling).