New light in loft. Identify a supply.

[arabbadot]

I am installing a new light in a converted loft area.
There are plenty of wires at 1.5 and2.5., but there are no junction boxes and no convenient ceiling roses.
I was proposing to cut into a 1.5 cable and install a junction box from which to run a light supply wire an a switch wire.
How do I identify an appropriate,or inappropriate cable to cut into?
All power off at fuse box of course

 

[martinxxxxxx]

I am going ask for some more information please.

When you have finished, will the junction box be accessible? (ie will it be under a floor board or behind a plasterboard)

 

[PrenticeBoyofDerry]

You need to identify the circuit you are working on.
This can be done by isolating individual circuits rather than isolating the whole system, you will also need to use a two probe approved voltage indicator to do this safely.
You also need to make sure the cable you use for the light is safely protected by a suitable fuse. The 2.5mm cable could well be on a ring final circuit that is protected by a 32A device. 1.5mm cable could be your existing light circuit, a spur or a power circuit for something like a immersion heater.
So some investigation on you part is needed.
The ideal situation would be the 1.5mm cable is existing light circuit and if you use 1.5mm cable for your alteration the rating should be safe.
Alternatives are break in to a suitable circuit and down fuse using a fused connection unit.
If the area is to be boarded or panelled any junction boxes you install must be easily accessible, so they can not be hidden or buried within the cosmetic fabrication of the building. This is so inspection, testing and remedial work can be carried out with ease.
There are maintenance free junction boxes available that the regulation will allow you to hide. Also if the cables are going to be concealed/buried within the walls and RCD protection is not already on the circuit you will need to have this installed.
You must also follow the permitted safety zones
//www.diynot.com/wiki/electrics:installation_techniques:walls
//www.diynot.com/wiki/electrics:installation_techniques:walls

 

[sparkwright]

The lighting circuit is the logical place to take the feed from.

It seems you need to know how lighting circuits work first.

Typically lighting circuits can be looped in at the rose, or looped in at the switch.

Looped in at the rose means permanent live and neutral is supplied to each rose. From each rose a switch cable is run to the switch.

You cannot get a supply from this switch cable as it contains permanent live and switched live - ie no neutral.

Looped in at the switch means permanent live and neutral is supplied to each switch. The neutrals are connected in a connector block. From the switch a cable is run to the ceiling rose.

You can't use this cable because it only becomes live when the room switch is on.

So, you need a cable that contains permanent live and neutral. With this supply you can fit a switch to turn the loft light on and off.

For lighting there is also the junction box method. With a junction box for each room it is very similar to the looped in rose method. Some systems have one very large junction box, usually in a central position, where each light and each switch cable run back to.

Many circuits are a combination of all the above methods.

 

[arabbadot]

I am going ask for some more information please.

When you have finished, will the junction box be accessible?
Yes the junction box will be visible and accessible.
The is a batch of cables running up from the fuse box.I am assuming they are all lighting at 1.5 and all power at 2.5.
One way would be to interrupt that run of a 1.5 and stick in a junction box.
I can easily identify if it is live ...er ...before I cut any problem with that?

 

[ban-all-sheds]

I am going ask for some more information please.

When you have finished, will the junction box be accessible?
Yes the junction box will be visible and accessible.
The is a batch of cables running up from the fuse box.I am assuming they are all lighting at 1.5 and all power at 2.5.
One way would be to interrupt that run of a 1.5 and stick in a junction box.
I can easily identify if it is live ...er ...before I cut any problem with that?

No he didn't.

The is a batch of cables running up from the fuse box.I am assuming they are all lighting at 1.5 and all power at 2.5.

Never, ever, assume.

I can easily identify if it is live ...er ...before I cut

How?

 

[arabbadot]

Thank you for all that. I was familiar with circuit box system and with looping in at the Rose. I was not familiar with looping at very crowded switch box.
I was hoping to identify the supply to a lighting point from the fuse box using a detector pen and switching on /off at the consumer unit.if the cable I identify is running straight to a switch it will have live and neutral and I can interrupt that supply with a junction box and run a new feed from that junction box to the light and also include a switch.
Don't worry I do not act on assumption but I theorise on the basis of assumption which I hopefully always check before acting.

The lighting circuit is the logical place to take the feed from.

It seems you need to know how lighting circuits work first.

Typically lighting circuits can be looped in at the rose, or looped in at the switch.

Looped in at the rose means permanent live and neutral is supplied to each rose. From each rose a switch cable is run to the switch.

You cannot get a supply from this switch cable as it contains permanent live and switched live - ie no neutral.

Looped in at the switch means permanent live and neutral is supplied to each switch. The neutrals are connected in a connector block. From the switch a cable is run to the ceiling rose.

You can't use this cable because it only becomes live when the room switch is on.

So, you need a cable that contains permanent live and neutral. With this supply you can fit a switch to turn the loft light on and off.

For lighting there is also the junction box method. With a junction box for each room it is very similar to the looped in rose method. Some systems have one very large junction box, usually in a central position, where each light and each switch cable run back to.

Many circuits are a combination of all the above methods.

 

[PrenticeBoyofDerry]

I was hoping to identify the supply to a lighting point from the fuse box using a detector pen and switching on /off at the consumer unit.if the cable

I would not trust a voltage pen with my life, they can give out false detections, not to be over trusted.

 

[ban-all-sheds]

I was hoping to identify the supply to a lighting point from the fuse box using a detector pen

You will also need to use a two probe approved voltage indicator...

 

[martinxxxxxx]

If you cannot pull a wire to the nearest ceiling rose ( this would be best I feel), Then you are stuck with having to trace the wire that is the lighting feed.

Forget using a voltage pen, those things will light when you put them next to a dead wire that is simply acting as an ariel and neglect to light on a live wire (with all of the dangers that that will cause).

You will need to switch off all of the power and prove it is off ( using safe isolation procedure in WIKI)

You are hoping that the wires going past are on the lighting loop. so if you separate the loop at the nearest ceiling rose and put a tone tracer on the wire, you might be lucky and be able to identify the loop wire. While the power is still off you could then cut into it and fit a JB. and then use a long lead to prove it is the wire that is going back to the ceiling rose, using low ohms range on a multimeter.

If any of the above is confusing then possibly you should consider getting some assistance.

Hope this helps