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

[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.

[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.

[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.

[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 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.....

[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.

[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!!

[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?

[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.

[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.

[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.

[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.,

[Sparks]

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

[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.

[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 .