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Electrical services

by brenfox
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Averagesupernova
#73
May28-14, 10:25 AM
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Ok. So I would say that is a #10. I don't see how that is saving copper.
sophiecentaur
#74
May28-14, 11:57 AM
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2.5mm copper is rated at 18.5A in the UK.
Ah, I may have spotted the misunderstanding - thats 2.5mm2
I was being sloppy and using the vernacular.
Averagesupernova
#75
May28-14, 12:16 PM
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Ok. That's different. So if part of the ring is broken, which could go for years unknown, the wire can be regularly overloaded.
sophiecentaur
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May28-14, 12:47 PM
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Yes. So it would; you certainly have a point there. But testing the system involves testing continuity of the ring.
Averagesupernova
#77
May28-14, 01:00 PM
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Ok so in the UK are there regular 'tests'? Here in the USA there are inspections at the time of install and that is it. I can see some advantage of a ring. I also see the potential of disadvantages. I would think that there would be more reason to use it on a low voltage system. I look at it this way: Break the ring at mid-point, fuse each half at 15 amps. 15 amps on a 240 volt circuit is double the power available compared to the USA. The one main disadvantage of the system in the USA is losing the neutral before the service. This gives potential of putting 240 volts on some 120 volt appliances. In the USA typically our transformer is located closer to the residences. As I understand in the UK they will run secondary wire up to half a mile. Is this the case? I doubt secondary wire would ever be run that far in the USA.
psparky
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May28-14, 01:08 PM
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Quote Quote by sophiecentaur View Post
2.5mm copper is rated at 18.5A in the UK.
Ah, I may have spotted the misunderstanding - thats 2.5mm2
I was being sloppy and using the vernacular.
Amazing how the same size copper wire safely conducts different amps in different locations.

In Chile Factories, 2.5 mm2 copper wire is rated at 20 amps.

If I take this same wire and install in USA, it only safely conducts 15 amps......and apparently 18 .5amps in UK.

Perhaps different types of copper are used? Perhaps copper behaves differently in different geological locations? Perhaps they are using different temperature rated insulators? Perhaps some are in conduit....perhaps some in open air tray? Perhaps the definition of "safely conducts" or "rated" differs from country to country?

Fascinating, indeed.
sophiecentaur
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May28-14, 05:18 PM
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Quote Quote by psparky View Post
Amazing how the same size copper wire safely conducts different amps in different locations.

In Chile Factories, 2.5 mm2 copper wire is rated at 20 amps.

If I take this same wire and install in USA, it only safely conducts 15 amps......and apparently 18 .5amps in UK.

Perhaps different types of copper are used? Perhaps copper behaves differently in different geological locations? Perhaps they are using different temperature rated insulators? Perhaps some are in conduit....perhaps some in open air tray? Perhaps the definition of "safely conducts" or "rated" differs from country to country?

Fascinating, indeed.
This is true because the environment of the cabling makes a difference to its operating temperature. It should be rated according to the routing - for instance, cable wound on a drum can be an embarrassment and they have thermal cutouts to protect it. I do not know the regs in detail but you should not swathe cable in fibreglass insulation but give it an air space. There is chapter and verse on all of this stuff. Many houses in the US are insulated much better than UK houses - which may account for the different rule-of-thumb figures.

@Averagesupernova: I take it that you really don't like the ring main system. Fair enough. It is bound to have its disadvantages - along with the two / split phase system.

The basic US and UK distribution systems are radically different and this, I am sure, is due to the different spacing between your average houses in the two countries. The UK does not give everyone their own transformer. The standard urban layout is for a 'large' 3 phase transformer and the three phases are taken along a road with houses fed 1,2,3,1,2,3,1,2,3 along the road. (Essentially a LV distribution) Only under special circs would a house be fed with three phases. One transformer would feed dozens of surrounding houses. There can be problems with progressive voltage sag along the path. I have experienced over-volts, right next to the transformer. I created such a fuss because lights kept blowing that they reduced the volts (I had a chart recorder installed for a week and they told me there was nothing wrong - but I read the chart and they did reduce the volts). I have also experienced low volts and lights going dim when the cooker was turned on. Clearly, one big transformer is cheaper but individual feeds from a long Intermediate Voltage line is necessary when the spaces involved are great.

The two voltage system in the US means that you can use many European white (power) goods with not much modification, I presume. But what about your 230V fusing? Are devices double pole fused? I remember getting US equipment that was double pole fused in the 60s and we had to modify it all to single fuses in the L feed. I can see that neither system is perfect.
We are, at least, not coming to blows about this. (Except I know my system is better than yours! )
krater
#80
May28-14, 06:05 PM
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Aha, so your 2.5 mm wire size refers to area, not diamater? I was also under the conception that we were talking about a 10AWG, not a 14AWG.

So you fuse a 14 at 32 amps? Jesus! I realize that it is ring connected and ideally should not be a problem but my concern would be exactly the problem described above, if one side of the ring goes open the circuit may continue to work just fine, however it is drawing the entirety of the load down one leg which is regularly overloaded.

