Wires runs through joists

[Sam]

Hello,

I'm planning to fit some two-way light switches in the downstairs rooms and also some mains powered fire alarms.

The fire alarms will be on their own circuit in 1.5mm^2 3-core and earth. I don't know how much current a fire alarm uses but I imagine it's very tiny.

The lights will be 1.5mm^2 T&E. In the grand scheme of things lights do not draw much current but I suppose its the sum of the lights which is important. Now even if the radial supplied a dozen 100w bulbs, that's only 5A out of a possible 16A for the cable (and of course the MCB is rated at 6A anyway).

Since these cables will carry low current, will they be ok to fit through the same hole in the joists? I ask because I am paranoid about drilling too many holes in joists, so the fewer the better. There's already notching for the CH. OTOH I don't want the cables to overheat and start a fire. You hear news stories about fires starting from faulty wiring.

Is it best to let each ring main/shower/cooker, i.e. big loads have one cable per load, but allow light loads to share a hole?

Thanks.

[A.Lee]

I'm not a sparky, so am not up with the latest regs on this, but in my own house, all of the cables come up from the consumer unit, then pass through the floor/ceiling void of the spare bedroom.(this was done 10 years ago) It is a bunch of around maybe 2 square inches. The upstairs lights and shower etc then branch off to go into the loft, but the 5 or 6 ring main cables are still tied together for another 8 feet or so until they branch off to go their separate ways. 5 or 6 2 inch round holes they go through to get to the hallway ceiling. Never had a problem, and I cant see how they would overheat if the cables are the correct rating for the appliance fitted to the various circuits. Though get confirmation from a sparky first. Alan.

[Bovvered?]

You are worrying unnecessarily, providing each cable is appropriately rated for the load it is supplying you can bundle them as tightly/closely as you wish. Also if you make any joints under the floor be sure to use a proper junction box.

[Andy Burns]

Actually the rated load of a cable *depends* on whether it is grouped with others.

[Dave Plowman (News)]

You should *always* keep data or signalling cables well clear of mains stuff - and where they must cross keep them at right angles or as near as possible.

The size of hole needed for data cables drilled in the centre of a joist will have little effect on the strength - especially if they have already been notched for pipe runs. Just keep that hole well clear of that notch.

[tony sayer]

In article , Dave Plowman (News) scribeth thus

Good advice;)...

[Dave Plowman (News)]

Mains cabling can carry all sorts of rubbish in the form of spikes created by dimmers and SMPS etc. If this gets induced onto data cables it can cause problems. Because even digital signals are not immune to corruption from interference.

Because it minimises induction or other forms of coupling at that point.

[Dave Osborne]

1. To prevent/minimise crosstalk/induced voltages in the data cables and to minimise the likelihood of the data cabnles becoming live in the event of a physical distrubance to the mains cable.
2. Because the IEE regulations requires you to segregate data cables by either an earthed metallic barrier or 50mm for the above reasons.

Because crossing at right angles minimises crosstalk/induced voltages.

There are rules about drilling joists which, no doubt Mr Rumm will give us a pointer to shortly (TIA John).

Rumble

[Andrew Gabriel]

I think the IEE regulation is just for safety, not data corruption, as the other method they allow is to insulate the signal wiring suitable for the higher voltages they are nearby.

Anyone tried running 100BaseT over a length of triple and earth? ;-)

[tony sayer]

In article , Dave Plowman (News) scribeth thus

Never had that problem. If its CAT 5 ethernet I think you'll find that the balanced mode of operation with the IP transmission protocol is very immune to interference.

In moist all installations in offices they use data and signalling in the same trunking very close to each other..

But they make Dado rail trunking in most all offices in the land where they don't cross cables at right angles their right alongside them. Are you saying they have that wrong?..

[tony sayer]

Well that covers quite a lot of all things going wrong at the once dontcha tink?..

I think that are excessive in that requirement..

Yeabut they make trunking where the cables are alongside each other they don't run them at right angles..

[tony sayer]

In article , Andrew Gabriel scribeth thus

It seems to me that their requirements are rather excessive..

You can do that it doesn't have to be twisted, its balanced operation which is very robust..

You won't need the earth;)..

>

[John Rumm]

Even then they should use segregated trunking that maintains a gap between mains and data. Also note that CAT5s are often used for voice services where hum pickup is undesirable.

[John Rumm]

There is a summary of the regs in the OSG:

1. Maximum diameter of hole should be 0.25 x joist depth.
2. Holes on centre line in a zone between 0.25 and 0.4 x span.
3. Maximum depth of notch should be 0.125 x joist depth.
4. Notches on top in a zone between 0.1 and 0.25 x span.
5. Holes in the same joist should be at least 3 diameters apart.

[Dave Osborne]

Indeed, and one can't even say yeabut all multi-compartment trunking has an earthed barrier, cos plastic dado trunking doesn't. I've never experienced interference "problems" with Cat5 or phone wire in three-comp plastic dado trunking, but I wouldn't want to, say, run microphone cables in close parallel with the power feed to a lift motor...

[Dave Plowman (News)]

'Data and signalling cables' covers rather more than ethernet cabling - or rather i meant it to. Things like telephones, alarms both burglar and fire, A/V circuits etc. However, decent microphones have balanced outputs too and I've oft heard the results of their cables running along with lighting ones in a temporary installation. You hear a spikey buzz on their output from lighting dimmers - cured by re-siting the cables. I've also experienced poor dial-up internet performance caused by dimmer buzz induced onto the phone line - which is balanced too.

But not bunched together as in passing through the same holes in joists. They are separated by a fixed spacer system. Nor would you see such a system in use where you were very concerned about crosstalk - like in say a studio. Mains and others will be kept separated by a much greater distance.

I'm not quite sure where the circuits would cross given they are in separate compartments?

[Ziggur]

if you make any joints under the floor be sure to use a proper junction box.

If you do use a junction box under the floor, the Regulations require that you must be able to access it,

[tony sayer]

Another balanced service with inherent rejection..

[John Rumm]

Not if you run three wire POTS service over it - the ring wire is unbalanced.

(For digital PBX equipment, then the usage typically is balanced).

[tony sayer]

You dont have to do that in the route from the exchange..