grouping of cables

[Fred]

Hi,

Can anyone tell me what are the regs/best practice when running cables from a CU?

I've got wondering after a thread I started called "shower plumbing and electrics", please take a read if you haven't seen it all ready. In summary: I wanted to upgrade an electric shower and wasn't sure what size cable was already there.

I then found that the shower cable and all the other cables from the CU ran up the wall together and under the bathroom floor together, even going through the same one hole in the middle of each joist. I thought you were supposed to use separate holes for each cable, spaced at three times the diameter of the hole?

There's more info in the original thread and I don't want to get in trouble for repeating myself here but I thought a change of title might attract a wider audience. Also speaking more generally, it seems to me that by design, all cables have to go to/from a CU, so in any house there will always be a lot of cables close together under the floorboards and down the walls near a CU.

Thinking about my own house, they all run up the wall from the CU and closely together under the landing for 3' or 4' before branching in their various directions. I can't see there is any other easy way to do it, so is it that it doesn't matter if you have tens of cables close together en route to the CU?

On the one hand I think I have read that cable heating doesn't really apply in domestic situations and I can see the logic behind that: the house may be fused at 80A or 100A, so you cannot have an electric shower, electric cooker, immersion heater, and the ring main at 32A all at once. And the cooker and shower should be on for short lengths of time. So does diversity mean that domestic cables never get hot?

On the other hand every now and again someone posts that there are x thousand house fires due to wiring each year. So I am unsure what to think. If grouping, heating, and derating in the home are not important, why include them in the regs. They must be there for a reason surely?

TIA

[Dave Plowman (News)]

I'm willing to bet the vast majority of those fires are caused by old (rubber) wiring where the insulation has failed, or by overloaded circuits using re-wirable fuses. Some degree of cable bunching is inevitable in the average house re-wire - it should just be kept to a minimum.

[ARWadsworth]

Nah, it's candles and chip pans.

Indeed, we have to have some bunching of cables or the joists would consist of nothing but holes!

With the common sense rule that cables used for water heating and other cables that can expected to be heavily loaded (eg kitchen ring, cooker, storage heaters) are not bunched then it is usually okay on a typical 3/4 bedroomed propery to not worry about bunching the rest of the circuits. Lighting circuits can often be ignored for the purpose if grouping.

Fred, the answer on this one is easy. Drill some new holes for the new shower cable so it is not bunched up with the others:-)

[David Robinson]

Separate holes for each cable? No! Good grief, the house would fall down (well, it would here!), and often there just wouldn't be the space to do it. You're supposed to use common sense, otherwise known as derating.

Yes, it's inevitable, and it seems to be common practice to ignore that couple of feet of cable.

You can. You can easily exceed that rating - just not all the time, or by a huge amount.

The cables should never get _hot_ anyway. For cables to get rather warm you need to run them at approx. 2x or 3x nominal rating for some length of time (e.g. for half an hour, not half a minute). That's still not generally enough to melt anything or break anything, though a bit longer use or a bit more current is getting close.

Yes, what Dave said! Grouping, heating and derating are important (you can't run all the cables from the CU in a tight bundle surrounded by

200mm of loft insulation for 2m!), but it's not competently installed new house wiring that causes these fires.

I think you're making this far more complicated than it is. You're swapping one shower for another one. The new one uses more power, but is still within the rating of the cable. If it's completely surrounded by insulation (in the loft or in the wall) then I'd be concerned, but otherwise, unless it's tightly bunched with other cables that are also likely to be used to capacity at the same time for tens of minutes, then it's probably not worth worrying about.

But if it will make you lose sleep to leave things as they are, then running a higher rated cable doesn't sound like a big job. Just be aware what a pain it is bending 10mm cable inside a tight space.

Cheers, David.

[Tim Watts]

Bunching or confinement or enclosure in insulation is not reckoned to be a problem over short distances (where short is around 150mm IIRC) - any built up localised heat will conduct away along the cable cores.

Confinment/bunching/insulation over non trivial distances should start taking various factors into account.

I wouldn't worry about cables clipped side by side on a wall - technically there should be inter-cable spacing if you look at the grouping pages in the regs, but you can also play the diversity card - if you loaded all your circuits to 100% you'd blow the main fuse in most cases. So at any one time some circuits may be maxed out, others may be doing bugger all.

A load of cables under the floor will usually have lots of air spaces through the bunch to aid convection, except where they go through holes in the joist and that's only a short distance.

Do what Adam says.

You have to remember the concept of grouping applies more to circuits where all circuits are deemed to be operating at 100% all the time and that will be more usually found in an industrial setting. That's not to say you should ignore it, but it is worth looking at the circuits that are likely to be heavily loaded, certain rings, cooker, shower.

[Fred]

Lots of replies all saying much the same thing, thanks everyone for the reassurance.

Sorry I didn't quite say what I meant. I was not suggesting one hole for one cable! What I meant to say was heavily loaded cables, e.g. immersion heater, shower, cooker should be in separate holes. I realise lighting can be grouped. I'm never quite sure whether the ring main can be grouped or not though.

There is a 32A radial to a cooker but it is not a big cooker so I doubt it is run at full load. There is also an immersion heater, so there are some "heavy" circuits in there. What surprised me was that there was only one hole in the joists and everything went through that. Sure, a dozen holes would be silly but I thought they would have shared the cables between say, three holes.

Having found out that the cable is 6mm^2, IIRC that is ok up to 9.5kW, so I was hoping to use the existing one. I just need to check about the length though. There is a limit isn't there. I guess to do with voltage drop? Because of the odd route the cable takes I guess it could run no more than 12m. Probably less because I am guessing rather than measuring. A more direct route would have taken it a third /quarter of that.

Thanks again.

[dennis]

I haven't read all this thread but bunching a few cables for a couple if inches is not going to do much. If they are cable tied together as they go from joist to joist that may be a problem. I doubt if there are many people that would do that.

[John Rumm]

Immersion is not usually a particularly heavy load - if its just a single 3kW heater - its a long term load, but well short of the cable's maximum rating (assuming its a 2.5mm^2 T&E so little heating). Same potentially goes for the cooker, diversity reduces the average load to significantly below the peak one. So the shower is the main one you would normally expect to run a little warm. Possibly the cooker on heavy use.

Generally, unless you are contending with too many other de-rating factors as well - like passing through insulation or in high ambient temperatures.

If doing now, quite possibly. However they have presumably been like that for some time, and assuming the insulation has not gone brittle and started cracking or there are other signs of overheating, it would suggest there has not so far been serious overheating.

7.3mV/A/m on 6mm^2. So a 9.5kW shower[1] gives 41.3A load at 230V. Your voltage drop limit is 5% of 230V or 11.5V

So your max cable length will be 11.5/7.3mV/41.3 = a rather generous 38m

[1] Most showers will be rated 9.5kW at 240V, so the actual rating for design purposes (i.e. at 230V) is often lower.

Can't see that being a problem...