shower cable

[theplumber]

Thanks for the many replys (very helpfull)

 

[ban-all-sheds]

Who says the starting point for the calcs has to be 230V?

The IEE.

 

[securespark]

But you can see from a layman's POV that the voltage figure you use to start the calc with could conceivably be anywhere between the parameters laid out by supply co, and of course in practice it is.

And the voltage value used determines the resistance value...

 

[BR]

I was trying to give a simple answer. Due to the short length or run here I believe 6mmsq to be adequate. 41.3A is well within the design overload characteristics of a 40A Type B MCB. Protecting 6sq with a 45A MCB is pushing it regardless of the installation method (with the possible exception of installation on a cable tray). The point of voltage rising is a valid one but unlikely to make a huge difference in this case.

 

[AL]

use 10mm2 stuff, i got a load that you can have for free (if you can collect from farnborough)

 

[securespark]

Off to Bramdean soon. How much have you got?

 

[ban-all-sheds]

But you can see from a layman's POV that the voltage figure you use to start the calc with could conceivably be anywhere between the parameters laid out by supply co, and of course in practice it is.

And the voltage value used determines the resistance value...

But can you see my point at all?

Why must it be @230V? If this figure changes (240 250 whatever) then the whole calculation goes t*ts up.

What I'm saying is that the whole basis of your calculations rest on a given I value (whatever that may be) at a voltage of 230. If the voltage value alters, then the figures differ. And voltage is the one thing that is prone to variation!

Who says the starting point for the calcs has to be 230V?

Get my drift? (He asks, hopefully!)

Sorry - 'fraid I don't see what your point is...

 

[securespark]

Ban ducking the issue again....

 

[Adam_151]

In our house there is an 8.5kw shower on a 6mm cable, in my mind that is pushing the cable a bit, for most of its length the cable runs in the loft, long the top of the joists but not clipped to them, it is not under the insulation, the insulation is below the cable. The cable run is about 20m (I am guessing)

When we moved in my dad changed the previous shower (7kw probably) to the current one, he said that 8.5Kw was right on the limit for the cable, and definatly would not consider a 9.5kw shower on the current (pun not intended) cable.

What is the general opinion on this, in my mind

A) It shouldn't really have been done with that cable size
B) Its unlikely to come to any harm though

 

[securespark]

As far as volt drop is concerned, it's fine, but I have not applied any other correction factors.

Plus, if it is protected by a rewireable fuse, then it is not OK, as rewirreable fuses are not the most quick-acting of protective devices.

 

[Adam_151]

Its a type 2 mcb

 

[AdamW]

The resistance of the shower does not increase when the voltage increases, it stays the same

Wow, you must have one of those superconducting showers!

As the voltage increases, the current increases as you say. However, because the power must thus increase the shower element becomes hotter. As you heat a typical shower heating element it's electrical resistance increases. So whilst the current increases, it increases slightly less than you would have us believe This would be more complicated if you were to build your element from a semiconductor, but if you do that then you deserve a hard life.

To sum up, the only situations where the resistance would remain constant are where it is zero (hence a superconductor) or infinite (in which case current is zero). Now if resistance is zero then it would be c**p as a heating element as you would get no temperature rise! If resistance is infinite then there is no current flowing hence no power and no temperature rise.

Note that I am only posting this to prove that I am not a complete gimp with respect to Ohm's law (after my last electric shower posts!)

 

[supersparks]

i have no idea why (its been a long day and ive hadda few) your all worried here
if the voltage increases so will the power of the shower (in terms of current)
but this wont matter to the cable because the distrabution voltage also rises..... and so there is less resistance in the cable.
like some very large systems might draw 990 000watts(example)
at 230 volts thats over 4304 amps...
at 400 000volts thats nearer 2.5 amps...
so with voltage rise resistance falls(as does temprature)
with the shower cable this would also apply as they will rise/ fall proportianly...


however like i said ive had a few and im possibly missing the mark alltogether...got to the stage where its taking me about 10 minutess to read a reply

 

[ban-all-sheds]

Ban ducking the issue again....

No - I really don't see.

The figure that you use is not arbitrary. If unspecified by the manufacturer (e.g. 9.8kW at 240V), then the figure that you must use is 230V.

Even when specified, AFAIK you should then convert that to what the current would be at 230V. Essential if the manufacturer's rating was at 220V.

What I'm saying is that the whole basis of your calculations rest on a given I value (whatever that may be) at a voltage of 230. If the voltage value alters, then the figures differ. And voltage is the one thing that is prone to variation!

That's the whole point of doing it. Either from the manufacturers data or from the official value of 230V you can work out I at 230V, and that is what you should use to design your circuit. However, for high current appliances like showers I think it is then a Good Idea to see what the current would be in extremis, when you may find that instead of being close to the limit for the cable you're far enough over it to warrant the next size up.

 

[ban-all-sheds]

Wow, you must have one of those superconducting showers!

No, but the key word is shower - and I did think about this...

As the voltage increases, the current increases as you say. However, because the power must thus increase the shower element becomes hotter.

Well now - I wonder if that is true? We don't know at what temperature the manufacturer has measured the consumption of his product - since it is in his interest for it to appear as powerful as possible, we might assume that he's done it at the lowest temperature possible, so in normal operation it may be that the resistance is already higher than V^2/P would indicate. Also, a shower element is very definitely operating in a non-adiabatic mode (at least, you'd better hope it is

), so all of the heat that it is generating is being transferred into and removed by the water. I'm not sure by how much the temperature of the actual element changes with the flow rate.

Even if it does vary a lot, we don't know what the user would do when provided with more power. If the water was already hot enough for him, he'd probably increase the flow rate, thus keeping it at the same temperature. If it wasn't then he'd accept the hotter water with pleasure.

I still don't know how that would affect the temperature of the heating element.

As you heat a typical shower heating element it's electrical resistance increases. So whilst the current increases, it increases slightly less than you would have us believe This would be more complicated if you were to build your element from a semiconductor, but if you do that then you deserve a hard life.

Now we should move on to the material used for the heating coil. Is it copper? It's probably not, as copper is too good a conductor - you'd need around a 1.5km of 6mm to get to 5-6 ohms resistance. An interesting feature of high-resistance wires used for heating elements, e.g. Nichrome, Magnanin etc is a very low temperature coefficient of resistance (probably not a coincidence). Whereas the figure for copper at 20deg C is 0.004, for Nichrome it's 0.00017, and for Magnanin it's 0.000015.

Unless the shower normally performed so pitifully as to be effectively useless, I doubt if the user would appreciate a temperature rise of more than 5 degrees, which would translate to a tiny increase in absolute resistance.

To sum up, the only situations where the resistance would remain constant are where it is zero (hence a superconductor) or infinite (in which case current is zero).

True. But I would be surprised if in the narrow temperature band under consideration the resistance of a shower element varied by more than it does due to manufacturing tolerances.

Note that I am only posting this to prove that I am not a complete gimp with respect to Ohm's law (after my last electric shower posts!)

That's the trouble with physicists - too theoretical.