The National Board of Fire Underwriters prepares tables showing the safe current ratings for sizes and types of conductors covered with various types of insulation. The allowable current-carrying capacities of single copper conductors in free air at a maximum room temperature of 30º C (86º F) are given in the table below. At ambient temperatures greater than 30º C, these conductors would have less current- carrying capacity.
Temperature Ratings and Current-Carrying Capacities (in Amperes) of Some Single Copper Conductors at Ambient Temperatures of 30ºC.
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 the table below.
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 resistance of pure metals, such as silver, copper, and aluminum, increases as the temperature increases. However, the resistance of some alloys, such as constantan and manganin, changes very little as the temperature changes. Measuring instruments use these alloys because the resistance of the circuits must remain constant to get accurate measurements.
In the table below, the resistance of a circular-mil-foot of wire (the specific resistance) is given at a specific temperature, 20º C in this case. It is necessary to establish a standard temperature. As we stated earlier, the resistance of pure metals increases with an increase in temperature. Therefore, a true basis of comparison cannot be made unless the resistances of all the substances being compared are measured at the same temperature. The amount of increase in the resistance of a 1-ohm sample of the conductor per degree rise in temperature above 0º C is called the temperature coefficient of resistance. For copper, the value is approximately 0.00427 ohm.
A length of copper wire having a resistance of 50 ohms at an initial temperature of 0º C will have an increase in resistance of 50 ´ 0.00427, or 0.214 ohms. This applies to the entire length of wire and for each degree of temperature rise above 0º C. A 20º C increase in resistance is approximately 20 ´ 0.214, or 4.28 ohms. The total resistance at 20º C is 50 + 4.28, or 54.28 ohms.
Specific resistances of common substances.
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