Question about electricity

[aluijten]

A question for the really knowledgeable:

How come I need a thicker wire to conduct say 1000 Watts though a wire @ 12 Volt compared to 1000 Watts @ 230Volt.

Clearly the wire gauge is not only dependent on the amount of power it needs to conduct, but also on the voltage/amps ratio. Why?

Cheers,

Arno

 

[AngusMcDoon]

A question for the really knowledgeable:

How come I need a thicker wire to conduct say 1000 Watts though a wire @ 12 Volt compared to 1000 Watts @ 230Volt.

Clearly the wire gauge is not only dependent on the amount of power it needs to conduct, but also on the voltage/amps ratio. Why?

Cheers,

Arno

Wire guage depends on current only. Doesn't care about the volts. Overhead power lines are thin yet transmit huge power. High volts low amps.

 

[sailorman]

A question for the really knowledgeable:

How come I need a thicker wire to conduct say 1000 Watts though a wire @ 12 Volt compared to 1000 Watts @ 230Volt.

Clearly the wire gauge is not only dependent on the amount of power it needs to conduct, but also on the voltage/amps ratio. Why?

Cheers,

Arno

its the restance & amps
1Kw @ 12v = 83Amps
1Kw @ 230v =4.3Amps

 

[halcyon]

A question for the really knowledgeable:

How come I need a thicker wire to conduct say 1000 Watts though a wire @ 12 Volt compared to 1000 Watts @ 230Volt.

Clearly the wire gauge is not only dependent on the amount of power it needs to conduct, but also on the voltage/amps ratio. Why?

Amperage, and Volt drop.

A 1000 watt at 12 volt = a nominal 83 amp. needs big cable to carry load, and avoid volt drop in cable run.

A 1000 amp at 230 volt is a nominal 4.3 amp, low current small cable.

Brian

 

[ap073]

Cable is designated by its current carrying capacity. The thicker the cable, the more current it can carry. Voltage and wattage do not matter (except in the calculations). If you need more detail just ask!

 

[aluijten]

OK, thanks so far.

Next question: electricity is the flow of electrons.
So related to the volt and amp; If the voltage is a measure for the density of electrons involved in the flow, what is the amperage a measure for?
In other words, why is the voltage not relevant in the required thickness of the wire.

 

[sailorman]

OK, thanks so far.

Next question: electricity is the flow of electrons.
So related to the volt and amp; If the voltage is a measure for the density of electrons involved in the flow, what is the amperage a measure for?
In other words, why is the voltage not relevant in the required thickness of the wire.

its akin to a water hose pipe
small bore pipe more pressure req for the same volume as a larger bore pipe

 

[AngusMcDoon]

OK, thanks so far.

Next question: electricity is the flow of electrons.
So related to the volt and amp; If the voltage is a measure for the density of electrons involved in the flow, what is the amperage a measure for?
In other words, why is the voltage not relevant in the required thickness of the wire.

Voltage is like pressure, i.e. how hard you are pushing the electrons.
Amps is a measure of flow rate, i.e. how many electrons per second.
Coulombs is how many electrons.
So a Coulomb is an Amp per second.
Power in Watts depends on how many electrons pass per unit time and how hard they are being pushed, so volts multiplied by amps.

Fluid flow equivalent...

Volts -> pressure, hPa in your part of the world
Amps -> flow rate, litres per second
Coulombs -> volume, litres
Power -> well you could drive a water turbine with lots of low pressure water, or a little high pressure water and get the same power output. But in the former case you would need a big pipe, which brings us back to your original question.

Amps already has time in it (coulombs per second) which is why when people are talking about batteries and they use units like Amps per hour they are talking bullocks.

 

[elton]

OK, thanks so far.

Next question: electricity is the flow of electrons.
So related to the volt and amp; If the voltage is a measure for the density of electrons involved in the flow, what is the amperage a measure for?
In other words, why is the voltage not relevant in the required thickness of the wire.

Amperage (current) is a measure of the electrical charge (in Coulombs) flow per second. 1 Coulomb = 6 x 10e18 electrons.

 

[aluijten]

Thanks guys, finally got the pressure thing straight in my head. Too much time elapsed since science class
Still think it's funny that the total power conducted is not directly a measure for the thickness of the wire. Somehow you expect it to be.

