10mm cable for shower - Why?

[sterose]

...that is why the regs should be followed by people who have no clue.

Is this the same Sterose that advises people to repair damaged cables (even shower ones) with junction boxes and then plaster over them because "its not always convenient to dig a cable out of a wall just to satisfy regs"?

Who says that you should earth plastic pipes, and radiators fed by plastic pipes, and will brook no arguments because he knows he's right?

Who's so in favour of regulations being followed even if they don't understand them that he thinks that Part P is b*11*cks and has no intention of using electricians because of it?

Who advises people to move their electricity meter themselves if the supplier asks £too much?

Who thinks that AC means that the neutral conductor is at 240V 50 times a second, and when people tell him he's wrong he accuses them of being mad?


Surely not....

"Is this the same Sterose that advises people to repair damaged cables (even shower ones) with junction boxes and then plaster over them because "its not always convenient to dig a cable out of a wall just to satisfy regs"?"

Ok ban-all-sheds. I admit that I said that.

However, I have performed this operation myself, and I deem it to be perfectly safe.

I do note, however, that I perhaps shouldnt suggest that others follow suit.
It was just a reasonable suggestion to that problem.


"Who thinks that AC means that the neutral conductor is at 240V 50 times a second, and when people tell him he's wrong he accuses them of being mad?"

I still think that it does. And I didn't use those words. I wouldn't mind someone who is a knowledgeable person / physicist perhaps filling me in if I am wrong on this point. My understanding is that the actual current flows in an alternate direction in AC, so therefore the neutral must become the "live" (in the loosest sense) when it flows backwards. Seems logical.


Incidentally, I have a grudge against the electricity board.
Perhaps I should follow my own advice and restrain myself from suggesting action that is beyond most peoples skill: i.e. moving meters.

 

[Big_Spark]

But doesn't that higher rating come about because the insulation of the cable will tolerate a higher temperature? If you install a circuit such that it runs with a higher conductor temperature than you would encounter with normal T&E, can you be sure that the switchgear and appliance(s) will be happy at that temperature?

NO it does not, the higher current carrying capacity of SWA comes about due to it being Hard Drawn Annealed Multi-Strand, T&E is soft drawn Multi-stranded and not annealed, as a result it heats up quicker for a given load than the cores of SWA or even Pyro, the latter is also a higher grade of copper, this is why a Pyro of a given size will carry more load than any other cable.

Doesn't 3-core SWA have a lower rating than 2-core? Or is that only on the assumption of multi-phase currents?

3 Core SWA has a LOWER volt drop rating than 2 core.

According to table 4D4B, the volt drop of 3 core cables is consistantly lower than that of 2 core cables, HOWEVER, this is assuming the cable will be used on a 3 phase supply in the Regs.

This is something I will be speaking to the IEE about in a few minutes to shed some light on this particular point.

 

[Big_Spark]

Sterose, what you say you have done, regarding making a joint in a shower cable and burying it in plaster is dangerous, foolhardy and makes you an out and out cowboy.

I can assure you that if you worked for me and you did that, you WOULD be looking for alternative employment.

Shower cables may be jointed, but you should use a proper BS approved cabnle joint kit to do it..but then ONLY if rewireing it is not a realistic option, economics are not a good reason.

As for moving Electricity meters, Sir if you have done this then you are a fool, not only is moving your incomer an Offense and lays you open to criminal proceedings, but is HIGHLY DANGEROUS.

You may believe you know what you are doing, I do not know if you are a qualified Electrician or a Bodging DIYer, but moving your incomer is foolhardy in the extreme.

That type of behaviour should NEVER be mentioned on a DIY forum as you are encouraging people to undertake tasks they do not have the skills for or the legal right to undertake.

 

[ban-all-sheds]

..My understanding is that the actual current flows in an alternate direction in AC, so therefore the neutral must become the "live" (in the loosest sense) when it flows backwards. Seems logical..

