Smoke detectors

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wwhitney said:
I don't have access to that document, but a 41' x 10' rectangle doesn't quite fit inside a 42' diameter circle. The diagonal of the rectangle is sqrt(41^2 + 10^2) = 42.2'. A 40' x 10' rectangle fits, however, as does a 41' x 9' rectangle.

Cheers, Wayne
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It is an artifact of the 0.7 multiplier for non-prescriptive coverage. It should actually be 0.707. Or really actually sqrt(2)/2 because the multiplier is based on the hypotenuse of the right triangle formed by the detector and the line perpendicular to one side and then extending to the corner. So 15^2 + 15^2 = 2*225 = 550. If we take the square root of that we get 21.213(etc) feet. Then 30/21.213(etc) is 0.707(etc). The wriggle room is that the prescriptive space is allowed to wander per A17.7.3.2.3.1 so it's +/- 5% or +/- 18 inches.
 
Dennis Alwon said:
Here is the instructions for BRK. I don't see where all this math stuff is necessary
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Ahh, in this case the manufacturer has added this restriction in the installation instructions. That changes things a little. Notice however that if the hallway is exactly 40 feet or a little less, according to BRK you could use only one detector.
 
gadfly56 said:
It is an artifact of the 0.7 multiplier for non-prescriptive coverage. It should actually be 0.707.
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Sure, I agree. But once the code writers choose 0.7, you have to use 0.7, not 1/sqrt(2).

gadfly56 said:
The wriggle room is that the prescriptive space is allowed to wander per A17.7.3.2.3.1 so it's +/- 5% or +/- 18 inches.
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Oh, is this some other part of A17.7.3.2.3.1 that you didn't excerpt? Obviously that changes things.

BTW, part (1) is really badly written, from a mathematician's perspective. "Spacing between smoke detectors" isn't defined, although we sort of know what is meant. Perhaps that section should be prefaced with "for a rectangular room with the smoke detectors laid out in a complete rectangular grid".

Cheers, Wayne
 
wwhitney said:
I don't know the scope of NFPA 72, but the excerpt is referring to smoke detectors. I thought smoke alarms were a horse of another color?

Cheers, Wayne
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I believe the op was referring to smoke alarms even though he said smoke detectors. Most people don't know the difference and call them by both names. My reference was smoke alarms because I thought that was what the op meant. Usually you see them in residences
 
wwhitney said:
I don't know the scope of NFPA 72, but the excerpt is referring to smoke detectors. I thought smoke alarms were a horse of another color?

Cheers, Wayne
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They are, but not too different. IFF you have interior floor space on a level over 1,000 ft2:

29.5.1.3.1* All points on the ceiling shall have a smoke alarm
within a distance of 30 ft (9.1 m) travel distance or shall have
an equivalent of one smoke alarm per 500 ft2 (46 m2) of floor
area. One smoke alarm per 500 ft2 (46 m2) is evaluated by
dividing the total interior square footage of floor area per level
by 500 ft2 (46 m2).

Notice that the 30 ft travel distance could be seen as more liberal that the 21 ft for smoke detectors, but the 500 ft2 limitation is more restrictive (900 ft2)

However, the OP wrote "smoke detectors" so that's what I went by.
 
gadfly56 said:
Notice that the 30 ft travel distance could be seen as more liberal that the 21 ft for smoke detectors, but the 500 ft2 limitation is more restrictive (900 ft2)
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But the quoted text says "or", so you get to pick which requirement you want to meet. Other than super long hallways (e.g. 7' x 70'), I would think the 30 ft travel distance requirement will be less restrictive.

Oh, I see now that you were comparing the two requirements of 29.5.1.3.1 with the two requirements of 17.7.3.2.3.1, not with each other.

Cheers, Wayne
 
wwhitney said:
But the quoted text says "or", so you get to pick which requirement you want to meet. Other than super long hallways (e.g. 7' x 70'), I would think the 30 ft travel distance requirement will be less restrictive.

Cheers, Wayne
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That is very true. I'm thinking that the CMP was typing faster than it was thinking also. :D
 
Dennis Alwon said:
I believe the op was referring to smoke alarms even though he said smoke detectors. Most people don't know the difference and call them by both names. My reference was smoke alarms because I thought that was what the op meant. Usually you see them in residences
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Us, old school guys, do call them smoke detectors.:slaphead: If I started calling them different, my builder, for one would just look at me. :jawdrop:
 
Strathead said:
If you are referring to this little discussion, then you haven't seen it when we really have a discussion!:p
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Yeah, it's not like we're discussing whether the receptacle should be installed ground up or ground down... :angel:
 
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