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Explaining wire thickness

J

Jimes

Jan 1, 1970
0
I was looking at the discussion here on Underground wiring, and one of the
comments there was to say ...
another idea would to use a couple of transformers, one on each end of
the run, to boost the voltage up to 480Vac and then back down on the
load end. Compare the cost of the transformers to the cost of putting
in a bigger pipe. Higher voltage = less current = smaller wire sizes
for the same load.

Some questions

(1) I understand the point about racking up the Voltage (ac) and as a
consequence reducing the amperage...but why does that mean a smaller
thickness wire is suitable? At the end of the cable, you have the same
energy available and by reducing the voltage, the amperage available goes up
as a consequence. It's the same 'energy' going in one end and coming out the
other, why does the wire size not have to be the same in both cases?

(2) Would there be any difference if one brings a DC Voltage into the
picture? I suppose the same technique cannot be used on a DC current as it
the very alternating nature of AC current that makes a transformer work. But
if one had DC devices (computers, lights, TV) what would be the
considerations of having a 12V supply running all over the house? Does one
get DC appliances (stoves / fridges / geysers) ?

Thanks
Jimes
 
R

R L driver

Jan 1, 1970
0
Jimes said:
I was looking at the discussion here on Underground wiring, and one of the
comments there was to say ...


Some questions

(1) I understand the point about racking up the Voltage (ac) and as a
consequence reducing the amperage...but why does that mean a smaller
thickness wire is suitable? At the end of the cable, you have the same
energy available and by reducing the voltage, the amperage available goes up
as a consequence. It's the same 'energy' going in one end and coming out the
other, why does the wire size not have to be the same in both cases?

(2) Would there be any difference if one brings a DC Voltage into the
picture? I suppose the same technique cannot be used on a DC current as it
the very alternating nature of AC current that makes a transformer work. But
if one had DC devices (computers, lights, TV) what would be the
considerations of having a 12V supply running all over the house? Does one
get DC appliances (stoves / fridges / geysers) ?

Thanks
Jimes
I was taught that the thermal losses in a cable are Volts x Amps = watts.

(VxI = W)

As the voltage can be represented by V = IxR then the heat loss can be
represented as IxIXR or I^2 R. So the heat loss is equal to the current
squared multiplied by the resistance of the cable.

So if you can keep the current down there are less thermal losses , if
you transform up then the current falls
so if we have a piece of cable with a resistance of 1 ohm


for 2 amps 2x2=4 watts lost
for 4 amps 4x4=16 watts lost
not double but 4 times !

doubling the voltage cuts thermal losses to a quarter

In the UK we use 132000 V for power transmission, so thermal losses are
Way low.

I guess its why here in Yorp we use 240 V and its why the lights on old
6 volt VW's are so crap

Steve the grease
 
J

JoeSP

Jan 1, 1970
0
Jimes said:
I was looking at the discussion here on Underground wiring, and one of the
comments there was to say ...


Some questions

(1) I understand the point about racking up the Voltage (ac) and as a
consequence reducing the amperage...but why does that mean a smaller
thickness wire is suitable? At the end of the cable, you have the same
energy available and by reducing the voltage, the amperage available goes
up as a consequence. It's the same 'energy' going in one end and coming
out the other, why does the wire size not have to be the same in both
cases?

(2) Would there be any difference if one brings a DC Voltage into the
picture? I suppose the same technique cannot be used on a DC current as it
the very alternating nature of AC current that makes a transformer work.
But if one had DC devices (computers, lights, TV) what would be the
considerations of having a 12V supply running all over the house? Does one
get DC appliances (stoves / fridges / geysers) ?

Thanks
Jimes

This may help:
http://www.windsun.com/Hardware/Wire_Table.htm
 
W

William P.N. Smith

Jan 1, 1970
0
harry said:
The key factor is non of these. The power lost in the cable causes
heating, if there's enough heating the insulation of the cable will
break down and ...boom!

That's "ampacity", the maximum current-carrying ability of the wire.
Hardly anyone uses ampacity in wire calculations, as you want to keep
losses low, which typically happens at way below the current limit of
the cables.

[I've used it myself as a limit in low voltage, high-current DC
supplies with remote sensing when I didn't care about efficiency, but
for siring a house or a solar power system, you care more about
efficiency than you do about initial wire cost.
 
D

daestrom

Jan 1, 1970
0
harry said:
Nevertheless this is how cables are rated. The actual power lost is
tiny compared with the electrical load (unless the cable is very long
when the volt drop caused may affect the performance of electric motors
etc)
As a matter of interest stranded cables carry more current for the the
same cross sectional area. This is due to "fringing" ie most of the
current goes down the periphery of the conductors & very little down
the middle. Dunno why :)

Like many 'old wives tales' this has a *hint* of fact and a lot of fiction.

Skin effect will cause AC currents to travel on the outer regions of a
conductor. This has the affect of reducing the amount of conductor metal
carrying current and thus increases the losses.

But skin effect for 60 Hz doesn't have much affect until the conductor is a
couple of cm in diameter (not mm, centimeters). In anything short of some
'serious' power transmission, it is truly negligible.

The second fiction is that 'skin effect' does *not* affect individual
strands of wire in a stranded conductor. The cause is the magnetic field
created by the current and that field doesn't affect the current
distribution within the individual strands. The one exception to this is a
very special (and costly) type of stranded wire known as "Litz wire".

For typical power conductors, there is no difference between solid and
stranded wire of a given total cross-section.

The last time the discussion of voltage-drop / power losses came up, it was
noted that at the low-voltages of some PV systems (12 or 24VDC), and a
significant cable run (350'), the voltage drop *is* much more significant
than the thermal rating of the cable insulation.

daestrom
 
J

Jimes

Jan 1, 1970
0
Thanks for all the feedback... let me see if I understand it correctly...

To get a certain amount of power through the wire, one has to look at three
factors: The Voltage (V) the Current (I) and the Resisitance (R).

Using the two relationships of Power = Current x Voltage and Voltage
= Current x Resistance....

Assuming the power remains constant, then as one increases the voltage, the
current demand component decreases as a consequence, and if the current
demand decreases, then one can get away with a higher resistance, so a
smaller wire becomes viable.

As one decreases the voltage, the current demand increases to deliver the
same power, and one finds that the resistance has to be lower, so the wire's
thickness becomes more important. Thicker wire = Lower resistance.

Does that about sum it up?

Thanks
Jimes
 
D

dnoyeB

Jan 1, 1970
0
Jimes said:
Thanks for all the feedback... let me see if I understand it correctly...

To get a certain amount of power through the wire, one has to look at
three factors: The Voltage (V) the Current (I) and the Resisitance (R).

Using the two relationships of Power = Current x Voltage and
Voltage = Current x Resistance....

Assuming the power remains constant, then as one increases the voltage,
the current demand component decreases as a consequence, and if the
current demand decreases, then one can get away with a higher resistance,
so a smaller wire becomes viable.

As one decreases the voltage, the current demand increases to deliver the
same power, and one finds that the resistance has to be lower, so the
wire's thickness becomes more important. Thicker wire = Lower resistance.

Does that about sum it up?

Thanks
Jimes


Thats pretty much it.

http://en.wikipedia.org/wiki/Voltage_drop
 
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