Ceiling insulation effect on electrical work

[duncan drennan]

I'm looking at having some cellulose insulation put into my ceiling. Basically they spray the stuff all over which results in good coverage.

From an electricians perspective, what impact can they have on the workability, and what should they do to make sure that everything is still up to COC spec?

Also, what is your experience of this type of insulation versus Aerolite or Isotherm?

[Electrick]

I can honestly tell you , that any insulated roof is " hell " to work in.
The spayed stuff , you're talking about covers all wiring , junction boxes,
ect. Point is , it's i'd prefer to work in an uninsulated roof.

The reality is that most roofs are insulated. so , go ahead and insulate your roof.

I wouldn't go for the spay though. ( This is just an opinion ) : It's like covering your ceiling with an inch of dust .

[Leecatt]

Originally Posted by Electrick

I can honestly tell you , that any insulated roof is " hell " to work in.
The spayed stuff , you're talking about covers all wiring , junction boxes,
ect. Point is , it's i'd prefer to work in an uninsulated roof.

The reality is that most roofs are insulated. so , go ahead and insulate your roof.

I wouldn't go for the spay though. ( This is just an opinion ) : It's like covering your ceiling with an inch of dust .

Look the answer here has to be that your cables are protected by a circuit breaker. The size of the breaker is determined by the conditions under which the cables are installed. For instance, if you have a 2.5mm cable running in a conjute from circuit breaker to plug then you protect it with a 20amp circuit breaker as the highest amperage it can draw is 24amps according to SANS (you do not get a 24amp circuit breaker). Now you go and toss that insulation over it and in my opinion it must now be seen the same as if it is mounted in an insulating wall. This means the amp rating on the cable gets derated to 19.5 amps and the circuit breaker should be replaced with a 15 amp version. Now try passing that one over on the seller!!

[Dave A]

Reading that, a thought occurs...

How much effect would ambient temperature have on the maximum current rating of the cable?

[Leecatt]

According to SANS the ambient temperature of 30 degrees celcius and a cable opertating temperature of 70 degrees celcius is taken into account when rating the cables.

[murdock]

i would say so long as it doesnt interfere with the integrity of the electrical installation it shouldnt be a problem...ie if there are no exposed wiring downlights etc in the roof there shouldnt be a problem.

the ambient temperature should be less because the wiring will be under the insulation the heat will be above the insulation...as mentioned this stuff is a nightmare for electricians...especially the pink aerolite...just my opinion i could be wrong.

[Dave A]

Originally Posted by Leecatt

According to SANS the ambient temperature of 30 degrees celcius and a cable opertating temperature of 70 degrees celcius is taken into account when rating the cables.

So the current is causing a fair amount of heat generation then.

My thinking was going something like this:
First, roof temperatures can get to 50 degrees celcius in summer, if not more.
From there, insulation might protect the cabling from ambient temperature, but you are also losing the ability to disipate heat generated by line resistance.

There must be fairly aggressive safety tolerances built into these calculations though, surely?

[AndyD]

The reason the cable gets derated in an insulated environment isn't because of the ambient temperature, it's because it's ability to loose heat is reduced. The cable has a resistance which generates heat along its length when current flows within it, this heat will cause higher temperatures if the cable is insulated than if the cable is in free air.

[desA]

Originally Posted by AndyD

The reason the cable gets derated in an insulated environment isn't because of the ambient temperature, it's because it's ability to loose heat is reduced. The cable has a resistance which generates heat along its length when current flows within it, this heat will cause higher temperatures if the cable is insulated than if the cable is in free air.

Very interesting. Learned something new today.

[Jacques#1]

my 2 cents worth....

-A cable rarely runs at its maximum CCC. This is even more the case if an installation is planned correctly and laid out correctly. Keep in mind that the heavy residential items on a non-dedicated S/O circuit like tumble driers, washing machines, garage Sockets (welding machines) does not run for very long, especially not all together. Maybe you'll run your washing machine and tumble drier together, and maybe you'll do that for 6 hours during the day but the fact is that even they draw current intermittently in their cycles.

-If you talk to the cable manufacturers they will give you the actual current carrying capacity of their manufactured cables. SANS in my mind sets an industry standard since not every second guy will phone Aberdare or who-ever to request their amp ratings etc.