Insulation and Elec Cabling

[Programmer]

Hi all, am currently installing batts into the ceiling of our Cal Bung (there was zip insulation there...:eek: )

Now we all know that you're not supposed to install the batts over the top of the wiring (risk of overheating etc etc). My problem is that between 50-60% of the cable runs underneath the ceiling joists and in many cases I cannot 'lift' it enough to get it on top of the batts, there's just not enough cable.:( This is probably due to some slack sparkies (or maybe they were trying to be nice and save the previous owners some $, who knows).

I know this seems like a silly question, but considering the wiring might be, ummm, about 20 years old, what chance is there of that wiring overheating? The cable appears to be in good condition, but there are the odd 'black' (old I gather) cables and they worry me. Should they?

Lastly, what makes a cable overheat? A fault? The constancy of electricity through the cable (ie length of time it's 'on'). At a guess, I suppose it doesn't matter cause that type of cable is always on?

Cheers all,

[ptrott]

Provided that you are not using oversized fuse wire (or breakers), ie only 10 amp wire for power circuits and 5 amp for lights, (15 amp fuse for a 15 amp power point if you have one somewhere) then in theory the fuse will blow before you draw enough current to overheat wiring that is not faulty or damaged. What you need to avoid is placing batts over junction boxes and any other joining connectors in the roof. (which should be in junction boxes) Heating can result when a join is loose or corroded and a resistance is present. When you draw current through a resistance, heat is produced. (like in your toaster, electric heater etc.) This can also occur if wiring is damaged so as to effectively make it smaller in diameter, like squashing it, or cutting it partially through. Very few houses that I have come across that have had insulation fitted post wiring stage have had the wiring lifted above the insulation, but most have it kept clear of joints. Hope that helps.

[soundman]

If you place is that old & you arent sure Its probably wise to get a lecky in to have a good look befrer you bury things.
In the past the rules had diferent ratings for various surroundings of insulation ( still do ). But now anything in a cieling cavity is deemed to be completely surrounded by insulation and has to be rated as such.
Also these days, the regs are more conservative, and certain considerations & requirements make a compliant domestic insulation much less likely to have any temperature issues with cabling.
In reality, unless things are being run close to the edge. insulation shouldn't make a blind bit of difference. BUT how do you know what the situation is.
I'd get it checked out anyway.
cheers

[Pestmaster]

After having crawled through 3-4 roof cavities per day during the course of my work (:eek: there must be a better way to make a living !) I can say that I've never noticed electric cables 'above' the insulation.
However, I'm not an electrician, so to be sure better contact one of them and ask their advice.

[Pulse]

Insulation reduces the ability of the cable to dissipate heat to the surrounding air. In the aust standards they classify the cable location as in air, partially surrounded by or fully surrounded by thermal insulation.

I wouldn't put the insulation on the cable directly but get some villaboard or gyprock offcuts. Pack these to form a small cavity around the cables then insulate over the top. Maybe two sheets as "walls" and a "roof" over the top which would give a cavity 20mm high to dissipate heat into.

To be sure get a sparky to have a look. As a rough guide I think 2.5mm2 can carry 20A partially surrounded. If it was buried direct in soil the capacity may double. There is a standard that lists all this info - part of 3000 series.

Cheers Pulse

[Pestmaster]

Pulse,
That makes a lot of sense, (allowing an air gap around cables) but someone ought to tell the insulation installers that because, I've never seen it happen.

[soundman]

Even if electrical cable was boxed in some form of tube then covered in insulation it si still "surrounded". Pointless.
There are all sorts of thisnd that "could be done" in cielings almost always they aren't.
That is why NOW, ALL cables installed in a cieling are considered surrounded.

Have it assessed.

[Pulse]

From AS 3008 part 1.1 selection of cables (australian conditions)

for 2.5mm2 sheathed cable, current carrying capacities as follows:
Partially surrounded by thermal insulation (includes lying on ceiling with insulation batts over the top) - 18A
Fully surrounded by insulation: - 13A
Buried direct in soil: - 40A
Buried in soil in conduit: - 31A
unenclosed in air, clipped to joist - 26A

This is supported by the simplified information in AS3018 - domestic installations.

For socket outlets:
2.5mm2 cable with 20A breaker cannot be partially or fully surrounded by insulation.
2.5mm2 cable with 16A breaker can be partially but not fully surrounded by thermal insulation. If installed this way, you are limited to 15 single power points.

For light circuits:
1mm2 with a 10A breaker may be partially surrounded only.
1mm2 with a 6A breaker may be fully surrounded.

Now to quote 3008.11 directly:
"partially surrounded installation - an installation where bulk thermal insulation is prevented from complety surrounding.... cable, such as where a ... cbale is clipped to a structural member or is lying on a ceiling.


After all of that, my initial advice is not that "kosher", so my new advice is to reduce the size of the breakers as needed and add new circuits if the existing ones keep tripping. (really crap solution I know). The lights may be OK because most 1mm2 installations have a 10A breaker. The power points may need some work though.

Cheers Pulse

[Programmer]

Thanks for the responses guys (and especially Pulse). I've done the insulation and have lifted all cables that I could above the batts. I'll also be getting some strips of board up there to create the air-gap mentioned. It's been up a couple of weeks with no 'surprises' but I'm not going to assume all is OK.

On another issue - Bloody sparkies! (Pulse with the detail you provided I apologise if you're a sparkie), I had some powerpoints installed and lights moved around. I said, "please ensure you fix the junction boxes up, away from the batts, if you need to run longer cable then sobeit, and I know it'll take a while, just do it. Guess what?

It's sorta like the expression "what part of no don't you understand"? I rang and the excuse was they thought I wouldn't want to pay for it! I told them that I had explicitly said "do it!" They then asked, "so, come back"? Couldn't believe it. "No, I want you to come and french polish my floors"! I know tradies are painfully aware of time vs costs vs worth with jobs, but I'd just about written these two a recipie. Next time I will.

The poor apprentice came back at 7:10am and had to do it.

PS. Pulse, just saw you're a surgical trainee? Bloody sparkies!