If we regard the neutral conductor as being at 0 volts, then at the start of a single cycle, the live, or phase, conductor is also at zero volts with respect to neutral. The phase voltage rises, and after 5ms it is at approx 340V with respect to neutral. It then starts to fall, after a further 5ms it is back to zero, and it continues to fall, until after another 5ms it reaches a value of approximately -340V with respect to neutral, which is still at 0V. Obviously with any potential difference you could argue that saying "B is at xV with respect to A" is the same as "A is at -xV with respect to B", and in terms of the PD between A&B, and therefore the direction in which current will flow, you would be right, but that would be ignoring any external frame of reference, and with the mains supply we do have one - earth. Neutral is, we hope, at more-or-less 0V with respect to earth at all times - it does not become live 50 times a second. It may be regarded as being at a positive voltage with respect to the live conductor for half the time, but that is very much not the same as it becoming live, not even in the loosest possible sense.

 

[securespark]

FWL

Just read your reply on page 2.

I only use 6mm (or re-use in the case of a shower upgrade) if the circuit is clipped direct, and not subject to grouping Temp or other factors - this is possible, where cable runs under floor in ducting etc..well ventilated.

I have never seen a shower with a cable clamp to cope eith a circular cable only 6-16mm2 t&E

It seems a lot of work, but if you are going to the trouble of sinking a box in the wall to terminate the swa in, you can do it without crimps (hate them!!!) by leaving your SWA tails long enough to exit the box and reach the term block in the shower. Hey presto, no joints!

 

[ban-all-sheds]

It seems a lot of work, but if you are going to the trouble of sinking a box in the wall to terminate the swa in, you can do it without crimps (hate them!!!) by leaving your SWA tails long enough to exit the box and reach the term block in the shower. Hey presto, no joints!

Presumably you'd run them in conduit? (Am I right in thinking that the cores on their own would not be allowed as they would only have a single layer of insulation?)

And would 3 singles (SWA cores + cpc) be clamped inside the shower OK?

The reason I'm asking all this is that sometime soon-ish (Before Oct ) I'm going to be upgrading my shower circuit from 6mm to 10mm. Or possibly SWA now....

 

[sterose]

..My understanding is that the actual current flows in an alternate direction in AC, so therefore the neutral must become the "live" (in the loosest sense) when it flows backwards. Seems logical..

If we regard the neutral conductor as being at 0 volts, then at the start of a single cycle, the live, or phase, conductor is also at zero volts with respect to neutral. The phase voltage rises, and after 5ms it is at approx 340V with respect to neutral. It then starts to fall, after a further 5ms it is back to zero, and it continues to fall, until after another 5ms it reaches a value of approximately -340V with respect to neutral, which is still at 0V. Obviously with any potential difference you could argue that saying "B is at xV with respect to A" is the same as "A is at -xV with respect to B", and in terms of the PD between A&B, and therefore the direction in which current will flow, you would be right, but that would be ignoring any external frame of reference, and with the mains supply we do have one - earth. Neutral is, we hope, at more-or-less 0V with respect to earth at all times - it does not become live 50 times a second. It may be regarded as being at a positive voltage with respect to the live conductor for half the time, but that is very much not the same as it becoming live, not even in the loosest possible sense.

Again communication has broken down.

Neutral is at 0V with respect to earth because it is earth.

Neutral to live is 240v 50 times a second.

Obviously you cannot be electrocuted without touching both conductors, which is why you dont get electrocuted by touching only neutral. You have to touch both, just as in the way only touching live without a path to earth produces no effect.



FWL: "As for moving Electricity meters, Sir if you have done this then you are a fool, not only is moving your incomer an Offense and lays you open to criminal proceedings, but is HIGHLY DANGEROUS."

I didnt move a meter. I had to cut the seals to work on my CU, because, without going into details, the Electric Board were being awkward.

"Sterose, what you say you have done, regarding making a joint in a shower cable and burying it in plaster is dangerous, foolhardy and makes you an out and out cowboy."
I did this job at home, not for a customer. Why would it be dangerous? I had already though of the implication of wet plaster getting into the box.


Incidentally, I'm curently a bodging DIYer.

