Electrical Cable Basics and Selection

Published On: October 19, 2023By Categories: Engineering Your Business, Pumps and Water Systems

Part 1. Wire and insulation types.

By Ed Butts, PE, CPI

An ignored or overlooked element of an electrical design is occasionally the proper selection and sizing of the electrical cables or conductors as designers often assume the NEC minimum sizes are adequate.

This becomes even more important for submersible pump motors where thousands of feet may be needed to effectively operate the motor. This month, as the first column of a three-part series, we will review the various types of wire and insulation used to transmit electrical power.

Basic Conductor Properties

Electrical conductors are defined by two, but opposite, values: resistivity, in ohms, is designated by the Greek letter rho (ρ) and is the resistance to the flow of electrical current. Resistivity for a material with a uniform cross section is determined from the following equation:

ρ = RA/l

where:

R is the electrical resistance

A is the cross-sectional area

l is the length of the material.

Conversely, conductivity, in mhos, is commonly signified by the Greek letter sigma (σ), but kappa (κ) and gamma (γ) are sometimes used. The reciprocal of resistivity (1/ρ), it is the measure of the amount of electrical current a material can carry or its ability to carry a current.

Copper vs. Aluminum

Figure 1. Resistance difference with solid and stranded conductors.

Most of us who work in the water well and pump industry know there are two basic types of electrical conductors: copper and aluminum. Except for silver, copper is the most common conductive metal used in industrial, agricultural, and residential electrical service as well as the established international standard.

The International Annealed Copper Standard (IACS) was adopted in 1913 to compare the conductivity of other metals to copper. Based on this standard, commercially pure annealed copper was defined as having a base conductivity of 100% IACS.

Commercially pure copper produced today will likely possess higher IACS conductivity values as refining and processing technology has significantly improved over the past century. In addition to copper’s superior conductivity, the metal contains greater tensile strength, thermal conductivity, and thermal expansion properties than other conductive metals.

Annealed copper wire used for electrical purposes meets the requirements of ASTM B-3: “Specification for Soft or Annealed Copper Wire” and ASTM B-8: “Stranded Conductors.”

Even though copper has a long history as the material of choice for conducting electricity, aluminum has certain inherent advantages that make it attractive and useful for specific applications. While aluminum possesses just 61% of the conductivity of copper, it is only 30% of the weight of copper by volume. This means that a bare aluminum wire weighs half as much as a bare copper wire but with the same electrical resistance.

Figure 2. Various electrical cable types.

Aluminum is generally less expensive when compared to copper conductors as well. These two factors make aluminum attractive for long-distance overhead and buried cables.

Aluminum conductors consist of different alloys known as the ASTM B-230: AA-1350 series and ASTM B-800 and B-801: AA-8000 series.

In the 1960s and 1970s, due to the high price of copper relative to aluminum, the AA-1350 grade of aluminum was used a lot for household wiring. However, due to low-quality workmanship at the connections and the physical and electrical differences between aluminum and copper, high-resistance and corrosive connections rapidly formed, often developing into a fire hazard.

As a response, aluminum alloys were developed to possess creep and elongation properties more like copper, resulting in the AA-8000 series. Solid aluminum conductors #8, 10, and 12 AWG are now required to be made of an AA-8000 series electrical grade aluminum alloy conductor material.

Stranded aluminum conductors #8 AWG through 1000 kcmil marked as Type RHH, RHW, XHHW, XHHN, XHWN, THW, THHW, THWN, THHN, service-entrance Type SEStyle U, and SE-Style R are now made of an AA-8000 series electrical grade aluminum alloy conductor material. These series of alloys are the only solid or stranded aluminum conductors permitted to be used according to Article 310 of the
National Electrical Code 2023 edition, which covers conductors up to and including 2000 volts.

Aluminum conductors are more malleable and potentially damaging than copper; therefore, adequate caution must be observed to not cut or nick the conductor during insulation stripping or termination. Nicks or cuts at terminations will result in a vulnerable connection that may result in possible breakage of the conductor or developing excessive heat.

In addition, electrical terminals with aluminum conductors must be frequently examined and re-tightened as the cycles of heat that develop will result in cycles of expansion and contraction which may cause loosening of the connection. In all cases, connectors and terminals used for aluminum conductors must be rated for use with aluminum wire.

