Hi might sound a daft question but does it matter how cable runs through insulation behind stud walls. From top hole on wall I’ve fed rod down at the back of insulation but rod pops out at the bottom hole on the wall at the front of the insulation?
Feeding cable through insulation
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yes it does matter, but we need more info
Skirting is removed and I’ve got a cable coming from under the floor boards through the bottom of the wooden stud frame with a hole cut out on the plaster board. I’m running the cable around 6ft above the opening at the bottom of floor that is cut out. 6ft high I’ve cut another patch out of plaster board. Behind the plaster board is thick insulation.
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Cable needs derating when in contact with insulation . Also,depends on how much of cable is totally enclosed in insulation. And thickness of insulation
Also is the cable protected with an RCD?
Also is the cable protected with an RCD?
Hi thanks for the reply, the cable is ran of the lighting circuit which is rcd protected.
The amount of cable totally enclosed at first point is 6ft then linking out under floorboards to next point which is also 6ft linking out under floor again to next point which is 6ft total in insulation.
Insulation is 3.5” thick
The amount of cable totally enclosed at first point is 6ft then linking out under floorboards to next point which is also 6ft linking out under floor again to next point which is 6ft total in insulation.
Insulation is 3.5” thick
Yes it’s only lighting. I’m linking from original switch out to photo cell then linking from there out to feed 4 led low watt lights. Cable to feed the Solar cell will be buried in 6ft of insulation and only 2 lights linking out after that will be in 6ft of insulation. The other 2 lights will be behind no insulation.
Does this seem safe enough to do? Should cable be at the front (plasterboard side) or at the back the exterior wall side?
Does this seem safe enough to do? Should cable be at the front (plasterboard side) or at the back the exterior wall side?
Sensai
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When you chose the cable you needed to factor in the insulation basically put because of heat and increased resistance in you cable. BS7671 includes tables how to do this if you haven't or if you Google it they can be found elsewhere. If you use conduit the factor will be different as less heat /resistance will build up in the cable.
Sensai
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The conduit has a lower rating to add in to your calculations. Without having any figures no-one else can really tell you what is safe and correct to use but conduit will mean less resistance and heat them through the insulation.
At my consumer unit my downstairs lighting circuit is ran from a 6amp breaker RCD protected. It runs 7 low wattage led lights atm and I want to add 5 more low wattage led lights. So 12 lights total will be ran from lighting breaker. 6amps can handle 1440watts. If I’m to derate by 50% as some cables will be in insulation or in conduct in insulation this would drop the wattage to 720watts? 12 lights led lights at say 5watts each would be 60watts = 0.25amps. Do these calculations seem correct in what I’m saying in which case I’m perfectly safe to run my few cables behind the insulated wall?
Any help would be great.
Any help would be great.
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From reading the above I'm not sure you understand the concept of de-rating..?
It is the protective device, (in your case 6amp MCB), that governs how much current can flow in a circuit & thus how much power, load can be connected.. (nominal voltage for calculations is 230v not 240v.. 6Ax230V=1380watts)
Irrespective of how you install the cable, the 6A MCB will still allow this current to flow in any circuit cables before it would disconnect the supply in the event of a fault..
So you have to ensure that your cable has sufficient capacity proportional to the protective device rating.
and with your 6A MCB you will always have the full 6A load capacity...
BUT.. you have to ensure all of your cables also have sufficient capacity..
All cable manufactures have specifications for the maximum capacities of their cables...
Then it is these cable capacities that are de-rated dependant upon the installation methods used.
IF the de-rated values are below the protective device rating,
Then either you reduce the protective device or increase the cable size, (& thus capacity)..
De-rating doesn't mean you just reduce your connected load..
The basic concept is that you should be designing every electrical alteration to fail-safe in the event of a fault..
which includes ensuring the protective devices disconnect the supply before any cables overheat and/or ignite..
And ensure protective devices disconnect the supply fast enough to reduce the risk of electric shock..
I haven't used 1.0mm as the main supply from an MCB for many years now..
I only tend to use it for switch drops, 2-way strapper's or as the final branch leg to a single luminaire..
I tend to use 1.5mm as the main permanent live feed throughout a lighting radial circuit.
I haven't got my books to hand at the moment to double check 1.0mm capacities..
As it is that time of day for some nibbles & an alcoholic beverage before bed.
Hope that helps a bit?
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that's not quite how it works to derate the circuit you should make the mcb smaller, or the cable bigger,
what size did you say was the cable you have is ?
the total length of the circuit matters and there is a very long list of things that we need to take into account when designing a circuit ,
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