Conductor Selection and Sizing

Conductor Selection and Standard Sizing in Electrical System Installations:

Copper, aluminum, and copper-clad aluminum are the most common material selections used for commercial and industrial wiring installations. Because of characteristics such as flexibility and good conductivity, copper (Cu) is the most widely used type of conductor material. Brief and concise introduction of Conductor Selection and Sizing is given below:

Copper as Conductor in Electrical System Installations:

Copper has better conductive properties than aluminum and connections made with copper are usually not as restrictive as they are when using aluminum or copper-clad aluminum. For example, wiring devices that are compatible with copper are readily available, while wiring devices for other types of conductors are typically more expensive and can require the use of anticorrosive grease as part of the installation process.

Aluminum as Conductor in Electrical Systems:

Aluminum (Al) is not as good a conductor as copper but is inexpensive and lighter, making it suitable for high-voltage applications such as overhead power lines.

Copper-clad aluminum conductors are also inexpensive and lightweight but have better conductive properties than aluminum conductors. Per the NEC, conductors must be aluminum, copper-clad aluminum, or copper unless otherwise specified. Improper connections between conductors can result in a high resistance connection.

Standard Sizes of Conductors:

Conductors are available and sized as either solid or stranded. A solid conductor is a conductor that is composed of a single piece of conductor material. A stranded conductor is a conductor composed of several strands of solid wire wrapped together to make a single conductor. Stranded conductors provide more flexibility for pulling and routing through raceways.

American Wire Gauge (AWG) Standard:

Conductors are sized based on the American Wire Gauge (AWG) system. The AWG system assigns a number for each size based on the conductor’s cross-sectional area. The AWG sizes range from 40 (the smallest) to 4/0 (the largest, pronounced “four aught”). See Figure 1.

Conductor Selection and Sizing: The smaller the AWG value, the greater the cross-sectional area and the heavier the conductor.
Figure 1. The smaller the AWG value, the greater the cross-sectional area and the heavier the conductor.

The definition of AWG sizes by cross- sectional area is based on a single strand. Sizes larger than 8 AWG are impractical as single, solid conductors, so they are only available as stranded conductors. To meet the same cross-sectional area, stranded conductors have a slightly larger overall diameter. This accounts for the tiny empty spaces between the strands.

Most wiring installations require only a few different sizes. For example, branch circuits in 120/240 V systems typically use even-numbered sizes from about 14 to 8 AWG. (Smaller conductors are typically used only in electronics.) Distribution circuits use larger sizes. Conductors larger than 4/0 AWG are sized based on their circular mil area (cmil). (A circular mil is the area of a circle with a diameter of one mil, which is one-thousandth of an inch.)

Conductor Sizing: Medium-voltage power cables are typically composed of one 37-strand, 500 kcmil conductor.
Medium-voltage power cables are typically composed of one 37-strand, 500 kcmil conductor.

Insulation in Electrical Power System:

An insulator is a material that has a very high resistance. Insulators (insulation) resist the flow of electrons (current). Common insulation material includes rubber, plastic, air, glass, and paper.

All electrical conductors must be protected against possible contact with other conductors, metal parts, and people. Conductor insulation protects the conductor from damage and isolates the electrical power contained within the conductor. However, not all energized parts of an electrical circuit are protected by insulation.

When energized parts of an electrical circuit are exposed, such as where conductors are terminated at fuse or circuit breaker panels, the distance, or air gap, is used as the insulator. The greater the distance between energized electrical conductors or parts, the greater the resistance. The higher the voltage, the greater the distance that is required to create a high enough resistance to prevent undesired electron flow, such as with deadly arc flashes.

Insulation in Power Cables:

The resistance of conductor insulation decreases when degraded by overheating. Therefore, the insulation for most conductors is classified by temperature ratings of 60°C (140°F), 75°C (167°F), and 90°C (194°F). See Figure 2. The insulation temperature rating required depends on the application and ambient conditions.

Some special conductor types have insulation rated for as high as 200°C (392°F) to 250°C (482°F). For example, fixture wiring for high-intensity discharge (HID) lamps can require a very high temperature rating.

Insulation for most conductors is classified by temperature ratings of 60°C, 75°C, and 90°C.
Figure 2. Insulation for most conductors is classified by temperature ratings of 60°C, 75°C, and 90°C.

During the manufacturing process, an insulation jacket is placed around a conductor to contain the current within the conductor. The composition and thickness of the insulation jacket are used to determine insulation ratings and applications.

Conductors are usually insulated. Exceptions include bare (non-insulated) conductors used in specific grounding, bonding, and overhead power line applications, and covered conductors used in certain distribution applications. See Figure 3. A covered conductor is a conductor that is covered with a material that is not recognized by the NEC as an insulation material.

Figure 3. Conductors may be bare, insulated, or covered.
Figure 3. Conductors may be bare, insulated, or covered.

The NEC specifies conductor insulation types, which depend on the application. Electrical drawings and project specifications typically specify the conductor size and the type of insulation required for a particular installation.

Type THHN insulation is a heat-resistant thermoplastic conductor insulation suitable for dry locations and is rated for 90°C. Type THWN insulation is a heat-resistant thermoplastic conductor insulation suitable for both wet and dry locations and is rated for 75°C. Copper conductors with THHN or THWN insulation are the most commonly used insulated conductors in commercial and industrial wiring installations.

Check out Transformer Design Hardware (a Practical example of use of conductor and sizing in Electrical Equipment)

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Ahmed Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA. His Profile Links: Facebook - Linkedin - Gravatar

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