Adding mains and lighting circuits to 1st floor radials

[loftconverter2011]

I'm hoping for a bit of advice here as I have followed guidelines thus far and come to a halt.

I've just completed the structural work on a loft conversion and have got as far as finding power sources for the lighting and mains circuits.

On direction I had a look at the existing setup which is a consumer unit running an 80A RCD with 6A MCB lighting circuits to the left (unprotected) and 32A MCB mains to the right along with cooker/showe protection.

I would find it very disruptive to access the consumer unit and add an MCB as the routing is very complex. Ideally I would like to tap into 1st floor circuits which are more accessible.

I have checked and the mains circuits appear to be radials (twin cables in, no continuity when powered down from RCD and multimeter bridging neutrals removed at the sockets back). This ties in with the information the last spark gave me when he tested the house.

I was hoping to tap into a double socket and extend upwards on 2.5mm cable to a 13A fused FCU and take sockets from there. However as its not a ring circuit I am unsure if this is advisable.

I feel stupid about asking regarding the FCU but the information I have seen so far is mainly covering ring only circuits.

Also, can I break into the existing lighting loop at looped ceiling rose and add 2 switches and 2 bulbs?



Please help!

 

[Chri5]

Are building control in the frame, beause if so it would be better to go the correct route via a spark.

If you do the work yourself they will charge to inspect it.

Lights- 1st floor 6 amp mcb

Taking a feed from here for 2 x lamps in the loft should be fine. Carp design because if the 1st floor mcb pops both loft and 1st floor will be in darkness.

If you really must then check the existing load via adding up the total wattage of all lamps on the circuit and add in the total lamps to be added on.

6 amps x 230v = 1980w load limit.

Ring- 1st floor

Taking a 2.5mm up to a f/spur with a 13 amp fuse will work, Again you need to assess the total circuit load AND that the intended circuit load in loft doesn't exceed 13 amp x 230v

What about smoke detectors ?
How will you heat loft space ?

 

[echoes]

The lighting circuit can be extended by branching the circuit at a convenient rose as you propose. Note that you will need to add an RCD to the lighting circuit unless you follow the rules for non-RCD protection, which will normally mean surface wiring or trunking.

As for the socket circuit: If it really is a radial circuit protected by a 32 A MCB, then you may extend it with minimum 4 mm² cable to supply more socket outlets, subject to de-rating / voltage drop / Zs limitations.

You need to ensure that it really is a radial - 32 A radials for sockets are not common. Have a look in your CU - what size are the cables for this circuit? are there 2 line conductors connected to the MCB?

If it is a ring, then you can supply multiple sockets via a 13 A FCU, but the spur will clearly be limited to 13 A load.

 

[loftconverter2011]

Thanks for the reply echoes.

I'm not sure they are radials.

On testing 3 different sockets I get no continuity and that's all I have to go on.

The MCBs are 32A and can be seen below.

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• loftconverter2011
• 12 Apr 2011
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• loftconverter2011
• 12 Apr 2011
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The mains MCB's are on the left and the cable seems to be considerable. It was rewired 5 years ago.

 

[echoes]

I'm struggling with those pics to see clearly, but they look like radials. Are the cables 4 mm² with stranded conductors?

 

[loftconverter2011]

Lights- 1st floor 6 amp mcb

Taking a feed from here for 2 x lamps in the loft should be fine. Carp design because if the 1st floor mcb pops both loft and 1st floor will be in darkness.

Thanks for the confirmation. Light isn't a problem as I have auxiliary (battery) lighting in the access areas/corridors.

Lights- 1st floor 6 amp mcb
If you really must then check the existing load via adding up the total wattage of all lamps on the circuit and add in the total lamps to be added on.

6 amps x 230v = 1980w load limit.

I'll do that

Ring- 1st floor

Taking a 2.5mm up to a f/spur with a 13 amp fuse will work, Again you need to assess the total circuit load AND that the intended circuit load in loft doesn't exceed 13 amp x 230v

Again, I don't mind checking that out. Most of the lighting is low wattage anyway for energy consumption reasons.

What about smoke detectors ?

I'm looking into interlinked mains wired systems. I did look at Kidde but got bad reviews and wireless interlinked (i.e. easier to site) are terribly expensive. Has to be done to comply though.

How will you heat loft space ?

