Fireproofing in cable calculations

What sort of fire proofing are you expecting around a steel conduit through an internal concrete wall? I would have imagined that and excess gap would normally be filled with sand/cement mortar - so very similar thermal properties to the wall itself and certainly not thermally insulating.


Intumescent firestopping is as far as I know only needed for plastic ducts/conduits that may melt during a fire leaving a hole in the wall - not a problem you're likely to have with steel conduit.


   - Andy,

Hi Andy,


 Thank you very much for your swift reply, and for the helpful information.


Yes, I had been incorrectly imagining intumescent fireproofing. Thank you for taking the time to put me right.


Not much experience here as you might imagine!


With much appreciation and thanks again,


Suki

You do need to understand that the derating factors are not the same for short lengths of cable passing through insulation. The temperature of a cable has thermal flow along the length as well as outwards, so the thermal loss is not the same as the whole cable under or in insulation. Steel conduit or trunking is also quite thermally conductive and will seek to equalise the temperature along the length. Either passing through a wall would normally be ignored, the temperature difference within 100-200 mm is tiny. As Andy says, concrete would normally be filled with sand/cement anyway, as it is cheap and convenient, and is quite fireproof.


It sounds as if you are worried about your design a little too much. Come back with more questions if you like.


Kind regards

David CEng etc

Yes, it is a bit more physics-ish than perhaps one is used to. If you take a bit of copper wire and heat one end in a flame, the thermal conductivity quickly heats quite a length to an uncomfortable holding temperature, moving the heat along the wire by thermal conduction. The same applies to conduit or trunking, we do not get an immediate "hot spot", just a spread out temperature which may be slightly higher than the rest, but not excessive. The wall also has a property called thermal capacity, a bit like the bricks in a storage heater which are made of a material with a very high value. This means that it takes a long time for this heating of the wall to happen, and of course more spread out, losing the heat to the atmosphere by convection and to a slight degree radiation.


If the wall is very thick and the expected temperature rise high, one might use somewhat thicker cables, which both reduce the resistive loss and provide more conduction along the length of the cable, to the outside of the wall. All this is the theory, in reality, one does not find damaged cables in short lengths through walls which is why the condition is not discussed very much in textbooks or lectures. Cables buried in insulation are a rather different thing, because the length is too long for much conduction outside the insulation, and in general cables surrounded by serious insulation are a bad idea (because they end up very large and therefore expensive) and it is wise to make a route without long lengths buried. This does not apply of course to lightly loaded cables, or short lengths dropping through for say a light drop in the roof space.


David

Single core cables in conduit in masonry is likely method number 59 Reference method B. For single phase column 4 of table 4D1A. Strictly speaking the surrounding material should have a thermal resistivity of not more than 2K.m/W but I would agree with the others that you could discount any minuscule  effect that fire stopping might have. Some fire batts do look very like thermal insulation and these can be used where a number of services are passing through a wall that is required to have fire resistance. I assume that your conduit is 32mm or less, otherwise may need internal intumescent  protection. Why not simply mention your thinking in a pre-amble to your design determinations and set out why you feel it is justified. Often such things make the difference between a merit and a distinction!

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.