# Interest group forums > Electrical Contracting Industry Forum >  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?  :Smile:

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AndyD (06-Aug-10)

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## 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  :Big Grin:  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  :Big Grin: 

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.

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## 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.

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## 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?"

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## 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

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## 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.

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## 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? :Big Grin:

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## berndj

> 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?


Are you looking at the table for rubber-insulated cables?  Because as far as I know we're dealing with PVC-insulated cables.  At 45 degrees I see the factor 0.79.  23A * 0.79 gives me 18A.

And no, Mr Quizmaster, of course the appliance doesn't save you.  As Sparks pointed out, the CB is there to protect your cable.  To hell with your appliance if it develops a fault!  :Smile:   But you're hinting at another odd thing I picked up - the standard specifically allows (can't remember the chapter and verse) for 16A socket outlets on a 20A circuit.  But I'm willing to believe that the 16A rating is a *switch* rating (i.e., can it interrupt that current), rather than a current-carrying capacity.

As for 45 degrees vs higher...  I dunno hey, it doesn't seem that crazy to me to expect at least an occasional 50 degrees.  I measured a 10 degree rise over ambient around noon on a sunny day.  40 degrees ambient isn't impossible - even in Cape Town.  I suppose with the 150 degree rated cable you can get away with the thinner wire.  I might use that for another project that'll come later.

Thanks for the engaging reply.  It's exactly tables 6.2 and 6.10 that I had in mind.

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## Sparks

To counter the voltage drop you would use a thicker cable thereby falling within what your 20A query. Something you need to keep in mind is that the specs in the tables are the max acceptable under legislation. This is not the max as per tests by SABS or cable manufacturers. The minimum requirements expected by SABS from manufacturers are higher than SANS tables. As for the temperature, I am regularly in the roof and having lived in the heart of the Karoo I can guarantee you that 50deg. is not unrealistic or unusual. A salesman at the counter is not qualified to give any technical advice. We live in SA. Every time I want a 3pole+N CB or isolator, I have to order it. In PE it is not considered movable stock because " no other electricians ask for it". If you take a closer look at SANS10142 and the ammendments you are guaranteed to find a bucketload of inconsistencies and contradictions, even on the new COC. With so much unclear in the legislation you can forget about finding consistency in the field. My new DB CB ratings are according to what I am informed the load will be. 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.

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123 (02-Aug-10)

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## Jacques#1

Nobody is wrong here.  It will depend on your installation, what cable you are using and your load factors.

Cable manufacturers also differ, it has got to do with the density of the copper etc.  So in some instances, a cable that they sell as 2.5mm or 4mm is in fact 0.3mm smaller etc.

The lebgth of the run with affect the volt drop, ohms law, since cables have an inherent resistance.  This in turn will affect the maximum output, but more importantly the actual power at the load.  Your equipment will break 10 times faster since i.e. that shaft needs so much power to be turned by that motor, which no has limited torque etc.  Heating up, catching fire and killing 50 people in the process.

Also consider how you determine the load factor for a house.  For socket outlets you use 5kW for the first 100m2 and then 1kW for every additional 100m2.  Why do you think this is?  Its because you never use all 50 or whatever socket outlets at the same time all loaded to exactly 15 amps or whatever the case may be.  Consider your ceiling using this knowledge, you would have to load that Socket outlet at 17 amps, exactly on the one day of the year its 40 DC outside.  I'm not saying this won't happen, but I am not too sure that you get anything on a plugtop except for a welding machine that uses that type of power.  Lets say you've got a plug in stove with two 1500W elements, and a fridge 1500W (which is buggered, so it starts up and runs for an hour at a time), all running at 13h00 in the afternoon on the hottest day.......then you have a problem, maybe, consider that you calculated your voltage drop and its withing spec, so you used a 2.5mm cable, but the amps is 19.XXXXXamps.  Yet, like you pointed out the temperature only rates the cable at 16 amps, adding the safety factor built into the cable, and the fact that you'll be cooking for no longer than an hour on both plates at maximum with a buggered fridge in the same room, the cable may get a little hot, but thats about the worst that could potentially happen.

I always use 2.5mm cable for my socket outlets, but I did do a job this past week using 4mm cable because my cable run was around 40 metres, and the outlets would be used for space heating.  Still overkill for the application, but I calculated the worst imagineable loads......20 amps be unit.  

Point is electricians will use a bigger size cable, if the conditions ask for it, and if you would rather wire in 4mm, than do it if you would prefer that.

