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

[berndj]

From reading some of the other threads, I'm the guy you guys love to hate: a homeowner who wants to add a socket outlet or two himself. By the way it's a single-phase installation, I remember a blurb in the standard about that being relevant.

I did, however, shell out for SANS 10142-whatever. I have my issues with some of the wording (ambiguity in places), but overall it's quite clear to me what I need to do to make my socket outlet(s) good.

In particular, wire size. By my reading, the correct wire size for an outlet on a 20A circuit has to be 4mm^2, not 2.5mm^2, because I'm using "installation method 3" (conductors in conduit in air, in ceiling space). That means I already can't use the 27A claimed for 2.5mm^2 wire on its packaging, and more importantly by the electrical goods sales staff, due to the method alone. Then I add some thermal derating, assuming a roof space temperature of 50C (I measured 10C higher than indoors temp, assuming worst-case 40C in Cape Town); I think the factor was about 0.65 or something - I don't have the spec in front of me. And then I find that the 4mm^2 wire is just *barely* adequate for such a 20A circuit.

It was actually quite funny, I'm normally quite a softly-softly type of guy, but there I was at the counter insisting that I wanted the thicker wire, and no, they can just give me what I asked for instead of the 2.5mm^2 they strongly suggested. In a room full of people who looked like they do this stuff all day.

Am I needlessly overengineering my new outlet? I don't think so. But what's with the supply shops claiming 2.5mm^2 is enough? When I looked at (some of!) my other pre-existing outlets, there seems to be a lot of the thinner wire around. Only a few with what looks to be a thicker wire - it might even be 4mm^2 wire from the quick look I gave it.

What's going on here? (And also, shouldn't supply shop staff be trying to get me to buy *more* copper than I need?

[murdock]

it all boils down to design and application...thats why i spent years in an apprentice school and at tech. and the doctor spent years at varsity.
if you are asking a saleman at the counter which wire to buy...good luck.

My son is like you he did 6 months with me then quit...got fired...what ever...now he gets all his spec from the saleman at the counter and does all his own electrical work.

why dont you just use rip cord so many diy people do....and as the saying goes i hear on a daily basis...but its worked for years

on a more serious note...it is very common for electricians to use 2.5 mm wire for plugs...depending on the age of the installation you will find in older installations they use to use 4 mm wire with a 30 amp circuit breaker...new installations normally have 20 amp circuit breakers...just remeber ohms law dictates everything in electricity...bigger is not always better.

[Sparks]

2.5mm cable may even be overkill. There are other factors to consider too. What will your end load be? That is the primary consideration. From there you can determine the size of cable you need which will then indicate what CB is required to protect that cable. Note: the CB protects the cable, it is not selected according to the requirements of your load.

[berndj]

Sorry, that doesn't really address what I was asking.

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. Let's take "installation method 2"'s ratings, which to me seems closest to the cable-in-conduit-on-roof-beams that is used in, at least, my house. That's already just 23A, at 30 degrees Celsius - not much margin. Now take thermal derating into account - I think 50 degrees up there in the roof is a reasonable assumption? That's a derating factor of 0.71.

23A * 0.71 = 16.33A

So I don't see a 20A circuit breaker adequately protecting a cable with a current-carrying capacity of only 16-odd ampere. Is there some rule that I haven't yet spotted that lets you wriggle out of this?

Note: I'm NOT asking, "what should I do?". I AM asking, "Is this apparently common practice kosher?"

[Sparks]

Like I said , look at the load. With the exeption of the geyser circuit you would seldom get a load in excess of 10A on a plug circuit. 6A less than what you consider max. Remember that the length of the circuit is also a factor. The geyser circuit usually being reasonably short it is acceptable. A socket outlet is only 16A, so before the 20A CB pops your switch will start frying

[berndj]

Hmm, gosh, am I speaking Martian or what? That wasn't my question.

Fact 1: most household DB's seem to be full of 20A breakers on plug circuits.
Fact 2: it seems to be popular to wire these circuits with 2.5mm^2 wire.
Interpretation 3: 2.5mm^2 wire (of any length) cannot safely carry 20A using typical installation methods.

Question: how to reconcile these?

Sparks, why do you suppose circuit length matters? Amps is amps is amps! Length is only an issue when considering voltage drop along the circuit. It doesn't affect the current carrying capacity of the 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.

[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]

From the regs a 2.5mm 2-core cable in conduit in a ceiling has a max current of 23A so long as your ambient temp is 30C or less. Only when you reach 45C or higher ambient temp does the max allowable current fall below 20Amps which is the standard rating of a socket circuit breaker.

45C is pretty damn warm for a ceiling space. If the temp is higher that 45C then yes, you'd need to go with a 4mm cable. But riddle me this Batman; if you just install a single socket on the end of your cable, would you be covered because it would only be possible to plug in a single appliance which would have a load of 16Amps or less because it has a limitation of 16A plug on it?