Power Cables

John
Henderson

Power cables connect equipment to a power supply, allowing energization and proper functioning. They are used extensively in facilities and plants for lighting, power and control circuits installed in the facility. In this article, you will learn about the types of power cables, the standards for power cables, installation concerns, as well and repair and maintenance considerations.

Types of Power Cables

There are many different types of power cables used in industry. The types of cables equipment can use depends on the load requirements, and the type of current the equipment requires.

Shielded Cables

At low frequencies, AC cables can be interchangeable for DC wires. However, at higher frequencies, the skin effect creates too much electrical noise, resulting in electromagnetic interference (EMI). Therefore, most AC power uses shielded cables. Shielded cables are made of one or more insulated wires enclosed by a common conductive layer. The shielding layer often composes of braided strands of copper or another metal, a non-braided spiral winding of copper tape, or a conducting polymer. This shielding layer allows the shielded cable to combat the skin effect of AC power. It also protects the cable from outside radio interference. As a result, high voltage power cables are often shielded.

Coaxial Cable

Coaxial cables consist of an inner conductor surrounded by a concentric conducting shield. Communication systems often use coaxial cables to transmit data between equipment. Coaxial cables also connect PLCs and other computers to a local computer network. The shielded design allows data to be quickly transmitted without succumbing to outside interference or damage. Coaxial cables are a cheap, low-cost installation for network devices.

Ribbon Cables

Ribbon cables consist of multiple insulated wires running parallel to each other on a flat plane. They often resemble a long slender ribbon hence the name. A ribbon cable usually contains between 4 and 12 wires. They are often used to interconnect network devices to a network. Unlike coaxial cables, ribbon cables transmit multiple signals of data simultaneously. They are also used to connect internal PLC modules to the CPU core.

Twisted Pair Cable

Twisted pair cables consist of pairs of copper wires twisted around each other. The number of pairs vary for different types of cables based on the required resistance of the cable. In general, the more pairs added to the cable, the resistance of the wire also increases. The cable also becomes more resistant to outside interference. Twisted pair cables are typically easy to install and inexpensive. Therefore, they are often used for low frequency AC power but are most common in telephone cables.

Power Cable Rating

Cable Ratings define the parameters for a safe usage of a particular cable. The three cable ratings used are temperature, current, and voltage. Temperature ratings define maximum conductor temperature, minimum installation temperature, and minimum flexible temperature of the cable. The voltage rating defines the RMS value between the insulated conductor and its covering, the RMS value between phase conductors, and the maximum system voltage. Finally, the current rating defines the normal maximum current rating and the short circuit current rating of the cable. However, there are different installations methods that change the current and voltage ratings. For example, a wall installed cable has a lower current rating than a cable of the same size, length, and material installed in a cable tray.

Conductor
Reference method C of Table 4E4A (clipped direct)
1 two-core cable, single phase a.c. or d.c.1 three or four-core cable, three phase a.c.
CSACurrent Carrying CapacityVoltage DropCurrent Carrying CapacityVoltage Drop
mmAmV/A/mAmV/A/m
1.527312327
2.536193116
449124210
6627.9536.8
10854.7734.0
161102.9942.5
251461.90a
1.85d
1241.65
351801.35a
1.35d
1541.15
502191.00a
0.98d
1870.87

When selecting a cable for equipment, it is important to know the current and the voltage drop listed by the equipment. The current rating of the cable should be higher than the load current. The voltage drop of the equipment must be lower than the voltage drop rating. If these requirements are not met, the size of the cable should be increased or the number of runs of the cable should be increased. However, increasing the number of runs is more costly than using a larger cable, so most often a larger cable is selected.

Construction

There are many construction considerations that need to be made. The National Electric Code (NEC) requires a certain amount of space between different types of cables, the length of cables as well as other requirements. Cables must also be protected from environmental factors. Often times, cables are installed inside raceways for protection from various factors. The NEC also regulates these raceways regarding the fill of the conduit and other factors regarding the strength of the conduit.

Repair and Maintenance

The power cables are arguably the most vital component of an electrical system. Therefore, it is critical that they are kept in good condition and inspected regularly. During an inspection, the presence of any exposed wiring is sought. Exposed wires can lead to short circuits, loss of power to equipment and if touched, severe injury and death. If an exposed wire is seen, the insulated material must be repaired with electrical tape or the cable must be replaced. When repairing an exposed wire, the wire must be completely de-energized. It is also important to make sure it is operating in its voltage and current rating so voltage and current measurements are taken.

In the United States, the NEC recommends a color code for power cable identification. For AC systems, the protective ground is green, the neutral wire is white, and phase wires are red, black, and blue respectively. And for DC systems, the protective ground is green, the positive end is red, the negative end is black, and other neutral wires are white. Canadian installations should refer to the Canadian Electric Code for color cables and refer to the International Electrotechnical Commission for European installations.