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123 (02-Aug-10)

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## 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]

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## 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.

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## murdock

so long as you dony use 1.5 mm or smaller wire

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## 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)?

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## 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.  :Big Grin:

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## 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?

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## 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 mmÂ² 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  :Wink:

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## 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...

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## 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.

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Dave A (02-Aug-10)

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## 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.

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## Dave A

> 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 mmÂ² 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


That's belts and braces thinking for you - they're catering for breaking the ring, I expect. But humour me one more time with this one:

What if you did a normal branch reticulation doubling up on 1.5mm all the way through?

(I am actually heading somewhere with this).

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## AndyD

> (I am actually heading somewhere with this).


Lol, and I thought you were still just poking the fire for the sake of it. :Big Grin: 

If you installed a standard skt cct in SA using 2 x 1.5mmÂ² FT+E cables all the way through I think it would be okay as far as conductor sizing would be concerned. It's one I've never come across before but without diving into the regs it might however raise other issues such as the occupation percentage of the conduit for one. This is also indirectly a temperature issue.

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## berndj

> If you installed a standard skt cct in SA using 2 x 1.5mmÂ² FT+E cables all the way through I think it would be okay as far as conductor sizing would be concerned. It's one I've never come across before but without diving into the regs it might however raise other issues such as the occupation percentage of the conduit for one. This is also indirectly a temperature issue.


What happens if exactly one of the conductors fails open?  Perhaps you got a bad batch of PVC glue, the conduit segments moved apart when the alarm technician installed his cobweb, and a rat fancied a nibble at the wire insulation.  What has to happen in order for you to know that it has failed?

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## Dave A

> If you installed a standard skt cct in SA using 2 x 1.5mmÂ² FT+E cables all the way through I think it would be okay as far as conductor sizing would be concerned. It's one I've never come across before


At least not in plug circuits - realistically because of cost efficiency. But in situations with big current, having multiple cables in parallel is not uncommon at all.

This now begs the question - doubling up in parallel is probably OK (from a regs point of view at least), so why isn't doing much the same thing but in a ring feed configuration a problem?



> What happens if exactly one of the conductors fails open?


Precisely. In such a situation a ring feed reduces the risk of such a failure causing catastrophic results.

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## Sparks

My plug circuits are minimum 2.5 but depending on load and length I do go thicker. As for ring circuits, not allowed in domestic installations, exactly as has been stated too many idiots in the field let alone the DIY crowd. The only time I will accept a 1.5 cable with a socket outlet on it is in a mixed circuit, obviously then it will not have a 20A CB.

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## AndyD

I have seen ring circuits before in SA but very few and a long time ago. I don't see them as illegal according to the regs and I don't see them as a hazard as Murdock suggested. They just have redundancy built-in and are obviously more capable of handling fault currents etc. 

Not being specified as standard would make them cost prohibitive to install as they require one extra cable and conduit back to the DB but in the context of this problem of a high temperature roof space it would be a reasonable alternative solution in my book. The cost difference between a 2.5mm ring circuit with 20mm conduit and a standard circuit on 4mm cable and 25mm conduit would probably be negligible.

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## Sparks

Spec yes, but in this country definately not "safe" therefor not meeting criteria for my COC.

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## AndyD

> Spec yes, but in this country definately not "safe" therefor not meeting criteria for my COC.


Please explain why it's 'definitely not safe'. Which part of the SA regs would make it unacceptable to install a ring main? Why would it be unsafe here in SA yet safe elsewhere?

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## berndj

> This now begs the question - doubling up in parallel is probably OK (from a regs point of view at least), so why isn't doing much the same thing but in a ring feed configuration a problem?
> 
> Precisely. In such a situation a ring feed reduces the risk of such a failure causing catastrophic results.


My worry would be that if exactly one conductor failed, your circuit would still appear to be "working".  But that single conductor is now grossly undersized for the current that it might be carrying, but you don't find that out until either you do a (routine) check (but realistically, how often does that happen?) or your building burns down - because your circuit breaker doesn't have a problem with the current.

With single conductors, if one fails, the whole circuit fails - it makes the failure very obvious.

As for why parallel is fine but ring isn't, I'd say it's because you're far more likely to get an equal distribution of current through the parallel conductors than in the case of a ring.  Think of a ring that's 50m long in total, with an outlet at 10m from the CB: those 10m will carry nearly all of the current, so to have a safe installation, you need the whole ring to be of a thickness capable of running the max current on its own.