Oxidation is a process that can negatively impact and degrade both an aluminum and copper conductor. Once the oxidation process begins, it is virtually impossible to effectively stop and increase the functional life of the conductor. Preventing oxidation by using a tinned copper conductor or anti-oxidizing joint compound (paste) with aluminum conductors is often the most plausible solution.

At times, aggressive oxidation of copper starts immediately after production and before it is even installed. Generally, the degradation process is almost invisible until it is too late, so prevention is critical.

A severely oxidized copper or aluminum conductor is no longer acceptable for use as it loses electrical efficiency, is more prone to breakage, and risks safety. An oxidized copper (copper oxide) generates a distinctive green appearance while the byproduct from aluminum corrosion (aluminum oxide) will typically possess a whitish, gray, pink, or brown appearance.

One of the most frequent and exposed locations for corrosion involves the direct connection of an aluminum and copper conductor. For this reason, specific types of connectors must be used.

Connections rated for use with aluminum conductors are also frequently able to be used with copper and are so marked with a dual rating, such as AL/CU or AL7CU, a connector suitable for use with copper or aluminum conductors at an operating temperature of 75°C (167°F).

Connectors used to connect copper to aluminum conductors must be approved for this application and equipped with a separating (isolation) saddle or barrel that prevents an electrochemical interaction between the two metals.

Without periodic attention, the heat developed at these connections will rapidly lead to oxidation and corrosion of the aluminum conductor or loosening of a pressure grip connector due to electrolysis from the direct connection between dissimilar metals. For applications where connections need to be soldered, copper or tinned copper is preferred as aluminum is difficult to effectively solder.

Unlike aluminum, copper conductors often have the ability to self-heal when the insulation becomes scraped or has minor damage. The oxidation layer that forms on an exposed copper conductor or connection will often help to facilitate isolating the conductor from additional electrical damage.

On the other hand, an exposed aluminum conductor will generally develop rapid corrosion from this oxidation and ultimately result in failure of the conductor or connection. This is the primary reason an anti-oxidizing paste or compound is used on aluminum wired splices, terminals, or connectors.

Conductor termination pastes are recommended for use on splice and termination connections of aluminum, copper-clad aluminum, and copper conductors. The paste is used to retard oxidation at the conductor to connector interface. These compounds do not harm the conductor metal, insulation, or equipment when applied following the manufacturer’s use instructions. Before using, though, verify the paste is UL-approved for the operating temperature, type of application, conductor, and the connection.

In situations where corrosion of copper conductors is also a distinct possibility, the use of tinned copper (ASTM B-33) may be suggested. Tinned copper is simply copper covered with a coating of tin (i.e., solder) to preserve the qualities of the metal when exposed to potentially harmful environments and extend its lifespan.

Tin acts as an extra layer of protection against corrosion, oxidization, and the detrimental impact of high temperatures and is frequently used in environments with exposure to high moisture levels, corrosive gases, and for direct burial applications.

The tinned copper wire may consist of several tinned copper conductors or a single wire. Because the tin is a silver-colored metal, the entire conductor displays a silver appearance. Tin is one of the most economical metals with the necessary protective properties.

Solid vs. Stranded

Electrical conductors are typically available in solid or stranded configurations, although larger conductors are generally stranded with various types of stranding construction. Solid wires are cost-effective, weather-resistant, easy to use, and anti-corrosive,

A solid wire, however, is more apt to fatigue and breakage if it is used on those applications where it is subject to more movement and vibration, such as motor connections.

If a large number of thinner wires are twisted or laid together to unify them, a single conductor becomes a number of thinner conductors and is now called a stranded wire. Stranded conductors are typically more flexible and forgiving with tight bends, although both types can break if exposed to frequent or severe bending.

Solid conductors have a slightly lower DC resistance than stranded conductors, but the difference is negligible and generally not a meaningful design issue. The resistance of the stranded conductor is slightly more than the solid conductor of an equivalent cross-sectional area.

Since a stranded conductor is spiraled, each strand is longer than the finished conductor. With larger conductor sizes, the skin effect plays a minor role in creating the difference in resistance. The skin effect is the tendency of an AC current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the conductor.

