The rate of temperature drop along a copper cored cable is very similar to the rate of rise along it when it enters insulation, so for thin domestic sized cables where the insulation is thick compared to the copper, a few hundred mm, while for larger sizes  between ten to hundred or so  cable diameters,  in the new environment, will be  enough to substantially establish a new steady state temperature.

Mike.

I am not sure about your set up or how you have calculated the csa required. However, if the loads are relatively fixed, then you might be able to claim that overload protection is not required and run with Ib rather than In.

Even if overload protection was required, advantage can be had if the circuits are not subject to simultaneous overload and the worst case extracted from;

If there is significant difference between In and Ib then the advantage will be significant.

Jason I think you are thinking this in the wrong way. Why do you want to put high rated circuits in trunking? Your 250A circuit for example would be much better as SWA, fitting 95mm cables in small trunking is not sensible unless it is a straight run. A better design would be to run a 95mm ring main to each panel, and put the CPDs there, and much more convenient operationally, particularly if you want RCD protection of the smaller circuits. I suppose you already have your main panel, but it might well be cheaper and better to forget this and do as I suggest. Obviously a 400mm cable is just plain stupid! Each of the smaller panels could then probably be a standard board with the various socket types around the outside, with breakers and RCDs as required inside. It sounds to me as if someone has tried to make a "pretty" job and this has forgotten that the users don't care, they just want to plug and play! Like this you have no grouping difficulties, the SWA is clipped direct so more than adequately rated, and any changes later (there will be!) are much easier to carry out.

Often overlooked is the fact Annex 4 gives you a choice of 3 methods to calculate grouping. Equations 2, 3 & 4.

So long as you've not got simultaneous overloads - which you can largely negate except e.g. ringmains - then you can use equations 3 or 4 which use design current.

Equation 4 is a bit of a headache but considers cases where design current is >30% & <100%.

When you have got rings then it's a bit more difficult.

Thanks Kier.

I looked at these, and they still require values for Cg - and using the tabulated values isn't really valid as we have multiple circuits of greatly varying sizes, and coming up with the Cg number is the bit that's particularly taxing!

Hi David - thanks for that.

Sadly is it precisely the aesthetic requirements that have got us into this pickle in the first place, and as a new build project the aesthetics are deemed important.

All the main panels and outlet boxes are made and fixed to the wall.

We have now in all cases separated the 250A circuits from the trunking with the other circuits in, and these are either running in SWA for some of their length or in a separate trunking which is deemed the best fix due to site conditions.

This is definitely a case of 'well if you want to get there then I wouldn't start from here'!

Jason.

Thanks Lyle.

Unfortunately as this is for power supplies to be used with temporary equipment then we can't assume that simultaneous overload is not possible.  I'd like to think that live event professionals would use distribution equipment connected the installed outlets that had suitably sized protection to prevent overload but it couldn't be guaranteed.

For Cg itself is simply a matter of counting and looking at Table 4C1. The problem I imagine you've got is that there’s several circuits so Cg is getting impractically small.

I’ve got a little excel spreadsheet I use that lets me play around. Sometimes for example if a circuit is currently 50% of its grouped rating then increasing the CSA on that circuit to bring it < 30% allows the other circuits to ignore it for the purposes of their own group factor [see note 9 to table 4C1]. This can have a cascade effect because suddenly another circuit that was previously 32% has had its grouping factor changed and is now < 30% so is also ignored.

And it’s perfectly permissible to have an increased cable CSA through an area with different thermal requirements and then revert to a lower CSA once the cables are in a space that the thermal requirements no longer demand that higher CSA. Reg 433.3(i) relieves you from the requirement to put new protection where the CSA is reduced to a level still appropriate for the existing protection, and 523.8 says you should work out the cables based on the heat dissipation requirements on each part of the route.

If I was you I’d be making a spreadsheet and trying to see what I can get away with. I take it you’re also a bit limited on conduit capacity factor and hence can’t go too far on CSAs? Appendix E to the OSG is entirely informational and is there to help design conduit and trunking that is easy to pull cables without damaging them as opposed to meeting any electrical objective – there’s no references from OSG Appendix E to the BS7671 and so long as you’re not putting too much mechanical stress on the cables then would be compliant to slightly exceed these but if you do then make sure the person doing the installation isn’t heavy handed use a cable pulling lubricant. 

In my mind simultaneous overload would mean more than the risk of users pushing the boat out too far - because that would typically be limited to one circuit.

Presuming the design assumptions are correct, and you're talking about serious amounts of power, surely the overload is more likely to be as a result of a user not properly distributing their load as opposed to a user overloading multiple circuits? I'd like to think that nobody connects up 100kVA of equipment without having some basic knowledge of what they're doing - I don't think anyone would last long in the events industry if they've got a habit of tripping overload protection for half the stage just as Gary Barlow is being flanked by a giant mechanical elephant.

When the regs ask to you to make these assessments then they there's an element of crystal ball gazing. But you're not being asked to negate the possibility of simultanious overload ever happening - without very specialist protection you never could and if the authors thought that any possibilty would be so serious then they'd have used stronger words.

I think what you're being asked to consider is if you've got circuits in parallel which tend to overload simultaneously. Maybe you've got a radial circuit which is looping down some trunking to an outlet and before continuing to the next outlet - that would clearly be subject to simultaneous overload. You might alternatively have loads such as heaters for the same space, but this would be fixed equipment and you'd already have done design calcs such that they never overload.

Thanks Kier.

I think Table 4C1 is only for groups of uniform cables equally loaded.  Our challenge is that we have 2 lots of cable groups - one with circuits ranging from 16A SP to 63A TP (5 circuits total), and the other from 32A TP to 125A TP (6 circuits total).

We could possibly use the method in 2.3.3.1 of annex 4 but that gives much worse factors than the the tabulated ones.

I'm going to take a look at Mikes' suggested method around a temperature calculation based on heat produced and using ambient correction factors and see where that works out.

We are indeed a bit limited on containment capacity.  We've already found solutions to put separate routes in for the 250A circuits in addition to the above as the grouping factors then were giving some very large cable sizes.

Simultaneous overload on reflection in a professional event environment I think could be ignored, but for these circuits we have to assume that Ib=In in the absence of knowing what form of mechanical elephants might get plugged in to it!

Jason.