Surface conduction at 60 Hz

Wire ampacity ratings are based on temperature. Nickel-plated copper can withstand higher temperatures than tin-plated copper and so higher ratings are possible. Keep in mind that nichrome wire is typically used in heater elements, whereas copper would typically be unsuitable for that application.

Skin effect, cross-sectional area, and conductivity get at resistance, which is not the primary factor in establishing current carrying capacity.

Does that make sense?

Chuck

Proctologically Violated©® wrote:

"Proctologically Violated©®" wrote in message news:_eWdnZrPoJBtCGLenZ2dnUVZ snipped-for-privacy@rcn.net...

...

Per cubic volume, or square if you prefer a 2-D visualization, the current passing thru a material is the same. A square foot cross sectional area of a material, regardless of its shape allows electron/hole flow at a consistant rate throughout its area.

Say you have a cross sectional area of 100 pieces of one square foot. Each square foot of area is allowing the same number of electron/hole flow as each of the others. Now, take those hundred squares and arrange them so they make an approximate circle (as in the cross sectional area of a wire). Each square foot of area still passes the same amount of current. Then, if you look at the very center, there is only one square foot of area tha can occupy the exact center. But, around the outside perimeter, it takes several square feet of area to make it all the way around the perimeter. Therefore, if you take an area on the outside of our gigantic wire on the outermost surface, it has many, many square feet of area, and thus a lot more current passing through it than the single square foot that occupies the center-most area. But, if you limited the area you're looking ato to jsut one foot in width, not the whole perimeter, it would be carrying the exact same current as the square foot in the center. THAT is why people get confused and think it's skin-effect, which it is not. But, obviously, around the entire full outside of the circle, there is a lot more current moving than in the tiny center because of the greater number of square feet it takes to make it round.

Hope that might help visualize it a little better?

Skin-effect is an entirely different phenominon and has no relationship to what this anecdote describes. With skin effect, current per square area of material is NOT the same - so, it's a lot different. And, it takes high frequencies to make tha t happen, but it's too much to go into here.

HTH,

Pop

Forget skin effect at 60 hZ. The effect at such a low frequency is of no practical concern. Even at the high end of audio frequencies ~20kHz the effect is negligible. Here's one audio site that makes that point:

Watch for line wrap in the above. Ed

I will take up a number of issues in sequence.

Forget about electrons, holes, and the like. They have no practical significance in almost all cases except when ELECTRONIC DEVICE operation is to be considered.

I don't believe the statement that nickel coated wire can higher currents than uncoated copper. In most cases, the limitation is going to be the maximum allowable temperature of the insulation. Skin depth is going to be greater for nickel than for annealed pure copper because nickel has a lower electrical conductivity. Countering that is the higher magnetic permeability of nickel which will reduce skin depth. The nickel plating will dissipate more power and reach a higher temperature than plain copper. In any event, I would like to se a reliable citation.

Skin depth at 60Hz in copper is about 6 mm. While that is large compared to typical wire radii in house wiring, it often cannot be neglected. That is why why power transmission lines often are fabricated with steel cores clad with copper or aluminum. Aluminum is preferred much of the time because of its low density and greater skin depth.

Bill

-- Ferme le Bush

Jeez, you've made some really bad rationalizations there! Better go back to class for awhile!

Short of vaporization, and ignoring insulation, which is most definitely not the subject of the OP's question, what exactly determines the ampacity of a metal wire?

If it is not temperature, then what else is there?

Skin depth relates to resistance or impedance. These may influence the suitability of a wire for a particular purpose, but they are not the basis for ampacity ratings. There is no direct relationship between resistance and ampacity if temperature is not considered.

A quick web search will reveal that nickel-plated copper wires DO carry a higher temperature rating than tin-plated copper wires. Or, put differently, they can carry higher currents at a particular temperature. Look here, for example:

Conductor Information - Wire and Cable

Chuck

Salmon Egg wrote: SNIP

Incorrect. It's > 9 mm

So you are saying that *the reason* 60 Hz power transmission lines are fabricated with steel cores clad with copper or aluminum is *because* skin effect cannot be neglected in that situation. And your reference for that is ?

Putting it another way, you imply that the transmission line could not be made out of the same amount of copper that is used in the cladded line you are talking about because of losses due to skin effect. Do you have numbers that support that idea?

With the lines you are talking about, would the greater ductability and lower strength of a purely copper cable (if it was made that way) preclude its use? Do you think this might be a more compelling reason for using steel core?

Ed

On Sat, 25 Feb 2006 21:09:06 GMT, ehsjr Gave us:

High tension lines are just that. Very high tensile forces are on the line. That is why it HAS to be steel cables. The point at which at attaches to the towers has several thousand pounds of weight hanging on it. Aluminum or copper either one would creep and cause breaks in the line. 5mm of Aluminum cladding reduces the resistance of a tower to tower traverse quite a bit at 60Hz. Way better than plain steel.

