Is it common to find too-thin wire in an installation?

[AndyD]

Are you looking at the table for rubber-insulated cables?

No, the tables attached are for PVC but the correction factors table has values for rubber as well.

Incidentally I hate the term rubber. It's such a broad term it practically has no real meaning in engineering terms. I wish they would just state the material correctly like 'silicone rubber' or even better give the cable code like HO7RNF if it's neoprene rubber for example. There's dozens of different rubber products in use and they all have different temperature characteristics.[/RANT]

[123]

I am quite happy to and quite often use 10A or 15A CB's for plug circuits. My logic for this being that the lower the breaker rating the higher the protection.

Have to agree, i do the same where ever possible.

[murdock]

so long as you dony use 1.5 mm or smaller wire

[Dave A]

I find this thread fascinating.

I just don't see a way to justify running a 20A circuit with 2.5mm^2 wire in South Africa - a pretty warm place, using installation methods that seem to be in common use.

Has anyone found scorched 2.5mm behind 20A protection (other than right next to a bad connection, of course)? The justification is it seems to do the job and it's the way it's always been done.

But the interesting thing is based on the evidence here, it does not comply with SANS 1507. Now when you sign off an electrical COC, what are you signing off on? That the installation is coping under existing loads or that it meets regulation (effectively standards set by SANS codes)?

[AndyD]

....what are you signing off on? That the installation is coping under existing loads or that it meets regulation (effectively standards set by SANS codes)?

Very much the latter.

'Existing loads' are very organic values and can change radically depending on season or even time of day so unless you use a power analyzer to obtain a max, min and mean value it's not predictable. The only thing that is predictable is the circuit protection tripping at 20 amps on a socket circuit so this would be the figure to work with when referring to the regs.

The only thing in doubt here is the maximum ambient temperature of the ceiling space and the corrective value that should be used accordingly. If you come across a ceiling space that's hot enough to warrant sufficient derating of the max current then you should fail the installation on a COC for this reason. The owner of the premises would no doubt install a R50.00 fan unit and invite you back for the retest on a cooler day.

[Dave A]

As you probably realise, I'm playing the devil's advocate here.

Here's another one- if I ring-feed the plug points on each circuit, can I use 1.5 mm?

[AndyD]

Ah ha. Now you're talking my language with a ring main. This is a standard installation in the UK but the British Standard spec there is also 2.5 mm² cabling and the socket outlets are only rated at 13 Amps not 16 Amps as in SA. I think we can safely say the answer to your question is 'no'.....but nice try though

[murdock]

yeah right ring circuit...you would find plenty electrocuted electricians in sa...they cant even get a standard circuit right...now yu want confuse them...cascading systems are bad enough...

[berndj]

I find this thread fascinating.

Has anyone found scorched 2.5mm behind 20A protection (other than right next to a bad connection, of course)? The justification is it seems to do the job and it's the way it's always been done.

Dave, I've often wondered about standards vs practice, and the resultant need for sometimes very generous safety margins in standards. This might be a manifestation of this effect.

Another possibility is that, perhaps, the 2.5mm^2 wire is damaged, but in a way that you can't easily see it, and that damage is cumulative. If the user is very disciplined and never trips the circuit, there's no damage. But if you just plug in heaters and washing machines and dishwashers willy-nilly, your wire could be creeping through the PVC as it repeatedly goes past its vitrification point at 80 degrees C.

With some mad physics-fu (*) and a few assumptions, I calculated the temperature rise of a wire during a 0.1s 20x overcurrent condition. I think I saw some graphs somewhere that suggested this might be the worst-case behaviour of "curve 1" breakers.

2.5mm^2, 15A circuit: 7.5 degrees
2.5mm^2, 20A circuit: 13.4 degrees
2.5mm^2, 25A circuit: 20.9 degrees

4mm^2, 15A: 4.7 degrees
4mm^2, 20A: 8.4 degrees
4mm^2, 25A: 13.1 degrees

My guess is that the 10 degree headroom between the maximum operating temperature specified in the standards, and the vitrification point, is exactly the room you need to survive an overcurrent condition without damage. Note that each of the calcs above which result in less than a 10 degree rise, are allowed.

Interestingly, notice also how 4mm^2 in a 25A circuit is marginally "safer" in this perspective than 2.5mm^2 in a 20A circuit.

(*) For 0.1s, ignore heat loss through radiation, conduction, or convection. It all goes into the copper, and only the copper. Take resistance per unit length, to calculate I^2*R losses per unit length. Multiply by 0.1s to get thermal energy dumped into the copper. Divide by copper's heat capacity to get rise in T.

[AndyD]

Another issue is that during current flow through a copper conductor most of the current flows at the surface. During overload this is also the case and the surface of the conductor would be where most of the heat would be developed. With a single conductor such as a FT+e cable the conductor has a lower surface area ratio to mass than say house-wire or panel-wire which is a stranded conductor with a larger surface area hence the house-wire should generate less heat. No allowance is made for these different conductor constructions.

You mention conductor creepage and vitrification of the PVC insulation. From experience, the first sign that a wire in a cable has been overheated is that the PVC insulation is bonded to it by the heat generated. At the termination point the PVC insulation may have retreated and shrunk back but I would suspect a higher temperature at the connection or where a ferrule or lug is crimped on would be responsible for this. I wouldn't see conductor creepage occurring in a multistranded wire which would have a twisted construction.

yeah right ring circuit...you would find plenty electrocuted electricians in sa...they cant even get a standard circuit right...now yu want confuse them...cascading systems are bad enough...

Both sides of a ringmain are fed from the same MCB so there's only one point of isolation. This shouldn't increase the risk of shock when working.