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Was reading an older thread and dont have any books to hand, but basically.

If a cable for a 10kw shower is to be run, the current drawn would be 43.4 A.

So if 6mm cable was appropriate for say 70% of the run that was clipped direct, but not suitable for 30% of the run due to de-rating factors, as it would be covered by insulation . Could you size up to a 10mm cable just before the cable hit the insulation part of the run, the drop back to 6mm after the insulation where it may be clipped direct again.

I would like to note that this a hypothetical question, and the thought behind it was that if you had a long run then being able to buy only 30% of the run in 10mm would save you monies on cables.

Also (on a separate note) with gu10 firerated lights is there a recommended clearance of insulation from around them i.e. in a loft where the lights come up from ceiling below, as I presume you would want as much insulation to remain as possible.

Hope that all makes sense, thanks.
 
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Lamps with aluminum reflectors help with the 2nd issue by deflecting all the heat from the lamp downwards, normally i just push the insulation aside to 6" or so , to be honest im not sure if there is a recommended distance across the board
 
S

sparks1973

and for how much (length) is the cable run in insulation?
 
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sparks1973

Table F2 OSG states that derating for insulation for 500mm and over is 0.50....so you have a 10KW shower...the design current (Ib) for this will be 43.47A...so the current carrying capacity (Iz) for the cable has to meet this AFTER any derating is carried out....
 
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sparks1973

look, if a portion of the run is ref. method 100 or method A......then why not just run the whole lot in 10mm?....
 
  • Thread Starter Thread Starter
  • #6
thanks for the swift replies,

to be honest I'm not really after the calcs and specifics just merely curious as to whether the derating factors only apply to specific parts of a run and if so is it acceptable to use larger cable for just this part, so you could have a JB or isolator switch to go from the smaller cable to the larger cable prior to the length of run within the derating area, for lack of a better term (which would be suitable after applying the correction factors in the given conditions)

If this is still unclear, im picturing 45A breaker, 6mm cable under floor boards, rise up beneath stairs, into airing cupboard, up into loft, across insulation (at which point im asking if, if this part of run would derate the 6mm then could you run 10mm until your away from insulation) down into 45A DP shower pull cord, then cable back up and dropped down stud wall cavity for 10kw shower...
 

Richard Burns

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Was reading an older thread and dont have any books to hand, but basically.

If a cable for a 10kw shower is to be run, the current drawn would be 43.4 A.

So if 6mm cable was appropriate for say 70% of the run that was clipped direct, but not suitable for 30% of the run due to de-rating factors, as it would be covered by insulation . Could you size up to a 10mm cable just before the cable hit the insulation part of the run, the drop back to 6mm after the insulation where it may be clipped direct again.

I would like to note that this a hypothetical question, and the thought behind it was that if you had a long run then being able to buy only 30% of the run in 10mm would save you monies on cables.

You could do this and meet the regs but the cost of buying the junction boxes /crimps and the time taken to connect the cables would likely outweigh the additional cost of the cable.

Also (on a separate note) with gu10 firerated lights is there a recommended clearance of insulation from around them i.e. in a loft where the lights come up from ceiling below, as I presume you would want as much insulation to remain as possible.
Follow the manufacturers instructions, they vary widely.

Hope that all makes sense, thanks.
See the notes above.
 
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  • #8
look, if a portion of the run is ref. method 100 or method A......then why not just run the whole lot in 10mm?....
I am aiming to hypothesise a situation that may never arise, but point being, if the run is 50m and only 10m is through insulation and needs 10mm, and the other 40m can be run in 6mm then why spend all that extra money to get 50m of 10mm as im sure 10m of 10mm and 40m of 6mm is cheaper that 50m of 50mm

I think I have made a simple question quite confusing....
 
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  • #9
Also sorry for alot of these posts as they keep springing up whilst im writing something
 
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  • #11
50m 10mm T+E £170

10m 10mm T+E £35
50m 6mm T+E £ 75

This is just a question i wondered to myself, and so the length of runs could vary but as long as it conformed to 7671 and the joint in cable was necessary due to derating, then on a '50m' run you can save £60 or 1/3 of the cost of cable by this method. I would again like to stress I am not contemplating this as an installation I was just wondering and thought someone may be able to satisfy my curiosity.
 
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sparks1973

well as long as joints were accessable for inspection/maintainence....but i wouldn`t do it....just pull a 10mm in....

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you would price accordingly....and if the client wont see it.....
 

ipf

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Have you thought about volt drop. If the run is 50 metres.....work things out.
 
D

drew35

Isn't there a reg about stepping up cable size? Thumbing my big green one now!!!
 
E

Engineer54

Isn't there a reg about stepping up cable size? Thumbing my big green one now!!!
Think your referring to stepping down a cable size!! The bigger the cable the better, except for your, or your clients bank balance!! lol!!!

Getting back to the topic, yes you can do it this way, but no self respecting spark would do so, your just adding unnecessary joints and weakness into a circuit. On top of that, it's going to probably work out cheaper to run the circuit as it should be done, in the lager cable size, when considering the overall costs...
 
Stepping up, or down, depends on which way you are looking at it :)
Personally, I wouldn't change cable size to allow for derating through a small section. Remember, a cable should be selected appropriately for all factors that affect it. If you take this hypothesis to the nth degree you could end up with several connections in one length.
 

ipf

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Stepping up, or down, depends on which way you are looking at it :)
Personally, I wouldn't change cable size to allow for derating through a small section. Remember, a cable should be selected appropriately for all factors that affect it. If you take this hypothesis to the nth degree you could end up with several connections in one length.
Maybe on an outside lighting circuit, for example, wired in, say, 6.00mm because of volt drop, some of the further points could be wired in smaller...?
 

imago

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As already mentioned it could be done, but could doesn't equate to should. The only time I would do something like that would be the run to and from the isolator. So you may have 6mm to the isolator and 10mm from the isolator to the shower as it runs through an insulated stud wall. Doing it that way though is more about convenience for running the cable and making it off than the cost of cable.
 
