Running cable in loft conversion

[martinxxxxxx]

To quote you

A horizontal wire will lose convective heat away from the wire. A vertical wire will lose heat into the same space as the wire above it, heating it up.

As I see it the basic physics is that there are 3 ways in which heat can be dissapated away.

1. Convection currents through gas or liquid, this is the heat rises case.
2. Conduction - through solids - in all directions equally.
3. Radiation - through everything (almost) - in all directions equally

So considering our wire.

Convection currents will be set up only after the heat has been dissipated to free air, in our case through the insulation it is surrounded by, so it matters not whether it is vertical or horizontal. Conduction conducts equally in all directions, as does radiation. So I don't believe there is a case for considering the orientation of the wire when deciding whether to use Method 103.

Even if we look Table 52.2 it would have us derate the cable by 0.51 if it were run in any longer length than 400mm which is 13.77A for 2.5mm t/e.
So we are back to 4mm.

Sorry if this appears to teach granny how to suck eggs.

Martin

 

[ban-all-sheds]

Would I be right in assuming that the reason the cable capacity is derated further in 103 to that in 101 (even though both diagrams have a cable clipped direct to the joist) is that in the 103 method it is enclosed top AND bottom by plasterboard? (wow what a long question)

I hadn't thought about it before, or looked at the similarity between the diagrams.

IHNI why the cable clipped to the joist on the left in diag 101 has a higher CCC than the one clipped to the stud on the left in diag 103.

It could be the orientation issue suggested by chapeau, but 523.7 (same derating factor as 103#) says nothing about that.

 

[ban-all-sheds]

Makes sense to me and might explain why the current carrying capacities are different, and why using methods that specifically say 'stud wall' should not be used in ceilings.

OK - the standard methods don't always apply, so I guess we should have gone by the description of what he said he was going to do, recognised that 100-103 didn't cover it, and gone with the figures from 523.7.

Still, not to worry, as the OP assures us -

i thought it was pretty straight forward.

i will find the correct solution and use the 2.5mm.

And don't forget - he's got an electrician prepared to falsify EICs and lie to the council, so the paperwork will be spot on.

What could go wrong?

 

[JohnL79]

Have you got 2 x 2.5mm cables going into loft space to create a ring or just 1 x 2.5mm cable for a radial?

If its 2 x 2.5mm cables do you not double the figures in the "Installation Reference Method" table?

 

[electronicsuk]

Have you got 2 x 2.5mm cables going into loft space to create a ring or just 1 x 2.5mm cable for a radial?

If its 2 x 2.5mm cables do you not double the figures in the "Installation Reference Method" table?

You need two cables rated at >20A after applying factors to run a ring. You could use something other than a 32A protective device, but you'd be on your own as far as circuit design goes.

 

[chapeau]

Convection currents will be set up only after the heat has been dissipated to free air, in our case through the insulation it is surrounded by, so it matters not whether it is vertical or horizontal

I agree with your understanding of basic physics.

But the insulation is essentially a lot of small air pockets surrounded by a material which traps them, and which itself is not a particularly good insulator. Most of the insulation comes from the trapped air.

So each cell of air will be warmer at the top than the bottom (convection), this will be transmitted via the insulation (conduction) to the cell above, which itself will be warmer at the top than the bottom (convection) - and so it goes on.

So the effect on a vertical wire will still be to warm the wire above.

Now I admit it won't be much of a warmimg effect, but if you look at the figures, the current capacity isn't too much different, but it is.

It's not a perfect insulator, and I suspect that any heat that does get out will be transmitted mainly by the method just described. And if you look at the orientation of the fibres in rockwook type insulation, they appear to be aligned to trap the air better when laid horizontally, so I suspect it is a better insulator when laid horizontally.

Just an idea.....

 

[marsaday]

update:

had the electrician in today and everything fine.

This is what we have done. Rewired the lighting circuit for the 1st floor as all wires were laying on insulation and over the joists.

they are now laying on the ceilings or going through the ceiling joists. insulation has been removed where these wires are now.

Above all this will sit my new floor. in between these 8" joists will be 200mm insulation.

There will be a 4" gap between the cables and the bottom of the insulation. i will lay netting over the cables so the insulation does not fall down and rest on them.

Sparky is very happy with this solution.

I await for BAS to haughtily put me down, scald me and make himself look oh so clever !!