Selecting a Conductor
Even in the design of a simple single insulated wire many factors must be considered: temperature, voltage, DC resistance of the conductor, insulation, O.D., required flexibility, physical properties of the conductor (tensile strength, voltage drop, conductivity, weight) and when necessary, specific electrical characteristics such as the dielectric properties of the insulating material.
Before selecting a particular insulated conductor, numerous factors should be considered. Those that pertain to the conductor are: size, stranding and material.
SIZE Determined with regard to DC resistance, current carrying capacity and breaking strength requirements.
GAUGE The most important consideration in calculating the individual AWG size is the CIRCULAR MIL area minimums as called out by ASTM (American Society for Testing Materials) to meet UL, CSA and military requirements, and by SAE (Society of Automotive Engineers) for most automotive products.
Gauge is expressed as AWG (American Wire Gauge) within the United States and Canada. An increase in gauge number results in a decrease of the diameter of a wire.
Size can also be expressed as CMA (Circular Mil Area), a term used to define cross sectional areas using an arithmetic short-cut in which the area of a round wire is taken as "diameter in mils (.001") squared.
MCM = 1000 Circular Mils ex: 500 MCM is 133 strands of .0613 size individual wires which have 3,757 circular mils each, this equals approximately 500,000 total circular mils or 500 x 1000 which is the same as 500MCM.
500 MCM = 133 strands of 3,757circular mil material (approx. 14 AWG) or 499,681 total circular mils.
Metric Equivalent Of AWG
STRANDED CONDUCTORS Developed as a way to overcome the stiffness of solid conductors, stranded conductors are composed of smaller gauge wire bundled or wrapped together to create a larger conductor. The Gauge size of stranded conductors is often expressed as a combination of the overall size and the size of the individual strand.
EXAMPLE: 16 AWG 26/30 - 16 represents the overall gauge size, 26 is the number of strands, 30 is the gauge size of each of the 26 wires. This can also be expressed as 26/.0100 using a decimal size.
Stranded conductors are preferred for several reasons:
CONDUCTOR FLEXIBILITYis much greater in stranded conductors, making them easier to install.
FLEX LIFEis longer than with solid conductors. Stranded conductors can endure more vibration and bending before breaking. Generally speaking, the finer the stranding is the more flexible the conductor will be.
SURFACE DAMAGEto stranded conductors, such as scratching or nicking, will be less serious than similar damage to solid wire.
STRAND COUNT affects both the flexibility and cost of a conductor. For any wire size, the more strands present, the more flexible and the more expensive the conductor becomes.
COPPER Either bare or tinned, copper is the most commonly used conductor metal.
In applications which copper is not a suitable choice, several options are available:
ALUMINUM This metal has many properties similar to copper; ductility, malleability, thermal and electrical conductivity, and the ability to be covered (extruded over) with almost any material suitable for insulating copper. While conductor costs may sometimes be reduced by using aluminum (especially in larger gauge sizes), savings diminish as gauge sizes are reduced. Aluminum is seldom used in OEM applications.
Drawback to aluminum conductors include:
- Aluminum has only 61% of the conductivity of copper, and therefore, wire diameters need to be 50% greater to provide equivalent current carrying capacity. This can create a significant increase in the outer diameter of a wire. Flex life is also 1/2 to 1/3 that of copper.
- The main advantage of using aluminum is a reduction in weight; aluminum weighs 1/3 as much as copper.
- Aluminum is difficult to solder to other metals.
- Aluminum may cause corrosion when in contact with some metals.
- Aluminum requires cleaning before termination, which can be a time consuming process.
- Aluminum is not usually drawn in smaller sizes.
BRONZE OR COPPER COATED STEEL When high tensile strength is required, such as coaxial cables or special cords, bronze or copper coated steel in the best choice.
HIGH STRENGTH ALLOYS While more expensive than copper or bronze coated steel wire, these copper alloy conductors permit significant size and/or weight reductions. High strength alloys offer high breaking strength and greater flex life with only a small increase in DC resistance. Cadmium-chromium copper, cadmium copper, chromium copper and zirconium are most frequently used.