Fireproofing in cable calculations

The problem isn't particularly widespread - most internal walls aren't required to act as fire barriers - e.g. in most homes the entire house would be one fire compartment - even in larger homes with three or more floors it's often only the floors/ceilings or possibly walls surrounding stairwells that need specific fire resisting properties. Even then when using small cables or steel conduit, the holes are usually filled with building material similar to the wall itself - e.g. mortar for masonry walls or wet plaster for plasterboard partitions - which easily maintain the fire resistance of the original wall. Very large conduits or trunking may need internal firestopping where they pass through a fire compartment wall - but that's quite rare for modest sized projects.


I think you've already understood some of Dave's points about heat being dissipated along the length of the cable run with the reference to 523.9 - but note that the factors in table 52.2 are based on the cable ratings for method C - so you wouldn't simultaneously apply the de-rating of other installation methods that already take thermal insulation into account in some other way.


Even where, say you were running cables in plastic conduit or trunking through a fire partition wall and needed something intumescent, rememeber there are many different firestopping and intumescent materials - and most are not as thermally insulating as mineral wool insulation - and just as importantly don't form a layer around that cable that's anything like as thick as the 'infinite plane' that's assumed by 523.9 - so in practice the cable will be able to loose some heat through the firestopping material into the centre of the wall.


You do right you keep your wits about you on this problem though - modern methods of constructions often put thermally insulating materials in placed were they traditionally wouldn't have been even thought of - e.g. in internal stud partition walls around bathrooms (to reduce the appearance of condensation) or as acoustic insulation, especially between floors, which as a dense form of mineral fibre also happens to have good thermal insulating properties. But by and large it's easier and better to avoid running many cables through such areas - design out the problem rather than designing a solution.


    - Andy.

The problem isn't particularly widespread - most internal walls aren't required to act as fire barriers - e.g. in most homes the entire house would be one fire compartment - even in larger homes with three or more floors it's often only the floors/ceilings or possibly walls surrounding stairwells that need specific fire resisting properties. Even then when using small cables or steel conduit, the holes are usually filled with building material similar to the wall itself - e.g. mortar for masonry walls or wet plaster for plasterboard partitions - which easily maintain the fire resistance of the original wall. Very large conduits or trunking may need internal firestopping where they pass through a fire compartment wall - but that's quite rare for modest sized projects.


I think you've already understood some of Dave's points about heat being dissipated along the length of the cable run with the reference to 523.9 - but note that the factors in table 52.2 are based on the cable ratings for method C - so you wouldn't simultaneously apply the de-rating of other installation methods that already take thermal insulation into account in some other way.


Even where, say you were running cables in plastic conduit or trunking through a fire partition wall and needed something intumescent, rememeber there are many different firestopping and intumescent materials - and most are not as thermally insulating as mineral wool insulation - and just as importantly don't form a layer around that cable that's anything like as thick as the 'infinite plane' that's assumed by 523.9 - so in practice the cable will be able to loose some heat through the firestopping material into the centre of the wall.


You do right you keep your wits about you on this problem though - modern methods of constructions often put thermally insulating materials in placed were they traditionally wouldn't have been even thought of - e.g. in internal stud partition walls around bathrooms (to reduce the appearance of condensation) or as acoustic insulation, especially between floors, which as a dense form of mineral fibre also happens to have good thermal insulating properties. But by and large it's easier and better to avoid running many cables through such areas - design out the problem rather than designing a solution.


    - Andy.