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Cables in heating voids

Martyn
Hi Folks,


'intresting' day today...


So at the moment I am replacing a 150kW gas warm air unit, a huge thing in a commercial building.


Today I finished ripping out the old unit (146kW) the system draws in fresh air from outside and distributes the warm air via a network of underground ducts, really they are just concrete tunnels large enough to crawl through.


So being inquisitive and not being able to resist this unknown hole to somewhere in the floor of the plant room I stuck my ladders down there and went for a wander (pig in sh*it comes to mind ?) as soon as I get in there are 4 25mm galve conduits (building lighting 10x 10A) and a 25 2c SWA (feeding a kitchen sub 100A) with a 10mm earth attached snaking through there, I wander further 25m ish still the armoured and conduits but also a few fire alarm cables have entered the scene.


Swa -90's

Conduits - either 73 or 90's


Clearly a very convienient route for cables and none of them are showing any outward signs of distress (cant see in the galv conduits ofcourse)


Thoughts? Just canvasing opinion...
  • 0
    It's common practice is some parts of the world to run cables in ventilation ducts - the Americans have specifically "plenum" rated cables for the purpose.  Obviously ambient temperature needs to be taken into account - but if it's not cooking people near the outlets it's presumably more "warm" than "hot". So perhaps a bit of a de-rating factor, but I wouldn't be having kittens about it.

       - Andy.
  • 0
    As Andy says, the ducts are distributing warm air rather than hot. The cables should be rated for an ambient of at least 30 degrees. The life may be reduced by having a higher average ambient through the year but it would still be years before I would expect to see a difference and if this is taken into account at the design stage it is certainly not a problem. The fact that they are better positioned for inspection or replacement may counter any disadvantage. The added advantage is that a bit of heat dissipation from the cables will marginally (but probably not noticeably) reduce your heating bill.

    Alasdair
  • 0

    AJJewsbury:

    It's common practice is some parts of the world to run cables in ventilation ducts - the Americans have specifically "plenum" rated cables for the purpose.  Obviously ambient temperature needs to be taken into account - but if it's not cooking people near the outlets it's presumably more "warm" than "hot". So perhaps a bit of a de-rating factor, but I wouldn't be having kittens about it.

       - Andy.




    Hi Andy,


    Cheers for the reply.


    The Δ t for the unit heater is 39.6 ᵒC, on high fire, most of winter to be fair, that said the losses to the concrete will ce significalt, The SWA is never really loaded to much above 40A, though it is on a 100A fus.


    I am not unduely worried as noting visible is dispalaying any signs of thermal damage, that said 50-60 ᵒC  in there would'nt be unusual.


    Cheers


    Martyn

  • 0
    Cables need then to be de-rated rather as of they were grouped with some other cable running hot. How much to derate much requires some understanding of the system, or probably more usefully some of those peak temperature stickers.

    Moving hot air is less of an issue, as it is not there long enough to get much hotter, More of a problem would be places that heat up but d not have much draught.
  • 0
    I am used to dealing with marine installations (ships) where the assumed ambient is 45 degrees (on the assumption they can be anywhere in the world so tropical conditions are expected), so this duct doesn't give me any concern (as long as the designer knew in advance what the temperature was going to be).

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