Splices go bad more often than one might like to think. Over here a bad splice, even when it happens on a neutral which leads to stuff taking both lines and burning up, tends to have the result of someone getting a call to service it. But if everything keeps right along humming who's to be the wiser until insulation starts to melt? I'm reminded of one old electrician who once told me that it takes a good 50 or 60 amp load before a 14AWG copper actually vaporizes. I didnt ask him how he knew that.

psparky, you certainly don't have to travel the world to see a 14AWG wire have multiple different current ratings. In fact, you can concievably see several different ratings even in the same structure. The conductor run in MC feeding an outlet next to the panel has to be on a 15A breaker. The other cable which feeds the air conditioning unit outside can be on a 20A breaker. The 10AWG in the inch and a quarter conduit which runs out of the panel along with nineteen other hots and neutrals might only be allowed on a 20A breaker as well, while the one feeding a motor might be connected to a 70A breaker and were it be used in a motor control circuit could legitimately be protected by a 150A fuse.

Sophie, overcurrent devices here are required to be installed in any ungrounded conductor. If the circuit has two then yes, it has to be a double pole OCP. I'm not sure how things work in other parts of the world, but one important point when it comes to electrical codes here is that they are NOT retroactive. You can install something legally by code that will be illegal to do the same way tomorrow, nobody can make you go back and change it and in fact even when new code is in force the standard a project is inspected under is usually the one the permit was drawn under. The NEC by its own definition is a set of minimum requirements, it is not a design manual, and honestly it is not of the code's concern if you want to keep using that 50 year old fridge, vacuum, or table lamp. The lamp is going to have a lousy cord, the vacuum will probably trip your AFCI breakers in your new house, and the fridge full of beer in your garage is going to trip the GFCI outlet it's plugged into. And if you burn your house down with something electrical it's between you, your homeowners insurance, and the manufacturer of the offending product which may or may not still be in business.

Maybe our different views of fusing come from our different experiences with it. For someone who grew up in a house with a fusebox identical to the one Jim posted a picture of a couple pages back, I just assume that anything which can be relatively easily meddled with simply will be. There was a 25A fuse in my panel which fed the 120v well pump, the fridge outlet, and about half of the first floor lighting and power oultets (there were maybe ten in all). Granted the load was not carried all at once on the same (14AWG I think, and old) conductors, but still the circuit was grossly overfused. Why? Because someone plugged something in and the smaller fuse blew. So they put in a bigger one and it didn't happen again.

Is this likely to happen with appliances? I guess not. Unless I suppose you knock your radio or something into the sink, the plug fuse blows, you go to the hardware store and buy the biggest they have and, well, what happened to the protection? I have no idea if this situation has any basis in reality, but an appreciable part of our code deals with situations that have happened maybe only once, someone submitted a proposal to the codemaking panel, and they were convinced it needed to be addressed.
jim hardy
#81
May28-14, 08:18 PM
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Quote Quote by brenfox
The potential voltage will be dropped across the load, so in theory the current will run along the neutral with no volts pushing it??
As MrSparkle said

Drop along the neutral wire is very small.

eg #14 is 2.5 milliohms per foot.

You indeed have a voltage divider between the load and the neutral
with the load devouring the tyrannosaurus' share of voltage.

A 50 foot #14 neutral carrying ten amps clear across the household will be elevated at the load end by
2.5 X 50 = 125 milliohms, X 10 amps = 1.25 volts.
About 1% in US, leaving load with 99% .

EDIT: Late Addition
Above was posted too quickly in an attempt to achieve minor clarification ( that's likely unneeded) .
Of course the "Hot" conductor carrying power to the load takes its bite as well, leaving 98% for the load.



old jim
Averagesupernova
#82
May28-14, 09:57 PM
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Sophie. In the USA I don't know of any single family residences that are fed with 3-phase. All single phase, and I would imagine all but the oldest of houses in the far reaches from civilization are 240 volt split-phase. Over here multiple houses share the same transformer. Different utility companies have different standards on who owns what. Some places the customer owns everything up until the transformer except the actual meter itself. The customer will own the meter socket. Other utilities the customer only owns up until the meter socket sometimes including the socket, other times not. 200 amp services are common in my location. 50 KVA transformers in my location are the largest you can get for 240 volt single-phase, at least for a single customer. However, it is not uncommon at all to have half a dozen 200 amp services on a single transformer of this size. With smart meters the utility can tell just how a customer loads the transformer. If a customers useage gets too close to capacity the utility will make you put in another transformer and meter and split your operation up.
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I see some advantage of a ring system. I just don't like the possibility of things going wrong and going unnoticed. I am not a big fan of multiple current paths anyway. I have had experiences on PC boards with a ring type ground. Normally ground currents would go mostly to the left for example from a certain component. Suppose that path breaks for some reason and now the ground currents go to the right. Now these currents are sharing a path that they did not before and if they are a high frequency or have fast switching current they can creep into places that they did not before. But, everything seems to be working. No catastrophic failures, just suddenly a noise that didn't exist before. Although it could happen with a star type ground if the PCB was laid a certain way, I recall hoards of circuit boards wrecked when there would be a ground current from an external source that opened up a ground trace in a ring type ground. The result was the ground pin for a 5 volt regulator floating up and putting 12 volts on everything on the 5 volt supply bus. This was just an example of an engineer who did not have a good grasp on design for mis-use or failure. Best case would have opened a single ground trace instead of frying every 5 volt component on the board. Something similar to this is in my opinion the largest drawback of the split-phase system. Losing a neutral ahead of the service (actually that is considered a shared neutral) has the potential to over voltage one side of the line and undervoltage the other side. However, it will never damage permanent building wires hidden inside walls.
AlephZero
#83
May28-14, 10:10 PM
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I think much of this is extrapolating from the faults in a system you know, to those in a system you don't know. That works both ways. As a Brit, the entire US system seems like it was designed by Rube Goldberg on a bad hair day....