I would like to emphasise though, that I have NEVER had an accident, in light of all the bodging I have done. I dont normally like to blow my own trumpet, although I have done many times on this forum, but I consider myself a fairly intelligent preson, and I have always had knack for electrics - amongst many, many other things.

 

[Big_Spark]

Sterose, regarding the Neutral, If you want a clearer understanding of it then I suggest you read my earlier post on this subject on page 1 of this thread.

Your understanding of AC is near to the mark, but not accurate.

 

[securespark]

Ban Wouldn't need conduit

FWL said mount the shower over the box, so the tails would go straight in.


FWL-

If 6mm2 SWA takes up to 63A, can't see a problem with 6 T & E running 46!!

 

[ban-all-sheds]

Again communication has broken down.

I think we're all trying to repair that breakdown..

Neutral is at 0V with respect to earth because it is earth.

This is true with a TN-C installation. I'm no expert (as I frequently demonstrate ), so I can't back this up with any explanation, but I don't think that with any other earthing system you can be sure that your Neutral isn't just "very close" to earth. But for the purposes of this debate it's irrelevant, and we can say that Neutral = Earth = 0V at all times.

Neutral to live is 240v 50 times a second.

Well, it's not, actually. A voltage difference between Neutral and Live of 240V occurs 200 times a second; 100 times Live is at +240V and 100 times it is at -240V. I can't bring myself to do the maths to work out at which points in the cycle it is +240V, and which it is -240V, but the two + voltages occur in the first half, either side of the +338V peak, and the two - voltages in the second half, either side of the -338V peak (or trough, if you're unhappy with the idea of a negative peak).

I'm afraid I still can't work out if you're now agreeing, or still disagreeing, that Neutral is always at zero volts, and that Live is at a positive voltage half the time, and a negative voltage half the time, but I suspect that if it's the latter then there's nothing more that I can say here in the way of further clarification.

 

[ban-all-sheds]

Ban Wouldn't need conduit

FWL said mount the shower over the box, so the tails would go straight in.

So he did - apologies to both of you for not picking up on that.

Bl**dy big hole to dig though - I think I'd screw my box to a ceiling joist, and run the tails in conduit down to the shower....

 

[ban-all-sheds]

But doesn't that higher rating come about because the insulation of the cable will tolerate a higher temperature? If you install a circuit such that it runs with a higher conductor temperature than you would encounter with normal T&E, can you be sure that the switchgear and appliance(s) will be happy at that temperature?

NO it does not, the higher current carrying capacity of SWA comes about due to it being Hard Drawn Annealed Multi-Strand, T&E is soft drawn Multi-stranded and not annealed, as a result it heats up quicker for a given load than the cores of SWA or even Pyro, the latter is also a higher grade of copper, this is why a Pyro of a given size will carry more load than any other cable.

Thanks for clarifying that - v. useful.

Doesn't 3-core SWA have a lower rating than 2-core? Or is that only on the assumption of multi-phase currents?

3 Core SWA has a LOWER volt drop rating than 2 core.

According to table 4D4B, the volt drop of 3 core cables is consistantly lower than that of 2 core cables, HOWEVER, this is assuming the cable will be used on a 3 phase supply in the Regs.

This is something I will be speaking to the IEE about in a few minutes to shed some light on this particular point.

OK - I was going by the table here (a similar one appears as Table 6E1 on p123 of the OSG), which does indeed show a lower voltage drop for 3- and 4-core cables but also gives lower current-carrying capacities. Although the cables in the table are not SWA, and so the absolute numbers are not the same, I would have expected the same relative relationship to apply to SWA. But if I'd looked more closely at the tables, I'd have seen that they do assume 3-phase working.

On the face of it a lower VD implies lower resistance which ought to mean higher current capacity.

Q1. Are we seeing a lesser contribution from reactance with a 3-phase cable? i.e. the total impedance is less, therefore the voltage drop is lower, but the resistance of the conductors is the same, and the grouping of 3 or 4 of them in the same sheath lowers their ability to dissipate heat and therefore lowers their current-carrying capacity?

Q2. Should I stop expecting the pros on this forum to act as unpaid electrical engineering lecturers?

