Thursday, July 26, 2018

Why Wire Diameter Is Important



Here Size Does Matter 

A metal consists of a lattice of atoms, each with a shell of electrons. The outer electrons are free to dissociate from their parent atoms and travel through the lattice, creating a 'sea' of electrons, making the metal a conductor. When an electrical potential difference (a voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the influence of the electric field.

The larger the cross-sectional area of the conductor, the more electrons are available to carry the current, so the lower the resistance. The longer the conductor, the more scattering events occur in each electron's path through the material, so the higher the resistance. Different materials also affect the resistance.

Simply stated… as electrons move across a wire, they constantly collide with atoms making up a wire. These collisions impede the flow of electrons and are what cause the wire to have resistance. Thus, if the diameter of the wire were larger, it would only make sense that the electrons don't collide as much, therefore creating less resistance due to a larger wire. This is all in accordance to Ohm's law.

The resistance is the ratio of the voltage difference across an object to the current that passes through the object due to the existence of the voltage difference (Resistance = Voltage /  Current). If the object is made of a material that obeys Ohm's Law, then this ratio is constant no matter what the voltage difference is. 

Consider a copper wire that passes some amount of current, say 1 Ampere (A), when a voltage difference of 1 Volt (V) is applied between the ends of the wire. Now consider an identical but separate wire connected across that same 1V source. You would expect that it would also conduct 1A  (R=V/A   R=1/1   therefore R = 1 Ohm).

Now think of joining those two wires together side by side into one, thicker wire. Much like using a thicker pipe to increase the supply of water, it is reasonable to expect that this wire should carry 2 A of current if the potential difference across the wires is still 1 V. Thus, the new, thicker wire will have a reduced resistance of ½ Ohm compared to the original wire with its resistance of 1 Ohm. (R=V/A   R=1/2   therefore R = .5 Ohm).
Why? Basically, a thicker wire creates additional paths for current to flow through the wire. This reduces resistance which results in less generated heat. This is one of the reasons you should use heavy gauge wires when, for example, running a voltage supply to your 12 volt mobile radio. If you don’t, you could find your expected 12-13 volts DC is actually significantly lower. 

American Wire Gauge 

American wire gauge (AWG), also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire. The cross-sectional area of each gauge is an important factor for determining its current-carrying capacity.

The steel industry does not use AWG and prefers a number of other wire gauges. These include W&M Wire Gauge, US Steel Wire Gauge, and Music Wire Gauge.
Increasing gauge numbers give decreasing wire diameters, which is similar to many other non-metric gauging systems. This gauge system originated in the number of drawing operations used to produce a given gauge of wire. Very fine wire (for example, 30 gauge) required more passes through the drawing dies than did 0 gauge wire. Manufacturers of wire formerly had proprietary wire gauge systems; the development of standardized wire gauges rationalized selection of wire for a particular purpose.
The AWG tables are for a single, solid, round conductor. The AWG of a stranded wire is determined by the total cross-sectional area of the conductor, which determines its current-carrying capacity and electrical resistance. Because there are also small gaps between the strands, a stranded wire will always have a slightly larger overall diameter than a solid wire with the same AWG.

Stranded wires are specified with three numbers, the overall AWG size, the number of strands, and the AWG size of a strand. The number of strands and the AWG of a strand are separated by a slash. For example, a 22 AWG 7/30 stranded wire is a 22 AWG wire made from seven strands of 30 AWG wire.

AWG 18 has a solid diameter of about 1mm. Adding 6 halves the diameter, Subtracting 6 doubles the diameter. Adding 20 divides the diameter by 10, and subtracting 20 multiplies the diameter by 10. The following table lists the minimum recommended wire gauge for the length of supply cable in high power radio systems.

Recommended wire gauge for a given amperage and length

Conductivity of Common Metals

The 15 most common metals are listed at right, in order of their conductivity. But, you may say, gold is always touted as best for contacts! It’s true… while gold is not the best conductor, it does not corrode like some other metals and therefore provides more reliable contact over a longer period of time.  

Another surprise is that lead and tin, two of the most common elements in solder are relatively low on the conductivity list. The reason for using them is the fact that lead and tin are used for solder because not only do they have low melting points, but more importantly they form a “eutectic” alloy which has a considerably lower melting point than either one individually (many metals form eutectics), at the disadvantage of higher resistance.

Because its conductivity is the second highest of any metal and its cost is relatively low, copper sees use in most wire, connectors, printed circuit foils and related electrical parts. The resistance of a 24-gauge copper wire 1,000 feet long at room temperature will be about 26 ohms.
Silver's higher conductivity and cost make it a niche product. It's used as wire and solder in specialty electronics. By comparison, a silver 24-gauge, 1,000-foot-long wire would measure about 24 ohms.

2 comments:

  1. Yes often larger wire is used in DC circuits due to voltage drop. A wire might be able to carry a said current, but if long enough there could be sufficient voltage drop resulting in lower than acceptable voltage at the load. In a 120VAC circuit where the same current is flowing and having same voltage drop might not matter for losing like 2-5V out of 120 will not affect much. Same wire with same current in 12V DC circuit and losing 2V will matter.

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  2. It's pretty clear that you are talking mainly about DC power supply wiring. For clarity and completeness, I would have also mentioned skin effect. - 73, Dave VE3WI

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