In this case it appears to be related to the type of stranding, with each type of concentric stranding showing a progressively higher resistance than a solid conductor.

Refer to Figure 1, with a solid conductor displaying a 15% lower resistance than the concentric round style of stranded cable. There is not a strong correlation in resistive difference with the actual areas of the conductor; rather, the stranding method and added length appears to be the main factor. Presumably there is some interaction between the contact that occurs between the strands and the strand configuration that causes an increase in resistance in the stranded styles.

Electric Wires and Cables

Electric wires are typically single wires made of aluminum or copper. They are either bare or insulated and typically covered in a thin layer of thermoplastic or thermoset insulation. The outer covering is colored to indicate whether the wire is a neutral, ground, or energized (hot) wire in the electrical installation.

Cables (Figure 2) usually contain a neutral wire, ground wire, and at least one energized wire that are twisted or bonded together. Depending on its purpose, the cable may contain many more wires. The single wires in a cable are contained in their own color-coded layer of insulation. The group of wires is then encased in an outer sheath to make a single cable.

Voltage Ratings

First, the allowable voltages applied to the conductors and cables are based on approval from the local “Authority Having Jurisdiction” (AHJ), which is another term for the electrical inspector.

Regardless of the installer’s assertions or backup from manufacturer data, the AHJ always has the final word involving electrical code interpretations, so it’s important to ensure that the voltage rating of the proposed electrical cables is consistent and in accordance with these judgments.

Voltage ratings for conductors and cable used in most water well work generally fall under one of four categories:

  • 600 to 1000 VAC
  • 1001 to 2000 VAC
  • 2001 to 5000 VAC
  • 5001 to 35,000 VAC.

While the NEC for decades defined a low-voltage power system as one with less than 600 volts based on the International Electrical Code (IEC), a low-voltage electrical system is considered as an electric system having a maximum root-mean-square alternating-current voltage of 1000 volts or less.

The maximum voltage rating of a conductor or cable will be a number embossed on the insulation, such as 600, which indicates the maximum voltage the wire is approved to carry is 600 volts.

AC system conductors operating at different working voltages are permitted to be routed through the same conduit, provided all conductors possess a common voltage rating for the highest rated voltage.

Dissimilar AC and DC system conductors should never be routed together as AC inductance may impact the DC conductor’s operation.

As the majority of agricultural, municipal, and industrial pumping systems utilize a three-phase power source with less than 600 volts (208, 240, 480, and 575 volts), most of the available electrical conductors and cabling are also rated for that maximum voltage.

For those who work with voltage systems higher than 2000 volts that operate larger horsepower and voltage motors, medium voltage cable (Type MV) is now included in NEC Article 311. This is defined as “a single or multiconductor solid dielectric insulated cable rated 2001 volts up to and including 35,000 volts, nominal.”

The requirements of this new article do not apply to conductors that form an integral part of the equipment such as motors, motor controllers, and similar equipment, or to conductors specifically provided elsewhere in the NEC.

Obviously, voltage losses will have a significant influence on cable sizing. During the cable-sizing process, you may find that the amperage of the motor cable is at such a low level that it can be used with a free air rating, allowing you to actually use the motor cable as a drop cable in the air, despite the fact that it is intended for submerged operation. However, this must be verified with the AHJ.

Insulation Ratings and Types

Wire insulation is a crucial outer layer applied to wires and cables to protect the wires from external conditions. The insulation or jacket simultaneously prevents current from leaking from the wires into the surrounding area.

To meet the needs of a diverse range of wire materials, settings, and applications—wire insulation is available in a variety of types and materials. The five different types of electrical cables and conductors based on the insulating materials are:

  • Thermoset (rubber) insulated cables
  • Thermoplastic (PVC) insulated cables
  • Paper insulated cables
  • XLPE insulated cables
  • Mineral insulated cables.

Insulation used to cover electrical wiring conductors and portable cords is rated for the maximum temperature it can withstand on a continuous basis. Standard ratings are indicated in degrees of Centigrade (°C), including:

  • 60°C (140°F)
  • 75°C (167°F)
  • 90°C (194°F)
  • 105°C (221°F).

