Anyone know the reason for Fire Wire being 18 gauge?

[benze]

Hi,

I've been looking into specs and regulations for smoke detectors and have noticed that although it is not a regulation in my area, the general consensus/recommendations is to run 18 gauge fire-rated cable for smoke detectors. However, I have yet to find anyone who could give me a logical explanation why different jurisdictions have adopted 18g as a standard.

I understand the need for fire-rated cabling; the last thing you want is a fire disabling the cable prior to the detector tripping. However, what I don't get is why the thicker wire. From a purely technical/electrical perspective, 22g would work perfectly fine. For that matter, even 24g would work fine, with the only exception being that most detectors are designed with terminals that accept 18-22g wiring. The only thing I can think of is that someone figured that 18g would take longer to burn/melt in the case of a fire. However, wouldn't that have more to do with the fire-proofing rather than the wire itself? Given the same specs for fire/heat resistance on the wire, wouldn't 22g be sufficient?

Does anyone have any logical explanations why fire wire is generally accepted to be thicker?

Just curious.....

Thanks,

Eric

 

[Sandpiper]

Hi,

I've been looking into specs and regulations for smoke detectors and have noticed that although it is not a regulation in my area, the general consensus/recommendations is to run 18 gauge fire-rated cable for smoke detectors. However, I have yet to find anyone who could give me a logical explanation why different jurisdictions have adopted 18g as a standard.

I understand the need for fire-rated cabling; the last thing you want is a fire disabling the cable prior to the detector tripping. However, what I don't get is why the thicker wire. From a purely technical/electrical perspective, 22g would work perfectly fine. For that matter, even 24g would work fine, with the only exception being that most detectors are designed with terminals that accept 18-22g wiring. The only thing I can think of is that someone figured that 18g would take longer to burn/melt in the case of a fire. However, wouldn't that have more to do with the fire-proofing rather than the wire itself? Given the same specs for fire/heat resistance on the wire, wouldn't 22g be sufficient?

Does anyone have any logical explanations why fire wire is generally accepted to be thicker?

Just curious.....

Thanks,

Eric

I have wondered the same thing for a long time. The NEC does not require 18 AWG, but many local jurisdictions do. I do agree with your guess that 18 AWG will last longer than 22 AWG in a fire. I did some field tests a long time ago in real fires which shows without a doubt that larger guage wire lasts longer. The larger copper is a better conductor of heat as well electricity. When you get a hot spot in a wire, the larger conductor can conduct heat away from the spot faster and delay the melting of the wire. Other than this, I have no guess why so many localites require the 18AWG.

 

[Digger]

Less Line resistance may be one reason. If I remember correctly 22 awg has 16 ohms per thousand feet where 18 awg has about half or quarter of that (I dont have my reference materials at home). This is more critical on 2 wire smoke zones and also when using detectors with internal sounders where a voltage drop could affect the sound level. It is also very critical in SLC (Signal Line Circuits) loops used mainly in Commercial applications however a few residential panels have SLC loops and more are coming out all of the time.

 

[MrDan]

Less Line resistance may be one reason. If I remember correctly 22 awg has 16 ohms per thousand feet where 18 awg has about half or quarter of that (I dont have my reference materials at home). This is more critical on 2 wire smoke zones and also when using detectors with internal sounders where a voltage drop could affect the sound level. It is also very critical in SLC (Signal Line Circuits) loops used mainly in Commercial applications however a few residential panels have SLC loops and more are coming out all of the time.

That and probably because it looks more substantial to the AHJ. The voltage drop issue is probably why... I have also seen a few where 16awg was called for...