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## Dave A

So let's sum up. I think we can acknowledge that:
Bernd has a point.There is science behind the numbers.You can't rely on the current rating per the packaging.The potential scale of the issue could be huge.
Why it hasn't caused widespread problems so far is probably a combination of eating into the safety margin and the fact that it has to be really rare that a plug circuit runs continuously close to maximum protection rating, due in most part to the start-up current draw profile of nearly all plug-in appliances.

Fortunately the numbers are not much on the wrong side of where they should be, but they *are* on the wrong side of where they should be under certain circumstances which runs contrary to regs and COC standards - and therefor the issue shouldn't just be ignored (particularly bearing in mind the changed potential for claims with the introduction of the Consumer Protection Act).

Some exposure on the issue would deal with new installations. It's the existing installations that are troublesome - probably on a number of fronts.

In terms of raw flack, there's probably enough fudge in the history of the widespread adoption of surfix to keep fingers pointing round in circles for decades.

(And here's where I was heading in the 1.5 mm ring-feed questions)

At a technical level, it's a case of either downrating the protection (much more nuisance tripping complaints), or upping the wire where it counts. Now given that the problem is really confined to roof spaces, I'd suggest a simple ring feed solution (could even be confined just to the roof loom) - adding a 2.5mm feed from the db to the furthest point on the circuit.

I only raised ring circuits at the 1.5mm level to demonstrate how powerful this solution would be.

Anyway - rough sketch of my thinking - I'm sure it needs ripping to shreds, refining and squaring away of the details/fine print etc.

Two more things perhaps worth mentioning:

1. If line resistance is primarily a function of surface area, round conductors are the most inefficient shape. You could up the rating of surfix simply by using flattened copper conductors.

2. I asked my IE what the maximum circuit breaker rating can be on plug circuits with 2.5mm wire - he said 25A. Only when I said Aha! did he say "Are we talking GP wire?"

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## Dave A

> My worry would be that if exactly one conductor failed, your circuit would still appear to be "working".  But that single conductor is now grossly undersized for the current that it might be carrying, but you don't find that out until either you do a (routine) check (but realistically, how often does that happen?) or your building burns down - because your circuit breaker doesn't have a problem with the current.
> 
> With single conductors, if one fails, the whole circuit fails - it makes the failure very obvious.


You face the same problem whether it's wired in parallel or ring. But in a ring the prospects for a single break being non-catastrophic are far more favourable.

I'm not suggesting we push this too far; I'm only looking for a workable patch solution that finds 10 - 15% for installations that are already in the hole here.

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## AndyD

> What happens if exactly one of the conductors fails open?  Perhaps you got a bad batch of PVC glue, the conduit segments moved apart when the alarm technician installed his cobweb, and a rat fancied a nibble at the wire insulation.  What has to happen in order for you to know that it has failed?


Firstly if a socket circuit is installed as a 2.5mm ring circuit then the failure of one of the supply conductor would leave the circuit with a single 2.5mm feed which is the normal installation today by SA standards. This just means that the failure of one side of the ring circuit is not critical or catastrophic, with the ring circuit there is a redundancy.
There would be no indication of this failure without the use of a clamp meter being used to check the loads on both sides of the circuit.




> .... so to have a safe installation, you need the whole ring to be of a thickness capable of running the max current on its own.


Correct. This is the case with a correctly installed ring circuit, the max load of the breaker is capable of being carried by one side of the ring circuit in the event of a failure.

I agree that in theory if the roof space is over 50C and the socket circuit is loaded to 100% of the 20Amps allowed then there could be an issue and (depending on the safety margins of the regs and assuming the cable has no manufactured safety margin and assuming the conduit is packed to the maximum allowable percentage of being full and assuming that these conditions all exist long enough for the full resultant internal cable temperature to occur) there could be minor heat damage to the cable insulation.

In real life I've seen a single phase motor installation (24Amp RLA) in high ambient temp conditions supplied with 2.5mm FT&E in conduit on a slow curve 32A breaker. Reportedly it was over six years in operation and the cabling which was very warm to the touch during prolonged motor run showed little to no signs of deterioration due to heat.

Realistically I think there is a considerable safety margin in the regulations, for example concern is not given to allowable power factors of appliances which are plugged in to socket circuits. Switch mode power supplies in computers for example often have attrocious power factors.

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Dave A (04-Aug-10)

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## Dave A

Anecdotal evidence on the ground suggests there isn't a problem. But is the IE operating within the regs when he/she issues a COC in these circumstances?