The aluminum, or copper cladding IS for better conduction. Even though losses at such high voltages are not that great, minimizing them is STILL part of the job, and in the case of high tension lines at 60Hz, cladding the cable in a better conductor such as aluminum or copper does make a better transmission line. Remember also that the lines are grouped in bundles of 3 or 4 cables, separated by a few inches of air, between towers. So they maximize the tower's capacity for weight, and they maximize the amount of aluminum or copper that is actually carrying flow, getting the most out of the tensile capacity the steel core wire has.

So three or four resistors (wires) in parallel from tower to tower to tower gives us the least lossy solution for passing the juice from point to point.

Please be more speciofic.

Bill

-- Ferme le Bush

The purpose of the steel core is to provide the tensile strength that allows longer spans (fewer towers) and less thermal expansion. Because of skin effect, copper in the center of a large conductor is wasted. Skin depth is about 6 mm. How much steel core and haw much copper or aluminum is placed on top of the core is an economic decision. Remember that the radii of conductors could be in the neighborhood of 20mm.

If copper were very cheap and spans small, wasting a bit of conducting metal would not be a big deal.

Bill

-- Ferme le Bush

Copper wire run at high temperature will oxidize badly and that is the limiting factor. I believe nickle-plated wire can operate at higher temperatures without oxidizing. Same is true with high temp crimp lugs. High temp nickle-plated wire will of course have high temp insulation.

bud--

Its not so much the nickel plating as the maximum operating temperature of the conductor and insulation. The nickel plating only serves to protect the copper conductor from reactions with oxygen at higher temperatures.

The NEC ratings are based on conservative calculations of the ability of a conductor to radiate or conduct the thermal energy away that is produced by I^2R losses and still remain within the temperature limits of the surrounding insulation. The are affected by insulation thermal resistance and ambient temperature, among other factors.

All correct, except that the reason for the multiple phase conductors has to do with the system voltage, not skin effect. At high voltages, the E field strength in the vicinity of a small radius conductor will exceed the ionization potential of the surrounding air. This ionization will result in power loss (it can be modeled by an equivalent resistance from conductor to ground per mile). A conductor bundle simulates a single conductor with a diameter equivalent to the bundle spacing. The field strength around this larger radius 'surface' is reduced below the ionization potential.

In addition, the use of bundled conductors appreciably reduces the series inductance of the line even though it does produce a small increase in capacitance to ground and ground level fields. This is not a negligable factor.

Unfortunately, except at DC, that is not true. For AC there are inductive effects and one result of this is a non-uniform current density distribution. For 60 Hz, except for large diameter conductors, this is negligable as the "depth" is larger than the conductor. A related phenomena is proximity effect where the current in one conductor modifies the current distribution in another conductor.

Exactly. The steel is required for strength, *not* to increase the amount of copper at the surface. The copper at the surface is required to reduce the electrical resistance of the cable, as copper or aluminium is a much better conductor than steel.

Your are comparing steel clad copper versus copper clad steel, by implication. Do you seriously believe that, for a given core diameter and a given cladding thickness, a copper core, steel clad cable would be as strong as a steel core, copper clad cable for the transmission lines we're talking about?

Whether copper in the center would be wasted or not is irrelevant. The need for a large steel core for strength puts the copper or aluminum on the surface in the first place, so there is no need to consider skin effect.

Ed

Skin depth is

It may be so, but it is not immediately obvious to me that parallel bundles of spaced wires will lower the electric field at the cylindrical surfaces. Locally, the cylindrical surface of the individual conductor is going to have the same radius as the conductor. To convince me, I would have to go through a Schwarz transformation to solve the two dimensional potential problem. I am not moved to do that.

Except for the skin effect, the velocity of propagation in the TEM mode along the line will be the speed of light. This means that the product of inductance and capacitance of the line stays relatively constant.

Bill

-- Ferme le Bush

I hope this statement is not attributed to me because I never considered steel clad copper.

Central copper in large conductors is wasted! Skin effect can be significant. Thin copper cladding on steel is often used for antennas where the skin effect would make thick cladding very wasteful indeed.

Skin effect also shows up in large ac electrical machines. To use the copper effectively, it is broken up into smaller separate conductors that are smaller than the skin depth. These conductors are insulated from each other and transposed. It is, in a sense, the machine analog of litz wire.

Bill

Even in 1930+ when the lines from the then Boulder Dam to LA were built, the advantages of a larger diameter conductor were apparent as was the extra weight of a solid conductor considering skin effect. Rudenberg was one who did an in-depth analysis of this. The conductors used there were about

2.5cm diameter and were made of twisted "barrel staves" about 3 to 4 mm deep (working from memory of a sample). ACSR simply gave us the advantage of the larger diameter with the strength of a steel core (not intended for current carrying). Bundled conductors were a further improvement in terms of surface fields and reduced inductance as well as mechanical and construction advantages.