I am building a timber frame extension and need of course to install a ring main. Sockets need to to be 400 mm high so I will be running the cable through walls packed with insulation board. What is the accepted practice for routing cables so that they are in sufficient free air?
ring mains in insulated walls
- Thread starter elmsoft
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Why "of course"? Have you considered whether a radial(s) might be more appropriate?
The accepted practice is as follows:
- Decide where the cables will run, having due regard to the rules for concealed ones in 522.6.
- Determine what Installation Method will apply, as per Table 4A2.
- Work out the current carrying capacity of the cable, given the Installation Method, and any other applicable derating factors, as per Appendix 4.
- Pick a cable size that's OK for voltage drop and complies with Ib ≤ In ≤ Iz as per Section 433.
- Install the cables, and test & certify them as per Part 6.
When you applied for Building Regulations approval, what did you say, or by default/tacit acceptance agree, would be the way that you would ensure the work complies with Part P?
In most cases, for sockets, ring circuits are better than radial circuits.
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Nah.
First floors run quite happily off a 20A radial.
As do ground floors.
And kitchens can have a 32A radial.
Ring finals are the spawn of the devil.
First floors run quite happily off a 20A radial.
As do ground floors.
And kitchens can have a 32A radial.
Ring finals are the spawn of the devil.
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It looks as if your list may need a little re-ordering, and other attention. It would be quite difficult to 'work out the CCC of the cable' before one had 'picked a cable size'. However, merely swapping (3) and (4) would introduce other anomalies - so a general re-think may perhaps be needed.
Kind Regards, John
As has been pointed out, it really is not as simple as that. For example, if the cable were surrounded by insulation and not touching the inner (plasterboard) surface of the wall, then the regulations wouldn't allow you to use 2.5mm² cable (or even 4mm² cable) for a ring final circuit.
Kind Regards, John
It must be touching the inner surface, i.e. the plasterboard, (not run through any thermal insulation) when its capacity is reduced to 21A which is satisfactory for either a 32A ring, with balanced loading, or 20A radial.
I like rings because they can serve a much larger floor area.
Indeed. I was probably wrong to talk somewhat 'in riddles', but that is essentially the corollary of my:
Kind Regards, John
Whilst that is generally true (because of VD considerations), do you often come across situations in 'ordinary houses' in which radials could not serve as much floor area as is required?
Kind Regards, John
I've never understood the relevance of floor area without reference to the shape or number of walls within it.
A 20 amp radial in 2.5 mm2 should only serve a 20 m 2 area.
That's about the size of a large room.
Though it's not always clear, you also need to keep the length of cable down too.
That's about the size of a large room.
Though it's not always clear, you also need to keep the length of cable down too.
Yes, I agree - that's why I said 'generally' (but it probably usually 'works'). Is the habit perhaps related to regs and guidances which talk(ed) about floor areas served by sockets circuits?
Kind Regards, John
The OSG (and even that's only 'guidance') says 50m² (and 75m² for a 32A 4mm² radial), which is not far off a whole floor of many a small houses.
Indeed - as I said, VD is likely to be the limiting factor.
Kind Regards, John
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