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

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.

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.

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?

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.

1. Yes I had overlooked the exception for <30% loaded circuits. I did suggest that a large DB like this would have circuits going in lots of different directions so it should be possible to segregate them to keep the grouping down. The trunking either side is a neat solution.

Good point Andy. That does give a good starting point when you're trying to decide on a sensible diversity to apply too. Having said that should we be accounting for diversity within the cable sizing calc?

Very good points Mike thanks. 

Yes I was thinking 36 SPN ways sorry I worded it badly. I had in my mind a fairly typical commercial / office install with trunking out from the DB and wired in 'singles'.

Completely ignored in domestics you're right. Though I think the point the others made about the incoming device limiting the amount of circuit load that can be present helps here too.

I did once see a document from a utility company where the diversity they applied to domestic properties for the purpose of transformer sizing meant each property was based on something like a 20A supply... this may have to be revised given the EV charging sockets appearing on a lot of houses though!

I thought about this some more this morning Andy and I think this is why it works from a practical point of view instead of simply applying the calculations from the regs.

If we have worst case grouping of 20x circuits going out from a 125A DB then the circuit loads must average at 6A each. Obviously there will be some higher or lower and that will vary during operation but going from burn's reply above a 32A ring circuit is going to be <30% loaded when carrying the average load so in the absence of more specific operational information there is a case for ignoring the circuit in terms of grouping factor.