Cable grouping factors

Farmboy:


Note 1 of table 4C1 in Apdx 4, says that the factors are applicable to uniform groups of cables, equally loaded. However, I see some publications that apply these factors to ccts, in trunking, for example, with no reference to cables being equally loaded (as though it's a belt-and-braces approach and apply the factors regardless). 


So what is your interpretation of equally loaded? It could apply to cables in conduit or trunking supplying heaters or conveyor belt motors, for example, all on for long durations. But what of cables supplying e.g. ring f. ccts, an EVC and other ccts, such as in domestic settings? Can they basically be ignored from such grouping factors or apply them regardless just to be in the safe side?

 

Equally loaded just means that that's what they've assumed when calculating the numbers in that table (i.e. every cable is carry the same and as much as it can without anything overheating). Trying to create tables for other conditions rapidly gets very silly as there are just too many combinations of how things could be arranged (90% fully loaded, 10% half loaded; 50% full loaded, 25% half loaded, 10% 10% loaded 15% unloaded....)


Where things aren't equally loaded then it's NOT saying you can ignore grouping - it's just saying that the figures in that particular table might not be entirely accurate - hinting that you might need to take other factors into consideration (i.e. apply some engineering judgement). As you say for other cases the tabulated values are likely to be erring on the safe side, so applying them directly if you don't really know much more about the loads probably isn't a bad approach. But where you do know something about the nature of the load .. or indeed any upstream overload protection ... you can often justify some significantly more favourable numbers.


In practice, for domestic socket and lighting loads, grouping factors are often ignored - but care is still needed especially for large long-duration loads (traditionally immersion heaters and night storage heaters).


   - Andy.

Chris Pearson:

Where I think that this concept fails is in ring circuits. By definition only one circuit, but imagine a circuit high up in conduit: down and up, over a door, down and up again, etc; or domestic under the floorboards, down and up (or up and down) under the capping, down and up again, etc. If the main load is half way round, that's the whole lot in two adjacent cables. Should that not be treated as two circuits?

I asked the nic about that, some time ago, and they said that for domestics they read it as for ccts, so wouldn't be concerned about individual legs if the ring was generally lightly loaded most of the time. However, for comm/ind they would apply the group ratings to the legs, depending on anticipated use/load. Others, of course, may disagree.

mapj1:

well, it should be treated as two adjacent cables carrying the same current,  and grouped, just the same as you would (I hope) for a hairpin of cables going up and down the wall to call in at a switch  for a heater  or similar on a radial as well.

Some of us sidestep this with 2 pieces of oval conduit or capping, each with one cable inside. I must admit I would not read it as 'circuit' literally, the same wire looped back on itself is still "grouped" from a heating point of view.

M.

 

presumably then each leg should also be run separately in floor joists - at the risk of making Swiss cheese out of the joists with all the cables run in separate holes, or upping the cable size based on rating factors. I'm still pondering how to alter the factors for cables that are not equally loaded but which shouldn't be ignored.

Where I think that this concept fails is in ring circuits. By definition only one circuit, but imagine a circuit high up in conduit: down and up, over a door, down and up again, etc; or domestic under the floorboards, down and up (or up and down) under the capping, down and up again, etc. If the main load is half way round, that's the whole lot in two adjacent cables. Should that not be treated as two circuits?

Farmboy:


...

presumably then each leg should also be run separately in floor joists - at the risk of making Swiss cheese out of the joists with all the cables run in separate holes, or upping the cable size based on rating factors.

...

 Extra holes not really required unless the parallel length of touching cables in the joist was so long that no significant heat conducted along the cable, and the fit so tight that no air could move between them - so perhaps a very thick joist like a ship's timber, but not the average bit of 2*8 or 3*10 - (even less  of a concern in the 'engineered joists' that are an I beam of 2 bits of what looks like roof batten attached to a strip of plywood)

If in doubt a slight slotting in the direction of the grain is enough to open up an air path, though to be honest T &E being flat, in a round hole, I'd not worry.

Because the heat generated (and in the simple systems the temperature rise ) is proportional to the square of current you  may find do not need to de-rate so much - 70% of the current in 2 cables is the same total heating as 100% of the heat in one cable (as 1.4 is sqrt of 2), but of course the surface area to sweat it out over is increased over the single cable case (but not quite doubled as the sides in contact are unable to lose heat in the way they would be if there was a gap between the cables) Also flat cables touching on the thin edge cool better than touching on the broadside - but this sort of thing rapidly becomes subject to too many subtle effects to be safely captured in a simple set of rules.

However a round bundle of cables is the worst case, for the one in the middle, and keeps getting worse the more cables are added, while if you can lay your cables  side by side, then only the nearest few neighbours to any given cable have much effect, and the effect on the grouping factor is that for more than a few cables the change in cg of adding another cable is small - the greatest steps are between 1 and 2 and then 2 and 3 cables, and a flat lay of 4 cables is almost the same as a flat lay of 40.

Mike


regards,

Mike.