Cable Specifications

One of the most important aspects of systems engineering is adequately designing the interconnections between components. In Mechanical Engineering, adequate bearings, belts, and gears must be chosen to transmit mechanical power within the system. Similarly, in Electrical Engineering, adequate cabling and connectors must be chosen in order to transmit both electrical power and electrical signals efficiently and without failure. This page is going to go over many of the different categories of electrical cables in use today and expound upon their uses, their strengths, and their shortcomings.

This page is going to be divided into three sections. The first two will describe factors to consider when choosing cables for the two primary applications of electrical cabling, power distribution and data transmission. Power distribution will be defined as interconnects for supply voltage rails and other high current loads such as motors. The data transmission section will discuss a system’s ability to effectively communicate analog or digital signals through different cables. Finally, the page will finish with a mention of several industry standard cable and connector solutions.

Terminology

In order to help explain the differences between many forms of electrical cabling, a set of commonly used terms will first be defined.

  • Wire - A length of conductive metal (usually copper or aluminum) surrounded by a plastic exterior (usually PVC or polyurethane).
  • Cable - A set of wires contained inside of yet another plastic housing or joined together by their plastic exterior.
  • Conductor - The metallic interior of a wire which carries the electrical current.
  • Jacket - The plastic exterior of a wire or cable, also referred to as Insulation.
  • Gauge - A shortening of American Wire Gauge (AWG) which refers to the cross sectional size of the wire. A wire’s gauge places it on a logarithmic scale where smaller numbers refer to larger diameters. Every three gauge decrease doubles the cross sectional area.
  • Breakdown Voltage - The voltage at which the insulation between two conductors will break down or arc over.
  • Short - An accidental connection of two conductors at different voltage levels. The low resistance of the connection leads to a large, instantaneous current draw which can have catastrophic results.
  • RMS - The Root Mean Square value of an AC signal is the DC equivalent signal that would dissipate the same amount of energy.
  • Baud - Unit of data transmission speed. One Baud is one bit per second.
  • Crosstalk - Signal interference between nearby conductors caused by the pickup of stray energy.

Power Distribution

Wire used for power distribution suffers primarily from the effects of wasted energy. Ideally, all of the energy passing through a cable should make it directly from the source to the load. However, the electrical resistance of the cable will be wasting some of the energy as heat as current passes through it. In most cases the resistance of connecting wires is so small in comparison to the load that this can be safely ignored, however, in cases where the source will need to be supplying many amperes of current or if the cable is especially long then ignoring these resistive effects can lead to catastrophic failure of your system.

Too Much Wasted Energy

Power dissipated in a resistive load is calculated most generally as (Watts)=(Volts)(Amperes), but can also be defined as (Watts)=(Amperes)2(Resistance) which will be a much simpler expression for our situation. As stated previously, the power dissipated in a cable can often be ignored but it is now clear to see that as the current rises, the power dissipated by the cable rises quickly with it. And unfortunately, the plastic jacket surrounding our conductor will also be acting as a thermal insulator, meaning that heat will not escape very quickly from our cable. If too much power is dissipated by our cable, the subsequent heat can begin to fatigue our conductors or melt our jacket, possibly leading to a short and a catastrophic failure.

Ampacity

The solution to the problem of wasting too much energy in your wires is to select cables with a sufficient ampacity. Ampacity, or Current Carrying Capacity, is the maximum number of amperes that the wire is able to pass through it without failure. These values are all estimated and depend heavily on the scenario in which the cable is installed, therefore, these values should be selected with large margins for error.

First, these values assume that the current will be flowing for long enough to heat up the cable and cause failure. Higher currents can be passed in short bursts as long as the RMS current is still below the rated ampacity. Second, several jacket parameters can have large effects on the ampacity of a wire. A thicker jacket or a more thermally insulating material could lower the ampacity dramatically. A generally nominal jacket is assumed and two values are given for bundled or seperate wiring.


Data Transmission


Specialty Cabling

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