Final thing. I heard a quick and dirty trick to calculate the wire diameter (in square mm) as being 1/3 of the current in Amps. Does that sound reasonable?
I know you should include the length of the cable into the calculation, but does it meet the rule of thumb approach? At least on moderate cable lengths. I'm not yet working on my 300+ ft yacht mind you.

 

[stuhaynes]

Thanks guys, finally got the pressure thing straight in my head. Too much time elapsed since science class
Still think it's funny that the total power conducted is not directly a measure for the thickness of the wire. Somehow you expect it to be.

Final thing. I heard a quick and dirty trick to calculate the wire diameter (in square mm) as being 1/3 of the current in Amps. Does that sound reasonable?
I know you should include the length of the cable into the calculation, but does it meet the rule of thumb approach? At least on moderate cable lengths. I'm not yet working on my 300+ ft yacht mind you.

Not a sparky, but as I understand it, each time you double the voltage the amperage halves. A load of 1KW at 12 volt equals about 84 amps. At 24 volts the amperage reduces to about 42 amps.

 

[CreakyDecks]

OK, thanks so far.

Next question: electricity is the flow of electrons.
So related to the volt and amp; If the voltage is a measure for the density of electrons involved in the flow, what is the amperage a measure for?
In other words, why is the voltage not relevant in the required thickness of the wire.

Voltage has nothing to do with density of electrons involved in the flow. The density of the electrons doesn't change, a wire is electrically neutral from a charge point of view whether there is current flowing or not. In the same way the density of water in your pipes doesn't change when you turn a tap on.
When talking about supplies it is probably better to use the term electromotive force instead of voltage.

 

[aluijten]

When talking about supplies it is probably better to use the term electromotive force instead of voltage.

That has been a long time since I've heard that expression (in Dutch it's EMK).
But it's a good point. If anything it's the Amps that say something about the density as I believe the speed of electrons does not vary in a conductor.

 

[AngusMcDoon]

My understanding is that it's the speed of the signal that does not vary much, i.e. from pushing an electron in at one end to getting an electron (different one though) out the other end, but that the speed that the electrons flow does vary. But I may be wrong.

 

[Sailfree]

Wire guage depends on current only. Doesn't care about the volts. Overhead power lines are thin yet transmit huge power. High volts low amps.

Thats why all power transmission is done at the highest voltage possible to keep the amps low and the wire thickness to a minimum.

 

[AngusMcDoon]

Thats why all power transmission is done at the highest voltage possible to keep the amps low and the wire thickness to a minimum.

Is that why God does his lightening bolts at zillions of volts? Keeps the bolt guage down saving money on bolt gas.

 

[wytco0]

And we havnt even got into surface (skin) effects yet ;-)

Keep its simple its the amps that matter in this situation, more amps = thicker wire.

 

[Billjratt]

Try not to get too involved in science (coulombs and ergs and all that)
The density of electrons - that's the first time I've heard it put that way - is not quite right,- they don't compress, so you need a wider path to get so many electrons past in a specific time - the waterpipe analogy comes to mind and is valid.
Voltage is the potential difference in electrical terms equivalent to the head of water in waterpipe terms. ie - pressure, the thing needed to make them flow.
As to your last question - ultimately, the thickness of the wire will depend on the length of the path from A to B and back again. If they've got plenty of room, the electrons won't even touch the sides and reach the other end without losing potential energy. If there isn't quite enough room, they will lose a certain amount every foot, so they won't all make it to the other end, and if there is a severe lack of cross-section, they will be queuing up to get onto the wire and jostling all they way down, getting all steamed up and frustrated, you will be able to feel the heat if you touch the wire...and there will just be a trickle at the far end.

 

[electrosys]

The other thing to bear in mind when trying to get your head around electricky flowing along wires is - just as there is no such thing as the perfect insulator, there's no such thing as the perfect conductor. Copper is good, but it has some resistance (under normal conditions) - if it was a perfect conductor, then you could quite happily run a couple of thousand amps along a very thin wire. But it isn't, so you can't.
Indeed, it's the conductor's resistance to electron flow that causes heating to occur, and thus poses an upper limit on the current which can safely be conveyed along it.

 

[cpedw]

Final thing. I heard a quick and dirty trick to calculate the wire diameter (in square mm) as being 1/3 of the current in Amps. Does that sound reasonable?

From the Tony Brooks website http://www.tb-training.co.uk/MarineE03.html your rule of thumb seems to be very conservative. That website suggests that area (mm squared) can be about Amps/9 for PVC coated cable.

Derek