Quote Quote by krater View Post
So you fuse a 14 at 32 amps? Jesus! I realize that it is ring connected and ideally should not be a problem but my concern would be exactly the problem described above, if one side of the ring goes open the circuit may continue to work just fine, however it is drawing the entirety of the load down one leg which is regularly overloaded.
It's theoretically possible, but I've never heard of it happening in practice in 50 years.

Splices go bad more often than one might like to think. Over here a bad splice, even when it happens on a neutral which leads to stuff taking both lines and burning up, tends to have the result of someone getting a call to service it.
Again, this probably comes down to the construction system as much as anything else. I don't know exactly what you mean by "splice", but again that type of fault just doesn't happen in the UK. (And it's impossible with the UK house wiring system for a fault to cause "double the voltage and burn up" in any case.)

Remember that we don't live in wood-framed houses either. Faulty wiring doesn't set fire to bricks or concrete wallblocks very easily.

Unless I suppose you knock your radio or something into the sink, the plug fuse blows, you go to the hardware store and buy the biggest they have and, well, what happened to the protection?
"The biggest fuse they have at the store" that will fit a standard UK plug will be 13A, which is the rating of the outlet it would be plugged into. So there is no safety issue with the house wiring. Fuses are to protect the wiring, not the equipment. If a UK appliance needs a low rated fuse for the safety of its internal circuitry, that fuse would be internal to the device and not user-replaceable. For example devices like "wall wart" power supplies typically don't have any user-accessible fuses.

I suspect lower rated plug fuses in the UK are a left-over from the old wiring system which had different sized plugs and sockets rated at 15A, 5A and (occasionally) 2A, each with its corresponding fuse. Even in my current house, I've still got an old 2A plug and socket that powers the water pump for the heating system. It's in such an inaccessible place that replacing it would be a serious building job, not 10 minutes' electrical DIY work.
Averagesupernova
#84
May28-14, 11:04 PM
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Quote Quote by AlephZero View Post
I don't know exactly what you mean by "splice", but again that type of fault just doesn't happen in the UK.
I suspect it does happen but goes unnoticed. You see that is the beauty of the ring system in that it is redundant and at the same time the drawback in that the failures never show up.
AlephZero
#85
May29-14, 05:39 PM
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The redundancy makes it more tolerant of a single "loose connection" fault. A loose connection that carries high current will generate heat and be an immediate fire risk. That hazard is removed by the redundancy.

Since the cable in a 32A ring is rated at 20A not 16A, a poor connection may not actually generate an overload condition in any case, depending how the ring is used - and remember everything is at 230V, so typical currents are half of the US 120V equivalents.

The wall sockets are designed for reliable connection of 3 sets of cabling, without any additional hardware for cable joints - i.e. the two "halves" of the ring, plus an optional spur connection to another socket.

It does introduce the possibility of other wiring errors though - e.g. adding wiring which cross-connects two rings, which means that a fault may leave 20A wiring protected by only a 64A fuse (2 x 32A in parallel).
sophiecentaur
#86
May30-14, 04:47 AM
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Quote Quote by AlephZero View Post
The redundancy makes it more tolerant of a single "loose connection" fault. A loose connection that carries high current will generate heat and be an immediate fire risk. That hazard is removed by the redundancy.

Since the cable in a 32A ring is rated at 20A not 16A, a poor connection may not actually generate an overload condition in any case, depending how the ring is used - and remember everything is at 230V, so typical currents are half of the US 120V equivalents.

The wall sockets are designed for reliable connection of 3 sets of cabling, without any additional hardware for cable joints - i.e. the two "halves" of the ring, plus an optional spur connection to another socket.

It does introduce the possibility of other wiring errors though - e.g. adding wiring which cross-connects two rings, which means that a fault may leave 20A wiring protected by only a 64A fuse (2 x 32A in parallel).
That's a good example of the high build quality of components used in UK Power distribution. The UK system looks a bit 'quirky' compared with what you find elsewhere but (with a few exceptions - available on some market stalls) it's made very well, with chunky brass components and large contact areas.
Or even cross connecting two rings, brought back to the consumer unit. But all systems are open to abuse by bad installation contractors.

It always amazes me that there is such a contrast between power and lighting circuits. You just cannot buy 'dodgy stuff' for UK power circuits but lighting fitments seem to come in all levels of quality and finish.


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