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Fireproofing in cable calculations

SBarton
Hi


I wondered if I could ask help from the forum with a long overdue project for the C&G2396 course. Struggling a bit.


In a nutshell, how would you derate for fireproofing when a circuit goes from room to room?


It's a lighting circuit wired in 1.5mm singles in steel conduit, passing through several internal concrete block walls, within the screed across floors and up and across wall chases to wall-mounted light fittings.


I gather concrete walls should be at least 90mm thick (Googling), so every time the conduit goes through a wall I guess it would be surrounded by 100mm thermal insulation (derating factor 0.78, Reg 523.9) because of the fire-proofing around the conduit. I thought maybe I should use this on top of Ref Method B for the bits in the screed and wall chases (59B non-sheathed or single-core cables in conduit in masonry). 0.78 x (16.5/19.5) = 0.66 [using Table 4D2A 1.5mm single will carry 19.5A clipped direct, 16.5A Ref Method B].


But then I worried that I should use Ref Method A (1A non-sheathed cables in conduit in thermally insulated wall) because of the fire-proofing. 14/19.5=0.72 [using Table 4D2A 1.5mm single will carry 19.5A clipped direct, 14A Ref Method A]. I was surprised it resulted in a better deal, as I'd been taught Ref Method A was the worst case scenario.


So I think I have it quite wrong.


...Also if Ref Method A makes sense here then doesn't that mean you wouldn't be able to use anything other than Ref Method A (worst case scenario) for any system that passes from room to room, anywhere at all?


Yes, I think I have it very quite wrong.


Any help would be greatly appreciated,


Suki

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    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
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  • 0
    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
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