Cable grouping

Question

An interesting point made by one of my colleagues today. 
 If we consider a 36-way TPN distribution board that is full. How can we apply a grouping factor to those circuits at the point they leave the distribution board (the worst case position) without it making the cables so large that they can't be terminated if some of them are lighting or ring final circuits installed in trunking? 
 Clearly we could use sub-distribution, etc. but I have seen plenty of installations exactly like this with fairly typical 2.5sqmm or 4.0sqmm cables on the lighting and ring circuits. 
 Is it simply that the point of exiting the board is ignored and the main run of the cable is used for consideration of the grouping factor? If the cables are de-rated on the basis of where they come together at the board how does any installation ever practically make use of a 36-way TPN board without substantially over-sizing cables?

burn · Accepted Answer

No , you don't ignore the grouping just because it's near the DB. Don't forget though, for the purposes of grouping you can ignore cables that are carrying less than 30% of their grouped rating. That should remove a significant number from the group. The second thing I would normally do with a larger DB, is to use two trunking drops, one from each side of the DB, up the wall and until the circuits split off into different directions. It usually only means a short extra length of trunking. 
 regards, burn

AJJewsbury · Answer

You can also note that the total outgoing current from the DB can't exceed the incoming - so rather than looking at say 20Ax36 = 720A things are probably limited to something closer to 3x100A=300A or 3x125A=375A - i.e. overall diversity is probably a lot more significant than considering each circuit individually would suggest. 
 - Andy.

mapj1 · Answer

Once in the board the cables are stripped, and hopefully not tied together, so they are spaced by more than a few wire diameters on average, and then airflow means that their cooling is good. On the way out there needs to be plenty of space. A single steel trunking run where you cannot get the lid on without leaning on, that erupts like wild spaghetti given half a chance, usually indicates an issue with cooling. 
 Do beware of folk who want to make a 'tidy' bundle of tightly laced up singles though - this can all to often "cook" in the middle - a number of spaced out smaller bunches or a flat ribbon are both vastly better. 
 As noted the upper limit on incoming power usually indicates the de-rating that really applies. It does mean that in practice there are rarely problems. 
 Mike.

Chris Pearson · Answer

Stephen Yewdall said: Is it simply that the point of exiting the board is ignored 
 I suspect so, although such a scenario seems a little odd for a 36-way (by which I assume that you mean 36 SP circuits/12 TP circuits) board in a commercial setting. How exactly do all the circuits leave the board - trunking, conduit, clipped direct even? 
 This certainly seems to be commonly ignored in the domestic setting with a whole bunch of cables going straight back into the wall, or through the ceiling, etc. 
 I doubt that this is much of a problem if it is simply that there is a very short (i.e. millimetres or a few centimetres) pinch point because of longitudinal cooling along the cables. 
 Is this very different from let's say, a couple of rings and a lighting circuit squeezing through a joist in somebody's home?

AJJewsbury · Answer

Is this very different from let's say, a couple of rings and a lighting circuit squeezing through a joist in somebody's home? 
 
 Taking the argument a bit further - domestic rings and lighting circuits very rarely run a full load and even then not for very long. If it had been a a few storage heater circuits and an immersion (or perhaps these days a couple of EV charge points, a heat pump and a hot tub) a bit more care perhaps would be sensible. I think we're pretty much all on the same page - it's not a case of ignoring grouping, but applying a bit more realistic version of diversity than the OSG tables might suggest (even if that often gives the same result). 
 - Andy.

Cable grouping

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