wiring a power shower

[AdamW]

what has a mountain to do with it?

I see what he is getting at, an analogy between potential electrical energy and potential gravitational energy.

If you jump from the top of the mountain... let's call it a cliff... you "flow" from the high potential to the low potential.







SPLATT!

 

[Damocles]

Got a case in point here. FWL, if you are still there, you said:

"If a persons knowledge of electricity and circuits is so low that they genuinly believe a larger cable will supply more power than a little one and will thus cause a piece of equipment to suffer damage, then I too must seriously question their ability to undertake the task they are asking about. "

Well actually a larger cable will supply more power. Thats why it's 2.5mm for sockets and 1 for lights.

I understand what you meant, but there is a danger of splitting hairs, as witness your own incorrect post.

Now as to your post about the dangers of electricity. I am sorry about your exeriences, but happily they are not the same as my own. The first sparks I knew delighted in telling stories about all the shocks he had had. His favourite being the one where he got the live stabbed into his thumb. As a boy I used to play with electricity and got quite a few shocks. People did not worry half so much.

Just one possible reason they did not worry so much might have been that people used smaller cables. When it finally comes down to shorting together the ends of the cable, then the amount of power delivered does indeed depend on the size of the cables. Having bigger cables than necessary may not matter while everything is working fine, but it can make quite a difference to the size of the bang if everything goes pear shaped.

Now where were those guys who had the misfortune to short out meter tails?

 

[ban-all-sheds]

Well actually a larger cable will supply more power. Thats why it's 2.5mm for sockets and 1 for lights

No no no - a larger cable can safely carry more current - it has no role in the "supply", it just meets the demand, with varying degrees of safety. If you seriously think that a 1mm² cable can limit the supply of current to an accessory, then try using 1mm² wire in fuses, and see what happens...

 

[Damocles]

Yes Yes Yes, have you never heard of voltage drop, resistance, prospective short circuit current?

just how big is the wire in a 100A fuse anyway?

 

[ban-all-sheds]

have you never heard of voltage drop, resistance, prospective short circuit current?

Yes, yes and yes.

But this argument was about whether an accessory could be damaged because a larger cable will supply more current. So in that context it is wrong to say "Well actually a larger cable will supply more power" - it won't, or at least within the limits of domestic circuits and domestic appliances it won't. If you put a 10.5kW shower on 2.5mm², then ignoring temperature coefficients, you could go up to 11.2m before VD exceeded the limit.

But all that is the other way around - appliance manufacturers do not design their products to require a voltage drop via thin cables in order to be safe. If you wired your lighting circuit in 16mm you would not find that your lamps were brighter and shorter-lived.

So no - a larger cable will not "supply" more power.

just how big is the wire in a 100A fuse anyway?

16SWG, which is 2mm²

 

[Damocles]

You are pandering to my pedantry here!

YES IT WILL! power =amps x Volts or V^2/R. v is the same, you make R smaller by having a bigger cable. And very probably higher current rated protective devices too.

The bigger cable will SUPPLY more power. The pump will not take it, but it IS AVAILABLE. So if you connect something different on the end of the wire (like yourself) then more power is available. You get a much crispier fried body afterwards if the available supply is greater, and that is the point.

I know this started as being about how much power an appliance will draw, but engineer muddled up the difference between available power and used power while he was trying to make a case about safety. It is NOT NOT NOT equally safe to have more power available than is needed by the circuit. Maybe it normally will not make any difference, but other times it could be the difference between a little bang and a big one, a hot cable or a fire, A death or a nasty shock.

I know I bang on sometimes about people overdoing the safety warning thing, but its because I want to see a bit of balance. Be aware of risks and understand them.

(added) the original poster was worrying about connecting thin wires from the pump to the thick wires coming from the ring spur. In essence he was exactly correct to worry. The solution to his worry was the fuse in the spur unit designed to limit the amount of current AVAILABLE from the ring.

 

[AdamW]

The bigger cable will SUPPLY more power. The pump will not take it, but it IS AVAILABLE. So if you connect something different on the end of the wire (like yourself) then more power is available.

You are forgetting a rather important factor: the fuse. This is an important issue. Due to the lower resistivity of 10.0mm cable over 1mm cable, the 3A fuse will blow more quickly with a fatter cable. So, you will probably get a worse injury from the more sustained burning from the 1mm cable on a 3A fuse.

 

[Damocles]

A good point. The wire in the fuse requires a certain amount of energy to melt. This could be supplied by a low current for a long time, or a high current for a short time.

The fuse is designed to isolate the fusewire somewhat so that heat is lost slowly, but yes a high current will blow faster and overall energy supplied during the fault will be less. I am not sure if it will be significantly less. It depends on the relative sizes of the rate of heat loss and heat gain. If the rate of heat gain is very much bigger than the rate of loss, then the loss rate can be ignored, and the amount of energy delivered to you will be the same. Whether a quicker delivery is better or worse on this timescale I have no idea.

But as I just added to my last point, the discussion rather ignored the existence of the spur fuse altogether.

And a MCB will take just as long to trip however big the current, once the fast trip threshold has been reached. (unless there is so much power it literally blows to pieces)

 

[AdamW]

Let's start a new thread: "Given the choice, would you rather get a shock off a thick cable or a thin one?"

Or, a 10.0mm cable with a 50mA RCD, or a 1mm cable with a 30mA RCD?

 

[pipme]

It is raining so cannot cut the grass today ... Such is life.

P

 

[plugwash]

while i thicker cable could supply more power to a fault (negligable difference in normal operation) the risks posed by this are generally considered to be negligable as you should be reaching the limit of the fuse long before the resistance of the cable becomes an issue in any properly designed cuircuit

also for the case of electric shock the human body has a high enough resistance that the difference in current between 1.5mm and 10mm feed cables into the human body would be negligable

 

[pipme]

The long arm of FWL_Engineer all the way from Tue Mar 09, 2004 2:05 am still causing ripples in the pool ... is he recruiting an army of sparks from Euroland

 

[Damocles]

It is now raining here too so I also can not cut the grass.