 

[sterose]

Neutral is at 0V with respect to earth because it is earth.

This is true with a TN-C installation. I'm no expert (as I frequently demonstrate ), so I can't back this up with any explanation, but I don't think that with any other earthing system you can be sure that your Neutral isn't just "very close" to earth. But for the purposes of this debate it's irrelevant, and we can say that Neutral = Earth = 0V at all times.

I didn't say that it wasnt....

Neutral to live is 240v 50 times a second.

Well, it's not, actually. A voltage difference between Neutral and Live of 240V occurs 200 times a second; 100 times Live is at +240V and 100 times it is at -240V. I can't bring myself to do the maths to work out at which points in the cycle it is +240V, and which it is -240V, but the two + voltages occur in the first half, either side of the +338V peak, and the two - voltages in the second half, either side of the -338V peak (or trough, if you're unhappy with the idea of a negative peak).

Oh dear oh dear. If it cycles at 50 times a second, then live is positive (relative to neutral) 50 times and negative 50 times.

I'm afraid I still can't work out if you're now agreeing, or still disagreeing, that Neutral is always at zero volts, and that Live is at a positive voltage half the time, and a negative voltage half the time, but I suspect that if it's the latter then there's nothing more that I can say here in the way of further clarification.

I'm disagreeing.

You're understanding is inaccurate. If live is at a negative voltage relative to neutral, then conversely the neutral is at a positive voltage to live.

If we think of it in terms of a water pipe with a pump connected in a loop of pipe (so it circulates the water), when the pump pumps forward, there is a pressure on the 'positive' side, and a vacuum on the 'negative'. When this is reversed, the opposite is true.

To continue the analogy, there is no point saying that there is a pressure on positive but not a vacuum on negative, in fact both are the case.

 

[ban-all-sheds]

Oh dear oh dear. If it cycles at 50 times a second, then live is positive (relative to neutral) 50 times and negative 50 times.

Indeed it is. But earlier you said "Neutral to live is 240v 50 times a second". Go back and read it. So which do you want to go for now? A voltage of magnitude 240V between neutral and live 50 times a second, or 100 times a second (50 positive + 50 negative)?

I'll save you worrying - neither of those is correct, the answer is 200 times a second. The peak voltage on our domestic supply is 338V, so in the first half of the cycle it is at +240V TWICE, once on the way up from 0V to +338V, and once on the way down from +338V to 0V, and then similarly it is at -240V TWICE in the second half of the cycle.

That's 4 times in a one cycle, and 4 x 50 = 200.

If you don't believe me get a piece of graph paper and plot a sine wave of peak magnitude 338, and see how many times the curve passes through the +240 and -240 values.

I'm afraid I still can't work out if you're now agreeing, or still disagreeing, that Neutral is always at zero volts, and that Live is at a positive voltage half the time, and a negative voltage half the time, but I suspect that if it's the latter then there's nothing more that I can say here in the way of further clarification.

I'm disagreeing.

With what are you disagreeing? There were two parts to what I stated, and I've highlighed them in two colours above, so let's see if we can get to the bottom of what it is you disagree with.

Do you disagree with "Neutral is always at zero volts"?

If so, then why at the start of your last post did you quote me writing "we can say that Neutral = Earth = 0V at all times." and then say "I didn't say that it wasnt.... "

Or do you disagree with "Live is at a positive voltage half the time, and a negative voltage half the time"?

If so, why did you say, above, "If it cycles at 50 times a second, then live is positive (relative to neutral) 50 times and negative 50 times

 

[Big_Spark]

I'll write a big explanation later about all this, however suffice it to say I will post this.

NEUTRAL = EARTH= 0V THIS IS WRONG

Neutral and Earth ARE NOT ONE AND THE SAME, they just have the same electrical potential in a balanced supply with a unity power factor.

My previous post on this was worded VERY badly, confused the issue, and I posted a couple of bits that are actually wrong..I must have had my head in my proverbial ...sorry.

I will write a detailed explanation of all this tomorrow, I have to go out now.