They are technically correct -- in practice there isnt a problem

So long as you have at least 300mm of insulation you will be ok

The current carrying capacity of a cable is related to its cross sectional area (1mm2 , 2.5mm2 etc) at a set range of temperature
if this temperature is likely to remain around the higher end of the scale ( as its covered in insulation) then this needs to be taken into account
when designing an electricial installation and choosing cable sizes
the installation method of the cable changes the rated current carrying capacity of each cable
If a cable is say clipped along a joist and surround by free area its rated to carry a higher current than say one covered in 100mm insulation
for example a 2.5mm2 twin and earth copper PVC cable clipped direct would be rated up to 27 amps. Above a plasterboard ceiling covered by not more than 100mm it would be rated at 21 amps
If then insulation >100mm then it drop to 17amps. which if in a ring circuit ( which is 2, 2.5mm2) protected by a 32 amp fuse is fine
If on a radial (single 2.5mm2) which should be on a 16 amp fuse, again its fine
This by the book
As long as your electrics is up to standard and you've a newer fuse board it shouldn 't be a problem

cheers Jim

Thanks, I hadn't considered the effect of temperature on current carrying capacity.

Any thoughts on my second question, i.e. foil backing for insulation? I'm leaning towards putting foil under the insulation, but I don't know if this is a waste of time. I hate going in the loft ;-)

Thanks
Simon

Never ever use 'rule of thumb' when deciding current capacity ratings of cables. Calculations must be carried out to determine if the cable used is of sufficient size to carry the required load (this will vary if the cable is covered in insulation or not, buried in walls, run through wall penetrations, run along side other circuits etc). In practice there may not be a problem, however complying with the 17th edition of the wiring regs is a legal requirement. If you are in a new build you should take some comfort in the fact that the installation should meet current regulations, if not, your developer has broken the law.

If you want to satisfy yourself that everything is ok, there are plenty of books to be found on amazon that give guidance to complying with the regulations and provide examples on how to calculate current carrying capacities of cables. Books providing guidance to complying with the regs are usually cheaper to buy than the regs themselves and much easier to understand.

[edited a few times to try and get this brain-dead forum software to display quotes AND hyperlinks that work. No such luck - the authors deserve a thorough kicking ]

Posted By: RoobarbThe loft in our new home is insulated with glass fibre blanket. Some of this insulation covers the electrical cables running through the loft, but our Reader's Digest DIY Manual claims that you should avoid covering cables with insulation "so that there is no danger of them becoming overheated". Is this correct? It seems a bit odd, because I can't see why keeping the cables warm would be dangerous. Surely electrical cables can withstand temperatures much higher than you'd get in a loft, even in summer?

The issue is that current flowing through cables make them warm, and if they can't get rid of the heat they get too warm, then the insulation softens, then where the cables are bent or not properly supported the conductors migrate through the PVC, and then eventually they touch, and Bad Things happen.

Insulation has a marked effect on current carrying capacity - a cable surrounded by insulation can only carry half the current that it can if clipped to the surface of a wall or covered in a bit of plaster.

Posted By: BluemoonA good reason for not covering cables with insulation is, you can find them afterwards. The contractors who put the glassfibre blanket in my loft told me that they are told to cover all except for the shower and cooker cables.

See here: http://www.diynot.com/forums/viewtopic.php?t=154936 for what happens when contractors are told to do that.

And if they've done it to you, you should get it sorted before Bad Things happen...

Posted By: jamesingramIf then insulation >100mm then it drop to 17amps. which if in a ring circuit ( which is 2, 2.5mm2) protected by a 32 amp fuse is fine

No it's not - 433.1.4 requires that the capacity not be less than 20A...

DaveK, 'never ever' is too strong. What's wrong with the rule-of-thumb that in a normal size house with normal size cables in a loft that only do lighting it will always be fine. There is no point doing calculations - we know what the answer will be: said calculation has been done done hundreds of times before. The law requires the regs to be complied with, not explicitly that calcs are done (SFAIK). After all, if you are building 10houses, you'll only do the sums once.

What are you all doing up at this time, anyway? Surely not the US elections...?

Quote from Tony "How come everyone uses 2.5 for ring mains when even running it down a new wall insulated to building regs and plastering it in doesn't comply? "

dont follow you ? You just size the MCB/fuse to suit
2.5 enclosed in conduit in a insulated wall is rate up to 20amps (2*20=40 so 32 amp MCB)

the only time you get a problem, is if the cable is in a stud wall filled with insulation and the cable is not touching the inner wall surface
its rated up to 13.5 amps in this case. for a ring you just use a 20 amp MCB
I recently did something similiar but ran radials,I then relised I had to put them on a 10 amp MCB . which is not ideal as they can only power equipment up to
2.3kw . eg. a kettle and toaster will trip it out

cheers jim

CommentAuthorban-all-sheds" No it's not - 433.1.4 requires that the capacity not be less than 20A..."

dont follow you sorry , 433.1.4 in the 17th ?, I cant see the bit you mean

Following 433.1.4 if the cables buried under the insulation then the protective device should not exceed 0.9 of Iz
so 34 *0.9=30.6 so a 20amp mcb should be used , is this what you mean?
or are you saying the capacity of the any cables in a ring should be no lower than 20amps?

regarding light circuits ,the amount of downlights people seem to go for now, 1mm2 might well not be up to the job in a superinsulated loft
and how many people do you know running thier 9.5kw electric shower on 16mm2 ?

cheers Jim

How much leeway d'ya get in all this?

My cables are carefully above loft insulation almost everywhere, but have to dive through it obliquely for up to 50 cm.

Also, above suggests major difference between surrounded by insulation, and resting on ceiling/clipped to joist and top covered by insulation. True?

Jacky R for reference
From the latest regs book, current carrying capacity of cable completely surrounded in more than 100mm of thermal insulation for more than or equal to 0.5m
(p282 table 4D5)

1 mm2 twin& earth 8 amps , a 6 amp MCB/fuse is used in this case for domestic lighting circuits . approx. max load 1380 watts (P=VA , 230*6) so OK
1.5mm2 T&E 10 A
2.5mm2 T&E 13.5 A (27A ring)
4 mm2 T&E 17.5 A
6 mm2 T&E 23.5 A
10 mm2 T&E 32 A
16 mm2 T&E 42.5 A

if its less than 0.5m the derating factors are . (in the above table, the values have already been derated by 0.5)
50mm =0.88
100mm=0.7
200mm=0.63
400mm=0.51

yes, there is a difference between cables surrounded and laying on insulation a one will lose heat quicker so can carry more current

as mentioned above electric shower cables are a potential problem, most other stuff should be fine

a 9.5Kw shower may use up 41.3 amps A=P/V (V=230) so in the above situation a 16mm2 cable should be selected
most showers are run on 10mm2 , and I've seen them wrongly run on 4-6mm2 as well
I once burn my hand on a dangerously undersized cable running a shower as it was so hot.

If your in any doubt you should get a qualified electrician to have a look

cheers Jim

probably old news to you , but a lot of the energy for washing machines is used in the water heating so , 30 or cold washing is the way to go
also if its a washer/dryer dont use the dryer bit , unless of course you're in a rush to dry that item of clothing for a night out on the town.

Average should be one load per person a week.

cheers Jim