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Properties of Conductors

In the previous Article," Introduction to Conductor Inspection ", I listed all the important terms and their definitions in one complete glossary.


Today, I will explain the different Properties of Conductors as follows.



3- Properties of Conductors 





The fundamental concern of power cable engineering is to transmit current (power) economically and efficiently. The choice of the conductor material, size, and design must take into consideration such items as:


  1. Ampacity (current carrying capacity),
  2. Voltage stress at the conductor,
  3. Voltage regulation,
  4. Conductor losses,
  5. Bending radius and flexibility,
  6. Overall economics,
  7. Material considerations,
  8. Mechanical properties.



As an Electrical Inspector, you will check to see that conductors are of the proper size, rating, insulation, and terminated correctly.



3.1 Type of Material


Not all materials conduct current equally well. Two materials of interest in electricity are:

  1. Conductors: they allow current to flow,
  2. Insulators: they oppose the current to flow.

The ease with which a metal allows current to flow is described as conductance



Definition
Conductance: it is a measure of the ability of a given material to conduct electrical current.


  • All metals are given a conductance rating which shows how well they conduct electricity, as compared to copper. The best conductor is silver, but copper is used more often because it is cheaper. Gold is a better conductor than aluminum but once again it is rarely used.
  • The type of material used as a conductor will affect its current carrying capacity. This is because different material elements vary in their amount of conductivity or specific resistance.

Definition
Specific Resistance: It quantifies how strongly a given material opposes the flow of electrical current. It is the reciprocal of conductance.
The specific resistance of a metal (conductor) is based on 1 circular-mil, one foot long.




To compute the resistance of a conductor of any metal, use the value given for the resistance of a circular mil-foot of the material and use the following formula.






Example#1:
If the resistance of 1 cmil-ft of copper at 23C° is 10.5, what is the resistance of 500 feet of copper wire with a diameter of .292?



Solution:



  
Note
For rough estimating computations of resistance of round copper wire in ohms per thousand feet, divide 10,500 by the size of the wire in circular mils.


Table-1 is a list of material elements and their specific resistance values. For example, a copper wire will pass more current than an aluminum wire of the same diameter and the same length. The copper wire has a lower specific resistance value.


Table-1: Specific resistances of conducting elements.(Resistance equals 1 cir-mil-ft in ohms)



3.2 Copper VS Aluminum Conductors


  • Although silver is the best conductor, its cost limits its use to special circuits. Silver is used where a substance with high conductivity or low resistivity is needed. 
  • The two most commonly used conductors are copper and aluminum. Each has positive and negative characteristics that affect its use under varying circumstances. A comparison of some of the characteristics of copper and aluminum is given in Table-2. 

Characteristics
Copper
Aluminum
Tensile strength (lb/in2)
55,000
25,000
Tensile strength for same
conductivity (lb)
55,000
40,000
Weight for same conductivity (lb)
100
48
Cross section for same conductivity (C.M.)
100
160
Table-2: Comparative Characteristics of Copper and Aluminum


  • Copper has a higher conductivity than aluminum. It is more ductile (can be drawn out). Copper has relatively high tensile strength (the greatest stress a substance can bear along its length without tearing apart). It can also be easily soldered. However, copper is more expensive and heavier than aluminum. 
  • Although aluminum has only about 60 percent of the conductivity of copper, its lightness makes long spans possible. Its relatively large diameter for a given conductivity reduces corona. Corona is the discharge of electricity from the wire when it has a high potential. The discharge is greater when smaller diameter wire is used than when larger diameter wire is used. However, the relatively large size of aluminum for a given conductance does not permit the economical use of an insulation covering. 
  • The choice between copper and aluminum conductors should carefully compare the properties of the two metals, as each has advantages that outweigh the other under certain conditions. 


The properties most important to the cable designer are:



A- DC Resistance


The conductivity of aluminum is about 6 1.2 to 62 percent that of copper. Therefore, an aluminum conductor must have a cross-sectional area about 1.6 times that of a copper conductor to have the equivalent dc resistance. This difference in area is approximately equal to two AWG sizes.



B- Weight


  • One of the most important advantages of aluminum, other than economics, is its low density. A unit length of bare aluminum wire weighs only 48 percent as much as the same length of copper wire having an equivalent dc resistance.
  • However, some of this weight advantage is lost when the conductor is insulated, because more insulation volume is required over the equivalent aluminum wire to cover the greater circumference.



C- Ampacity


The ampacity of aluminum versus copper conductors can be compared by the use of many documents. See Chapter 9 for details and references, but obviously more aluminum cross-sectional area is required to carry the same current as a copper conductor as can be seen from Table-1.



D- Voltage Regulation


  • In ac circuits having small (up to #2/0 AWG) conductors, and in all dc circuits, the effect of reactance is negligible. Equivalent voltage drops result with an aluminum conductor that has about 1.6 times the cross-sectional area of a copper conductor.
  • In ac circuits having larger conductors, however, skin and proximity effects influence the resistance value (ac to dc ratio, later written as ac/dc ratio), and the effect of reactance becomes important. Under these conditions, the conversion factor drops slightly, reaching a value of approximately 1.4.



E- Short Circuits


Give consideration to possible short circuit conditions, since copper conductors have higher capabilities in short circuit operation.



F- Other Important Factors


  • Additional care must be taken when making connections with aluminum conductors. Not only do they tend to creep, but they also oxidize rapidly. When aluminum is exposed to air, a thin, corrosion-resistant, high dielectric strength film quickly forms.
  • When copper and aluminum conductors are connected together, special techniques are required in order to make a satisfactory connection. 
  • Aluminum is not used extensively in generating station, substation, or portable cables because the lower bending life of small strands of aluminum does not always meet the mechanical requirements of those cables. However, it is the overwhelming choice for aerial conductors because of its high conductivity to weight ratio and for underground distribution for economy where space is not a consideration.
  • Economics of the cost of the two metals must, of course, be considered, but always weighed after the cost of the overlying materials.

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