I'm lucky enough to have planned ahead and the space is thoroughly insulated with Recticel. Also when fitting new central heating I routed pipes into the loft which just need tapping into for a radiator.

 

[loftconverter2011]

I'm struggling with those pics to see clearly, but they look like radials. Are the cables 4 mm² with stranded conductors?

Yes, 4mm (or slightly more) with copper stranded conductors. Is there any way (better/closer images) to determine more accurately?

 

[ban-all-sheds]

Are building control in the frame?

They'd better be.

lc2011 - when you applied for Building Regulations approval, what did you say would be the way you'd comply with Part P?

Do some research here to find the tales of woe from people who said one thing (or by default agreed that they would do one thing), and then did something else, and you'll see that that is an important question....

 

[loftconverter2011]

Are building control in the frame?

They'd better be.

lc2011 - when you applied for Building Regulations approval, what did you say would be the way you'd comply with Part P?

Do some research here to find the tales of woe from people who said one thing (or by default agreed that they would do one thing), and then did something else, and you'll see that that is an important question....

Building control have been involved at all stages. It's been a drawn out process and I have liaised with them on the structural side as we needed calcs from engineers due to the size of the conversion and dormer. I've put in internal walls and the staircase plus all insulation and plasterboarding. I am an amateur but quite capable in most respects provided the information I have is complete

I would of course be getting an electrician in to check/test all circuits and have taken appropriate advice concerning cable routing/insulation gaps etc.

I can not risk deceiving BC as I have foster children living here which requires certification including the smoke alarms (recommendations for these would be very welcome).

This is why I am enquiring here. I merely wish to cut down the costs by routing cables/cutting in boxes where appropriate. Connection will be left to the electrician.

BTW - I don't take offence to these comments, having read a few of these threads I am completely aware of the botch element of new posters

 

[echoes]

I'm struggling with those pics to see clearly, but they look like radials. Are the cables 4 mm² with stranded conductors?

Yes, 4mm (or slightly more) with copper stranded conductors. Is there any way (better/closer images) to determine more accurately?

Measuring it wilth a ruler is useless - it's the area you need to determine, not the diameter.

But it looks like this is indeed a 4 mm² radial circuit. The cable sheath may be marked with its cross-sectional-area. That would confirm it.

On the basis of the above, you may branch/extend that circuit without needing a FCU, provided that the cable is not subject to any de-rating factors (like being installed in contact with insulation) and its length is such that the voltage drop and Zs are within limits. You will need to use
the same size cable, i.e. NOT your 2.5 mm².

Hooray for radials!

 

[loftconverter2011]

Thanks again echoes.

I couldn't find any information on the sheathing other than the manufacturers details.

Given that 4mm it is!

Do I 'extend' from the last socket on the branch and daisy chain from there? I have identified where this is on the wall.

I only need 3 doubles for running a computer and things like bedside lamps, alarm clock, hair dryer etc.

 

[echoes]

Thanks again echoes.

I couldn't find any information on the sheathing other than the manufacturers details.

Given that 4mm it is!

Do I 'extend' from the last socket on the branch and daisy chain from there? I have identified where this is on the wall.

I only need 3 doubles for running a computer and things like bedside lamps, alarm clock, hair dryer etc.

You can branch from the most covenient accessory anywhere along the circuit. If it saves cable and is easier, then you can have further branches, branches off branches etc.
Tip: Use deep back boxes: you might get 2 x 4mm² cables in a 25 mm deep box if you're neat. 35mm deep for 3 cables.

Please confirm the cable size of that circuit before proceeding: you need to know, and building control will want to know. It's no good to 'assume'.

 

[loftconverter2011]

That sounds great - there is a double socket right below the stairwell, easy to access and good to chase in new cable as it has to be plastered above anyway.

I got my measuring gear out and the diameter is above 4mm per core. I say above because I would have needed to compress the insulation jacket considerably to get to 4mm - but its not 6mm

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• loftconverter2011
• 12 Apr 2011
• 2

 

[echoes]

Your calipers are measuring the diameter of the cable + insulation. This is not what you need. You need the cross-sectional area of the conductors.

 

[Taylortwocities]

To do this with stranded cable::

Unwrap the strands so you have just one strand
Measure the diameter of the single strand, halve it to get the radius
Work out the area of the single strand ( πr² )
Multiply by the number of strands

A very rough way to ESTIMATE the CSA is to measure the outside of the cable sheath (with the insulation). There are guide figures and more info here