Wire Ampacity - Reference Question

Classic Toy Trains Forum
1

Dumb mechanical engineer type question here that you electrical types out there should be able to answer.

Is the ampacity of solid wire the same as stranded wire of the same gage?

I think I remember learning somewhere that electricity is mostly conducted on the surface of the conductor. If that is the case, stranded wire should have a higher ampacity than solid wire due to the fact that it has more surface area?

2

Bill, I found this online:

http://www.rowand.net/shop/tech/wirecapacitychart.htm

“Stranded vs. Solid WireThis one is a bit of a mind-boggler, but it’s important. When electricity flows through a wire, it mostly flows on the surface of the wire, not through the middle. This effect is more pronounced on high frequency AC than it is on DC or low frequency AC. This means that a “wire” of a given size that made up of many smaller strands can carry more power than a solid wire - simply because the stranded wire has more surface area. This is one reason why battery cables in your car and welding cables are made up of many very fine strands of smaller wire - it allows them to safely carry more power with less of that power being dissipated as heat. However, this “skin” effect is not as pronounced in a typical 12V DC automotive application, and the wire and cable used there is stranded for flexibility reasons.When looking at a chart or description of wire capacity, take note of whether it is referring to stranded or solid wire - some charts may not specify but instead assume a default based on the typical wiring used in a given application. For example, almost all automotive wiring is stranded while almost all home wiring is solid. For most applications, flexibility or the lack thereof will be more important, but for very high frequency AC applications, stranded wire might be a requirement.”

3

It is the same. The skin effect depends on the resistivity of the material and the overall diameter of the wire. The slightly larger overall diameter of stranded wire, which increases the surface area, is exactly compensated for by the slightly higher average resistivity resulting from the voids included among the strands. [National Bureau of Standards, U.S. Department of Commerce, Circular C74, Radio Instruments and Measurements, U.S. Government Printing Office, Washington, D.C., 1937, pp. 300 and 306-307] In any case, the skin effect is completely negligible at 60 hertz for any wire size that you are likely to use for toy trains. (There is no skin effect at all for the DC current in battery cables.)

4

As Bob said, there is no need to worry about “skin effect” when wiring your layout. When used at 60 Hz, the “skin depth” of a solid copper wire, that is, the distance from the wire surface at which the current resistance stays low, is close to a centimeter, which far exceeds the radius of any wire you would ever use.

5

The same ampacity, (I had learned this from Bob in the past), but also, the stranded wire is more flexible. that is its advantage.

Dennis

6

Dittos here. The skin effect is proportional to the frequency. The higher the freq. the more skin effect. We just covered this in one of my electrical classes. In fact their is a radio station at our school and they have a flat cable for the grounding conductor on the broadcasting aerial.

“Grounding a piece of equipment operating at 60 Hertz, for example, may may be as simple as using a grounding rod and a piece of 6 AWG copper conductor. Grounding a piece of equipment operating at 20 megahertz, however, may require the use of wide copper tape or wide, flat, braided, cable.”

(Quote from Delmar’s Standard Textbook of Electricity. 4th edition, pg. 460)

7

Thanks for the answers guys.

I figured that the power levels we typically see on our layouts any difference between stranded and solid wire would be unnoticable.

My preference for stranded wire is the improved flexibility.

8

Check this site for just about anything you’d like to know about wire.
There is a little application you can download and put on your computer.
Or you can just use the site:

http://www.wiretron.com/nicrdat.html

9

The only thing I can say for solid wire is that for a 500 foot role(awg 12) it can be $10.00 cheaper a role. Stranded wire is easier to work with as others have said.

Think about this, would you want your electric stove cord to be solid wire? Once you positioned it, it would be difficult to bend, as most stoves use awg 8 or 6 with three or four wires, can you say Hercules training?

Lee F.

10

The reason for using braided cable for grounding v. #6 cable is that the braided cable has a much lower inductance than the #6 cable. The #6 cable has the strands twisted together which creates an inductor. From 1967 to 1978 I worked in and managed large analog and hybrid computer simulation labs. We found by experimentation that 2" flat braided cable was far superior to any other ground including solid bus bar. However, for electric trains, forget the braided cable and use regular wire.

11

The skin effect is caused by the E field within the conductor. A high frequency signal on a stranded wire will flow only on the outer surface of the outer conductors. It will not flow on the inner conductors.

12

The skin depth in copper at 60 hertz is 1/3 inch, whereas the heaviest wire any of us is likely to use is 10 AWG, which has a radius of only 1/20 inch. So, as “Servoguy” says, it need not concern us in wiring toy trains.

“8ntruck” is correct in reaching the same conclusion, but for the wrong reason. It is not the power level that is important, but rather the current. Our power levels are much lower than for a typical residence; but that is because the voltage is lower; and power is the product of voltage and current. The currents that we use are comparable; that is why we often use the same gauges of wire that are used to wire our houses. But the skin effect on those wires is no more important on a train layout than it is in house wiring.

As for stranded wire, there are two very different kinds which are often confused. “Litz” wire is a special stranded wire in which the individual strands are insulated and woven together in a pattern that migrates every strand between the inside and the surface of the wire. It, like flat conductors, is an effective way to reduce the skin effect. But ordinary stranded wire is as vulnerable to the skin effect as solid wire. Even the slight increase in overall diameter produced by the voids among the strands is, as I described above, exactly cancelled by the decrease in average conductivity caused by the same voids. But all of this is irrelevant at 60 hertz for the size of wires that we use.