loft insulation
- Thread starter Perth
- Start date
Yes, preferably clipped clear.
If not, correction factors would need to be applied. This may or may not require the cable size to be increased, depending on the size to begin with, and the loading of the cable.
If not, correction factors would need to be applied. This may or may not require the cable size to be increased, depending on the size to begin with, and the loading of the cable.
so if the cables are clipped clear what about the section of cable that runs through the insulation to be the bedroom light for example - here there would be say 225mm of cable running through the insulation to go throught the ceiling.
is it a problem in a standard house with say 3 x 1.0mm cable lighting circuits with max 800w of lights on each and 5A fuse.
Plus ring with 2.5mm that dosen't span more than 100m2
Can I read more about this anywhere?
is it a problem in a standard house with say 3 x 1.0mm cable lighting circuits with max 800w of lights on each and 5A fuse.
Plus ring with 2.5mm that dosen't span more than 100m2
Can I read more about this anywhere?
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Interesting question. Would people installing insulation be expected to know this needed to be taken into account? Which, I guess, amounts to asking if insulation installation needs to be subject to the same controls as electrical installation!!
Was going to run elec cables to inside (room side) of wall insulation so that they would be accessible if necessary. Present cables run I think under rafter insulation which is also the slope in the bedroom roof upstairsand down inside of insulation. There is no loft insulation and I wanted to install - so cables should go above.
Is going from inside to outside of inuslation not good?
With the back of socket boxes and ring cable the backs will be infront of the insulation as will the ring cables - is this ok?
Is going from inside to outside of inuslation not good?
With the back of socket boxes and ring cable the backs will be infront of the insulation as will the ring cables - is this ok?
For a lighting circuit, with a 5 A fuse or 6A breaker, the cable is very under-run if it is 1mm (the normal lighting) and even more so if it is 1.5mm. (see the cable selector charts referred to in 'for reference')
There is only a real problem if the cable is well insulated, and is running above its rating for that mounting condition - the cable feeding any one lamp holder isn't in that category. Possibly a cable feeding all the rest of the lights in the building needs to be considered more carefully, but there is usually only the one. In practice the risk to lighting circuits is very small.
If in doubt, check with the cable tables.
There is a greater risk from some brands of polystyrene used in roofs and cavities reacting adversely with the PVC insulation over many years, and causing it to go sticky and flow. In a well ventilated place this doesn't happen.
regards M.
There is only a real problem if the cable is well insulated, and is running above its rating for that mounting condition - the cable feeding any one lamp holder isn't in that category. Possibly a cable feeding all the rest of the lights in the building needs to be considered more carefully, but there is usually only the one. In practice the risk to lighting circuits is very small.
If in doubt, check with the cable tables.
There is a greater risk from some brands of polystyrene used in roofs and cavities reacting adversely with the PVC insulation over many years, and causing it to go sticky and flow. In a well ventilated place this doesn't happen.
regards M.
Have read the kevin boone cable selector in For refernce only to find the reader wouldn't show the tables of cables! will try a friend comp.
Can I just clarify what you mean by the cable being under-run.
We have 3 5A lighting fuseholders in the CU, each circuit is run in 1.0mm cable and has about 8 lights on each, on each circuit are 2 or 3 x4 term junction boxes and 1 larger junction box at the end, all in the loft and light/switch cables travel down from loft to their location. This is all existing. I have removed some of the lights in the house and re-directed some and altered the manner in how some of them switch. Thought after a few questions that this seemed simple enough.
However I gather from all this that now it seems that there must be a problem with what is existing if we want to put insulation in the loft and that our lighting is "under-run". If this term could be clarified it would be helpful - so that when I finally give up and get the electrician in i can ask him exactly what to rectify. Surely the PIR he did should have picked this up ?
Can I just clarify what you mean by the cable being under-run.
We have 3 5A lighting fuseholders in the CU, each circuit is run in 1.0mm cable and has about 8 lights on each, on each circuit are 2 or 3 x4 term junction boxes and 1 larger junction box at the end, all in the loft and light/switch cables travel down from loft to their location. This is all existing. I have removed some of the lights in the house and re-directed some and altered the manner in how some of them switch. Thought after a few questions that this seemed simple enough.
However I gather from all this that now it seems that there must be a problem with what is existing if we want to put insulation in the loft and that our lighting is "under-run". If this term could be clarified it would be helpful - so that when I finally give up and get the electrician in i can ask him exactly what to rectify. Surely the PIR he did should have picked this up ?
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Perth said:
Since nobody else has answered this... Under-run just means that the cable is capable of carrying more current than is typically the case in normal operation.The current carrying of 1mm twin and earth is a fair bit more than your 5A protection device, so even if there are some correction factors to apply then usually you'll still be OK. (In other words, it's basically a good thing
)Neal
Hi, thanks for picking up on my 'casual' terminoligy
To expand on what has been said
To under-run or under-load - to use at less than full capacity, so asking it to carry a lighter load than maximum. In a good design there is always some underrunning to allow a safety mrgin, and leave spare capacity in case someone adds something extra later on . A good thing
To run at the limit - to have no spare capacity, any more load or an increase in running temperature (by adding insulation) would be dangerous
To over-load (or over-run) To use beyond the makers recommended maximum, a potentially dangerous condition if this persists.
What I meant was, from what you described, your lighting circuit was fine, and even has some unused capacity for more lamps to be added later if you want. Sorry if I scared you!
As an aside 1mm cable is good for more than 10 amps (2400 watts - thats a lot of light bulbs!) if it is well ventilated, and more than 5amps even if sandwiched between a wall or ceiling and insulation. Most house wiring is conservative in the sense that the lighting wires could be thinner, and still safe but the problem with anything thinner is mechanical strength, not overheating.
For ring main cables, as a rule of thumb, the usual 2.5mm is just about good enough when well insulated in a wall or whatever, but more than enough if open to air on one side to allow cooling. However, the real detail is in the tables of course.
regards M.
To expand on what has been said
To under-run or under-load - to use at less than full capacity, so asking it to carry a lighter load than maximum. In a good design there is always some underrunning to allow a safety mrgin, and leave spare capacity in case someone adds something extra later on . A good thing
To run at the limit - to have no spare capacity, any more load or an increase in running temperature (by adding insulation) would be dangerous
To over-load (or over-run) To use beyond the makers recommended maximum, a potentially dangerous condition if this persists.
What I meant was, from what you described, your lighting circuit was fine, and even has some unused capacity for more lamps to be added later if you want. Sorry if I scared you!
As an aside 1mm cable is good for more than 10 amps (2400 watts - thats a lot of light bulbs!) if it is well ventilated, and more than 5amps even if sandwiched between a wall or ceiling and insulation. Most house wiring is conservative in the sense that the lighting wires could be thinner, and still safe but the problem with anything thinner is mechanical strength, not overheating.
For ring main cables, as a rule of thumb, the usual 2.5mm is just about good enough when well insulated in a wall or whatever, but more than enough if open to air on one side to allow cooling. However, the real detail is in the tables of course.
regards M.
Different story with shower cables though - if you have those in the loft then they must not be insulated.
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