Cable calculation in ducts

I don't think there's any such get-out clause in the regs - the closest I can think of of is 523.9 (and table 2.6 in appendix 4) for cables running through thermal insulation - which seems to suggest that there's some allowance for heat transferring length-wise along a cable to cooler areas - but the distances are comparatively small - some derating needed after 50mm, down to half rating above 0.5m. Numbers are only given for comparatively small cables (≤10mm²) but you could perhaps deduce a principle of few multiples of the cable diameter and scale things up a bit for larger cables. I'd doubt you could justify no-derating at all for 2m though.

Other than that, the principle in the regs seems to be that every part of the cable length of cable (even 1cm) should be treated according to the conditions at that point.

   - Andy.

I don't think there's any such get-out clause in the regs - the closest I can think of of is 523.9 (and table 2.6 in appendix 4) for cables running through thermal insulation - which seems to suggest that there's some allowance for heat transferring length-wise along a cable to cooler areas - but the distances are comparatively small - some derating needed after 50mm, down to half rating above 0.5m. Numbers are only given for comparatively small cables (≤10mm²) but you could perhaps deduce a principle of few multiples of the cable diameter and scale things up a bit for larger cables. I'd doubt you could justify no-derating at all for 2m though.

Other than that, the principle in the regs seems to be that every part of the cable length of cable (even 1cm) should be treated according to the conditions at that point.

   - Andy.

problem is that heat mostly flows out the sides of cables, not along them and a length of just a few cable diameters is enough to support quite a noticeable thermal gradient. Copper is a good conductor of heat, but not that good. (as every plumber with a gas torch can demonstrate.) The best you can easily do is have 90C cable in the hot sections, and the ends cooler

How near the limit is expected rating/ temperature rise in the ducted section - I assume you have looked at the ERA recommendations and have cores spaced etc.

If not there are some extra cases there not covered in the example 'methods'  in '7671 that may help.

Mike

Thanks for your answers

In the regs there does not seem a method to install single AWA cables buried in ducts. 

We will need to install 8 singles per phase which mainly run on ladder. when they reach the ducts, each duct would need to take L1,L2,L3 & N

Or is there a table in the regs that I have missed

Andy

From that description, there probably isn't quite what you want in the regs - which is at best about  three quarters of real situations. You are then left muttering 'it is similar to ?' or maybe making some measurements, or as alluded the ERA guides, but even they wont cover 8 cables per phase ! - What the heck are you doing that needs that, and how do you terminate the ends ?

Do keep the cores bundled so that  currents in any one duct/ containment / bundle  cancel as far as possible, - so LLLNE in one duct, and then LLLNE again in the next as well, not one duct all L1, and another one one all L2 , you need for EMI reasons to try and form the smallest possible un-cancelled current loops.
Mike.

Thanks fort he response

The supply is 2800A for a data centre, Ends are lugged and bolted to LV switchgear

hah! well so long as at neither end no poor so and so is trying to get 8 lugs onto one terminal ;-)