Correct fuse/cables for lighting

[Ambergreen45]

Two simple questions:

1) I need to install an outside lamp. I believe I need to connect a 2.5mm cable from the double plug socket to the FCU. I then need to change the fuse from 13A, but I'm not sure weather to go for a 3A or 5A since I don't know if my circuit or ring or radial? Is it then safe to check my connection at this stage which will show a red light on the FCU when switched on?

I need to do a bit of plastering and put a new skirting board before proceeding to the second stage, which will be to connect a cable from the FCU to the outside light. Does it matter if that cable is 1mm or 1.5mm?

2) I want to move a lighting switch to the other side of the door. I very difficult to know if the cable from the mains is 1mm or 1.5mm (with my glasses on and a ruler). Which size should I use from the existing switch to new one on the other side of the door, 1mm or 1.5mm, or doesn't it matter? Just want to make sure.

Many thanks!

 

[riveralt]

Two simple questions:

1) I need to install an outside lamp. I believe I need to connect a 2.5mm cable from the double plug socket to the FCU. I then need to change the fuse from 13A, but I'm not sure weather to go for a 3A or 5A since I don't know if my circuit or ring or radial? Is it then safe to check my connection at this stage which will show a red light on the FCU when switched on?

The size of fuse will be determined by the manufacturer of the outside lamp - normally 3Amp for a standard lamp - sometimes 5Amp for a floodlight.
There are specific rules if the socket you are spurring from is already a spur. No if its a ring circuit - Yes if its a radial circuit.
If the socket is already a spur there are ways around it.

I need to do a bit of plastering and put a new skirting board before proceeding to the second stage, which will be to connect a cable from the FCU to the outside light. Does it matter if that cable is 1mm or 1.5mm?

No.

2) I want to move a lighting switch to the other side of the door. I very difficult to know if the cable from the mains is 1mm or 1.5mm (with my glasses on and a ruler). Which size should I use from the existing switch to new one on the other side of the door, 1mm or 1.5mm, or doesn't it matter? Just want to make sure.

Preference is to use the same size as the current cable but for lighting circuit 1mm² is fine

 

[Ambergreen45]

Manufacturer says 3Amp for ring, 5Amp for radial (but don't know which circuit I've got).

The double socket has 2 red cables, 2 black and two green/yellow. I have no skirting board in that room at present and I cannot see a metal conduit coming from the concrete floor. It looks like the double socket is a spur coming from the double socket on the same wall from the room opposite.

There was previously an FCU with a 13Amp for the outside light, but I just want to change the position of the new FCU and put a new outside light.

In the light of the above, would I be safe using a 5Amp?

 

[riveralt]

Manufacturer says 3Amp for ring, 5Amp for radial (but don't know which circuit I've got).

The double socket has 2 red cables, 2 black and two green/yellow. I have no skirting board in that room at present and I cannot see a metal conduit coming from the concrete floor. It looks like the double socket is a spur coming from the double socket on the same wall from the room opposite.

A spur will normally have one set of cables; a Ring Final Circuit will normally have two or three sets of cables if it feeds a spur; a Radial Final Circuit will normally have two or one sets of cable depending on where it is in the circuit. Though radials can also have more than two sets of cables depending on how the circuit is designed.

You might want to check the other socket to check how many cables are present.

While I wouldn't advise this personally - you could check inside the Consumer Unit and look at the number of cables coming out of the circuit breaker for this circuit - assuming its a 30/32Amp you would normally have two cables for a Ring FC and one for a Radial which should be no more than a 20Amp breaker.
I say normally because there are legitimate ways to have more than one cable set and it still be a Radial FC.
Just remember that even if you turn the CU switch off parts of the CU's inside are still live.

There was previously an FCU with a 13Amp for the outside light, but I just want to change the position of the new FCU and put a new outside light.

In the light of the above, would I be safe using a 5Amp?

The purpose of the fuse is to protect the cable so either 3 or 5Amp would be fine.

 

[EFLImpudence]

Manufacturer says 3Amp for ring, 5Amp for radial

That doesn't make any sense.

 

[Ambergreen45]

Thank you! I don't think I will touch the consumer unit . I'll use a 5Amp.

Manufacturer says 3Amp for ring, 5Amp for radial

That doesn't make any sense.

According to the manufacturer's instructions, if I were to have a radial circuit a 5Amp fuse should be used on the FCU for the outside light, if I were to have a ring circuit a 3Amp should be used. I'm not sure why they are saying that.

 

[EFLImpudence]

I'm not sure why they are saying that.

And neither are they.

The fuse will be to protect the cable YOU are connecting to the circuit, not the circuit.
(and possibly to protect the fitting if it has not been manufactured well enough)

 

[Ambergreen45]

I've been trying to carefully twist the cables using a pair of long nose pliers and they have scared the copper a little, so I stopped before causing any damage. Is there really a need to twist every set of cables together?

 

[EFLImpudence]

No. They should not be twisted.

If the terminal seems too large bend the end of the wire back on itself.

 

[Ambergreen45]

A final question. My downstairs flat has a concrete wall and ceiling and the plaster on the walls is no more than 15mm thick! It will look pretty awful to have a pvc conduit run over the door frame, and if I can avoid the same from the double socket to the FCU. As I am renovating I have already created channels through the plaster. I have bought a narrow pvc conduit but realised it's too soft and would break when bending in corners. I was thinking of a copper tube as a metal conduit (if allowed), but I wouldn't be able to cover it completely in plaster. Not sure if there is an alternative. Can I just safely bury the cables in plaster?

 

[riveralt]

Can I just safely bury the cables in plaster?

Yes you can provided it buried at least 50mm deep and/or the circuit is protected by an RCD in which case below the plaster line.
The alternative is to mechanically protect the cable - I would consider copper pipe not to be capable of fulfilling this.

Other options are steel wired armoured cable and other similar variants.

 

[EFLImpudence]

Can I just safely bury the cables in plaster?

Yes you can provided it buried at least 50mm deep and/or the circuit is protected by an RCD in which case below the plaster line.

That's a bit confusing.

If buried less than 50mm. deep it must have RCD or mechanical protection.

 

[Ambergreen45]

So my understanding is I can bury my cables in plaster not because the plaster is only 15mm thick but because my Consumer Unit is equipped with an RCCB (RCD). Where would I be able to find some documentation? I have tried a search on the Blg Regs but probably missed it. It's just in case one day, if I'd sell the house... Sorry to be a pain.

 

[EFLImpudence]

//www.diynot.com/wiki/electrics:installation_techniques:walls

 

[Ambergreen45]

Thanks again, you've a great help!