rubber and foam fire stops.

[Indy]

Hi,
I have been hearing different things with regards to rubber and foam fire stops( rectangle with circular holes) when a cable is passing through a wall. Some people are saying these count as thermal insulation and some people are saying they don’t. So when cable sizing to bs7671 would a derating factor for thermal insulation be used?

Thanks

 

[ScottyUK]

Rubber is a pretty good thermal insulator, foam is a great thermal insulator so I would say yes. Does this link help?

https://www.voltimum.co.uk/articles/thermal-insulation-and-flat-twin-and

 

[Indy]

Thanks for this. We have a 22kw vsd approx 43A the cable will pass through a fire barrier of approximately 150mm. The run is approximately 30m The rest of the run is on perforated tray. This would increase the cable size to around 35mm. This seems excessive for the load. Provisional fuse rating is 50A.

Thanks

 

[ScottyUK]

That cable size doesn't feel right, although my regs book is at work and I can't check. 35mm² on rack should be good for about 150A or so. What de-rating factor have you applied, and how did you arrive at it?

 

[waross]

That's a good point.
But;
In many years I have never seen an AHJ comment on ratings through a fire stop.
I suggest that with a thicknes of 150 mm and 75 mm from the center to the edge of the fire stop, the thermal conductivity of either copper or aluminum would prevent too much temperature rise in the center of the fire stop.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Indy]

Thanks for the replies, rechecked the cable size. I was originally applying all derating factors for the cable run which increased the cable size considerably.

Thanks again

 

[jraef]

When people say that those fire stop devices (officially called "cable transit seals") are providing "thermal insulation", they are probably referring to the fact that because they actually seal tightly around the cables, they prevent air from passing from one area to another, as it relates to HVAC issues. Not only air, but gasses, vapors etc. as well, so most can be used when cables pass from classified to Non-Classified areas too. I've never heard of having to de-rate cables as they pass through them because of an increased thermal insulation issue around the wires, the distances are too short to be of concern in my opinion.


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