That's the bit that is really troubling me on this.

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## Jacques#1

I would tend to agree with Andy.  Two weeks ago I had a callout to a house.  They had 2 pool pump motors on one CB.  The one pool pump was fried, and when you start it up, it hums for 9 seconds and the CB trips.  Tested the load, and it ran for around 5 seconds at 29Amps.  2.5mm cable, 20 Amp breaker.  No damage to the cable whatsoever, no damage to the control wiring (which, if you know these installers  :Roll Eyes (Sarcastic):  was 1mm cabtyre.......no damage).

Also, I am a bit rusty on my regs of head, but the new law does not allow for a cb rating of more than 20 amps on a socket outlet circuit.  Anything higher must be dedicated i.e. a welding plug with 4mm cable and a 30amp breaker.....

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Dave A (04-Aug-10)

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## AndyD

> Anecdotal evidence on the ground suggests there isn't a problem. But is the IE operating within the regs when he/she issues a COC in these circumstances?
> 
> That's the bit that is really troubling me on this.


If the ceiling space was over a certain temperature (and assuming the IE had equipment on hand to measure that temperature) then he should fail the installation accordingly. I've never heard of this happening however.

I had another thought regarding ring mains and how some people feel they're unsafe. If a ring main developes an open circuit neutral on one of the supply cables from the DB, the neutral in the whole circuit still remains at zero volts WRT earth. If a standard South African socket circuit developes an open circuit neutral on the supply from the DB then the neutral in the circuit becomes 220v WRT earth as soon as even a small load is on that circuit. I've seen more than one sparky with a surprised look on his face when he gets a whack from a neutral on a faulty circuit. Surely this would also mean a ring fed circuit is safer for this reason also.

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Jacques#1 (05-Aug-10)

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## Jacques#1

> If the ceiling space was over a certain temperature (and assuming the IE had equipment on hand to measure that temperature) then he should fail the installation accordingly. I've never heard of this happening however.


If you would have to factor in a possibility or chance for everything to fall into place at exactly the same time.....what would that calc give you?  Were talking 1 in a million, just on the current installation, and then you still haven't even touched the safety features inherent to all the equipment.  As far as I'm concerned, I would only consider this for a house with a steel roof, and aerolite on the ceiling, no higher than 1 meter (hot air rises), with the cable lying on top of the aerolite, in Thabazimbi or somewhere where even the dogs were caps on their heads.





> I had another thought regarding ring mains and how some people feel they're unsafe. If a ring main developes an open circuit neutral on one of the supply cables from the DB, the neutral in the whole circuit still remains at zero volts WRT earth. If a standard South African socket circuit developes an open circuit neutral on the supply from the DB then the neutral in the circuit becomes 220v WRT earth as soon as even a small load is on that circuit. I've seen more than one sparky with a surprised look on his face when he gets a whack from a neutral on a faulty circuit. Surely this would also mean a ring fed circuit is safer for this reason also.


This will happen with a loose or corroded connection, you don't necessarily need a open connection, trust me, I've learned my lesson the hard way, added that it took me a few hours to find the "not tooooo great" connection!!

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## Sparks

Ring circuits are not a problem for me personally. The safety question comes in when you consider a ring circuit being fiddled on by the SOUTH AFRICAN public. After they have been there, and you know they will go there, what nightmare awaits us when we arrive on site. With only one live conductor they already set death traps for us, not to mention themselves. Then again the "hand-langers" who decide they know enough to work for themselves. They don't bother connecting the earth, steal neutrals, and even use the earth as neutral or when they do not want to buy a 3core+earth just use the earth wire as a return.Who knows how many taps going to dead ends will be in the circuits then?

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## berndj

> If you would have to factor in a possibility or chance for everything to fall into place at exactly the same time.....what would that calc give you?  Were talking 1 in a million, just on the current installation, and then you still haven't even touched the safety features inherent to all the equipment.  As far as I'm concerned, I would only consider this for a house with a steel roof, and aerolite on the ceiling, no higher than 1 meter (hot air rises), with the cable lying on top of the aerolite, in Thabazimbi or somewhere where even the dogs were caps on their heads.


You mean on a hot summer's day when the aircon is running full tilt and your wife is doing the washing so it can dry before the braai?  And just at that moment an aircon motor winding shorts out because the heat causes the varnish to burn, to dump an extra 15 degrees into your cable.