E

Engineer54

Maybe on an outside lighting circuit, for example, wired in, say, 6.00mm because of volt drop, some of the further points could be wired in smaller...?
Virtually every street/road lighting arrangement is installed this way. as the load get's lighter the supply cable size is reduced. Lucy and other street lighting protection equipment suppliers, have standard post equipment that facilitates this arrangement....
 

ipf

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Virtually every street/road lighting arrangement is installed this way. as the load get's lighter the supply cable size is reduced. Lucy and other street lighting protection equipment suppliers, have standard post equipment that facilitates this arrangement....
Exactly my point.
 
S

silverfox

50m 10mm T+E £170

10m 10mm T+E £35
50m 6mm T+E £ 75

This is just a question i wondered to myself, and so the length of runs could vary but as long as it conformed to 7671 and the joint in cable was necessary due to derating, then on a '50m' run you can save £60 or 1/3 of the cost of cable by this method. I would again like to stress I am not contemplating this as an installation I was just wondering and thought someone may be able to satisfy my curiosity.
Its not that simple. Are we pricing for a junction box as well. How long would it take you to mount the box and make it off.
 
Virtually every street/road lighting arrangement is installed this way. as the load get's lighter the supply cable size is reduced. Lucy and other street lighting protection equipment suppliers, have standard post equipment that facilitates this arrangement....
My apologies. I thought we were hypothesising about a standard domestic situation.
I will get my coat... :waving:
 

telectrix

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cheaper to rip out and bin a couple of yards of fibreglass, and use 6mm throughout.
 
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sparks1973

Think your referring to stepping down a cable size!! The bigger the cable the better, except for your, or your clients bank balance!! lol!!!

Getting back to the topic, yes you can do it this way, but no self respecting spark would do so, your just adding unnecessary joints and weakness into a circuit. On top of that, it's going to probably work out cheaper to run the circuit as it should be done, in the lager cable size, when considering the overall costs...
quite right eng...the voice of sense....
 

telectrix

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not on my jobs he don't. amazing the reaction to a handfull of fibreglass down the back of his y fronts.
 

Strima

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amazing the reaction to a handfull of fibreglass down the back of his y fronts.
Oi, non of that talk on here kinky, what you get up to in your own loft is your business, not mine... :lol:
 
  • Thread Starter Thread Starter
  • #30
Ok, so basically you could, but whether you should or would is different. That is fine, as this isnt a real situation there is no need to panic.
Also with regards to the junction box costing extra money and time, surely pulling a 10mm cable over a '50m' run (with v.d excluded and not being taken into account) would be more tedious than pulling a 6mm cable...

Nonetheless my question has been answered and thanks for that !

.....one more thing ha, if you were to design a circuit with all things considered and no insulation around any cable, then mr ecosulation man comes and dumps a crap load of insulation on your cable thereby severely underrating it and a fire starts would he be solely liable and responsible?
 
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sparks1973

well thats just like suggesting that volt drop isn`t a factor......jesus..lol...
 
ultinator;579554 then mr ecosulation man comes and dumps a crap load of insulation on your cable thereby severely underrating it and a fire starts would he be solely liable and responsible?[/QUOTE said:
Imho yes. You have designed properly and installed, inspected and tested to 7671 and have copies of the paperwork to prove it. Mr Insulation comes along with his ignorance and screws everything up.
I'd say that ignorance of our regs is no excuse, after all we're supposed to be aware of all building regs so why shouldn't he?
 
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Silly Sausage

.....one more thing ha, if you were to design a circuit with all things considered and no insulation around any cable, then mr ecosulation man comes and dumps a crap load of insulation on your cable thereby severely underrating it and a fire starts would he be solely liable and responsible?
Normally I would say, if your install complies at the time you did it, no problem, you can't be held responsible for what happens to it in the future.
But...if you were laying cables in an uninsulated attic on the ceiling below (you know what I mean!), for example, there is a good chance that cables would be covered over in the near future, especially with these free loft insulation offers going. Had mine done for freebies a couple of years ago.
 

telectrix

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quite right. would he pile a foot of fibreglass on top of the kitchen hob, or cram it roung the C/H boiler. ... NO. so why should the muppet treat our installations with such disrespect.
 

tigerpaul

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If you can reasonably foresee a situation where thermal insulation could compromise the integrity of your installation, then you should take the appropriate course of action to protect the electrical system.
 
S

sparks1973

quite right. would he pile a foot of fibreglass on top of the kitchen hob, or cram it roung the C/H boiler. ... NO. so why should the muppet treat our installations with such disrespect.
because he`s a `green industry` muppit...or is that puppet?..
 
So much easier to keep cables above the insulation in a loft, then take them through the insulation in oval conduit. Saves a lot of worry.
 

telectrix

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If you can reasonably foresee a situation where thermal insulation could compromise the integrity of your installation, then you should take the appropriate course of action to protect the electrical system.
good thinking. a 12 bore in the attic with a piece of string attached to the hatch. that should sort it.
 

telectrix

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then again, in a house less than 40 yrars old, it would probably collapse the roof trusses,, explode the water tank that the kind plumber has placed in the only space that you can crawl through, bonus free sprinkler system.
 

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