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