For what it's worth, I actually measured (yes, measured, not just guessing) the temperature in my roof.  A cellphone might not be as accurate as a gas thermometer, but I reckon it's close enough to make these sorts of decisions.  I left it on one of the trusses, exactly where a conduit might rest, and pulled it down 30 minutes later.  It showed 10 degrees higher than outside ambient temperature.  Even in Cape Town 35 degrees isn't all that unusual, no need to go to Pofadder to get 45C, even 50C in the roof.  Mine's a tile roof, for the record, and yes, there's glass fiber (yuck!) insulation on the ceiling.  But the conduits aren't resting on it.

Sorry, I don't buy this idea of eating into the safety margin.  Coping with substandard installation is not its purpose.  Sure, I'll still sleep at night knowing that there are (slightly) undersized cables in my roof, but no way will I let an electrician work on my wiring and have him tell me it's "okay" to put in underspecced wires.

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## AndyD

It's just going to cost more if the installer uses 4mm wiring and possibly 25mm conduit and it might not be necessary in many installations. With the prevelence of the cheapest quote mentality I don't see this becoming the norm unless it gets a special mention in a future regulation ammendment.

This issue is a bit of a grey area and this has certainly been a very worthwhile and thought provoking thread.

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## Jacques#1

> You mean on a hot summer's day when the aircon is running full tilt and your wife is doing the washing so it can dry before the braai?  And just at that moment an aircon motor winding shorts out because the heat causes the varnish to burn, to dump an extra 15 degrees into your cable.
> 
> For what it's worth, I actually measured (yes, measured, not just guessing) the temperature in my roof.  A cellphone might not be as accurate as a gas thermometer, but I reckon it's close enough to make these sorts of decisions.  I left it on one of the trusses, exactly where a conduit might rest, and pulled it down 30 minutes later.  It showed 10 degrees higher than outside ambient temperature.  Even in Cape Town 35 degrees isn't all that unusual, no need to go to Pofadder to get 45C, even 50C in the roof.  Mine's a tile roof, for the record, and yes, there's glass fiber (yuck!) insulation on the ceiling.  But the conduits aren't resting on it.
> 
> Sorry, I don't buy this idea of eating into the safety margin.  Coping with substandard installation is not its purpose.  Sure, I'll still sleep at night knowing that there are (slightly) undersized cables in my roof, but no way will I let an electrician work on my wiring and have him tell me it's "okay" to put in underspecced wires.


Look, all I am saying is, looking at historical evidence, consider the following.  Best guess, there is around 60% of residential houses in this country wired with 2.5mm, another  say 20% informal settlement with less than 2.5mm wire, lying on a hot tin roof in the blazing sun, any location, any type of building structure etc.  The other 20% is for argument 4mm and "old" type wiring which was around 3.1 or something (the older guys knows these old cables). 

My logic suggests, that if this was really a problem you would expect at least a 1-2% failure rate, therefore last summer when virtually the entire country was blazing hot, we should have seen at least one in every 20 houses catch fire.  I am no news boff, but I did not see one which was due to electrical cables (apart from informal settlements) specified correctly, but with a lower value due to high temperatures.  Just my opinion, with no credible evidence whatsoever.

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## murdock

> I would tend to agree with Andy.  Two weeks ago I had a callout to a house.  They had 2 pool pump motors on one CB.  The one pool pump was fried, and when you start it up, it hums for 9 seconds and the CB trips.  Tested the load, and it ran for around 5 seconds at 29Amps.  2.5mm cable, 20 Amp breaker.  No damage to the cable whatsoever, no damage to the control wiring (which, if you know these installers  was 1mm cabtyre.......no damage).
> 
> Also, I am a bit rusty on my regs of head, but the new law does not allow for a cb rating of more than 20 amps on a socket outlet circuit.  Anything higher must be dedicated i.e. a welding plug with 4mm cable and a 30amp breaker.....


i take it you know why the pool pump hums like that...the capacitor is normally cooked...it could be a whole lot of things but the majority of callouts i get pool pump will not start...the first thing i ask is does the motor make a humming sound if yes ...i take a cap with.,

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## Sparks

Unless the fan blade is obstructed or the motor siezed up.

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## AndyD

Snails can cause this problem especially in Cape Town. The little critters get in the fan at the back of the motor, they jam it and cause it to burn out or the thermal overload on the motor trips if there is one.

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## Jacques#1

This was theoretically almost a dead short as the resin around the windings melted, just enough not to turn the motor, but enough to lower the resistance so much that the motor turned into a heater  :Stick Out Tongue: .

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