The most common ratings for submersible cable are 75°C and 90°C. The fundamental current-carrying capability of a conductor is a function of the following:

  • Type of conductor: copper or aluminum
  • Cross-sectional area of the conductor in circular mils
  • Insulation temperature rating in degrees Centigrade (°C)
  • Ambient temperature in °C.
  • Number of energized conductors in raceway or cord.

Derating for greater ambient temperatures and conduit fill based on the number of energized conductors is frequently required and all conductors are potentially exposed to this process. This will be outlined in next month’s column.

The ampacity of a conductor of any size is increased as the rating of its insulation is increased. However, as the ambient temperature is increased, the ampacity of the conductor must be derated from its base ampacity at 30°C (86°F).

For example, a No. 6 copper conductor with a 90°C-rated insulation will be theoretically capable of a higher continuous current than a No. 6 copper conductor with a 60°C-rated insulation. However, the terminal’s temperature rating, where the wire connects on both ends, must also be rated for the greater operating temperature. Otherwise, by default, the conductor must be limited to the equipment or terminal’s rated temperature reading.

This is often an issue when applying higher heat rated conductors to older equipment with a lower temperature rating, such as older circuit breakers. Many installations have been rejected due to this oversight.

Remember, there’s no single wire insulation that’s ideal for every application. Rather, it’s best to look at the requirements of each application—including the electrical equipment’s allowable operating temperature, derating requirements, and amp rating—and select the wire insulation type that fits best.

Insulation types for electrical conductors are typically classified as thermosetting (rubber) (UL 44), thermoplastic (plastics) (UL 83), or fluoropolymer (UL83A). Examples of thermoplastics and thermosets are shown in Table 1.

Thermosetting plastics and thermoplastics are both polymers, but they behave differently when exposed to heat. Thermoplastics can melt under heat after curing while thermosetting plastics retain their form and remain solid under heat once cured.

Thermosetting is a material that will not soften, flow, or distort when subjected to heat and pressure. Once extruded over a conductor, these compounds will not re-melt, but they can be burnt or deteriorate due to excessive heat.

The most common types of electrical wires in electrical applications typically use nylon-coated thermoplastic insulation with a high-heat resistance of 75°C or 90°C. These wires are often identified with the dual THHN/THWN type label, material, maximum voltage rating, and gauge.

The meaning of each letter used in wire labels is shown below:

  • T: Thermoplastic insulation, a fire-resistant material
  • H: Heat-resistant; able to withstand temperatures up to 75°C (167°F).
  • E: Elastomer jacket
  • F: Feeder
  • HH: Highly heat-resistant; able to withstand temperatures up to 90°C (194°F).
  • W: Wet or approved for weather (i.e., damp and wet locations); also suitable for dry locations
  • X: Thermoset insulation; made of a synthetic polymer or rubber equivalent that is flame-retardant
  • N: Nylon-coated for added resistance to oil and gasoline
  • NM: Non-metallic, sheathed cable; thermoplastic cable
  • O: Oil-resistant outer jacket (portable cord)
  • OO: Oil-resistant outer jacket and inner insulation (portable cord)
  • R: Rubber insulation
  • U: Underground cable
  • SE: Service entrance
  • V: Vacuum cord (portable cords); rated for 300 volts
  • -2 Suffix: Approved for 90°C-wet locations (such as THWN-2).

To illustrate, a UF rating signifies an underground feeder cable; a THHN rating is a thermoplastic, highly heat resistant insulation with a nylon outer coating; and an RHW insulation designates a rubber, heat-resistant insulation approved for wet locations.

It’s important to understand fully each letter within the designation to properly apply to the wire.

For example, one of the most common wire types, THW, is made to handle temperatures up to 75°C in wet locations, a common submersible cable designation, but type THHN, while approved for 90°C, is only approved for dry locations as the W is missing.

On the other hand, type THWN can usually handle higher temperatures in both dry and wet conditions. All submersible pump cable insulation should include a W designation in its description.

In addition to classification of conductors, portable cord types are also assigned a unique identification:

  • Type G: Heavy duty round mining service portable cord with three or four conductors. Rated for 2000 VAC
  • Type J: Junior hard service (portable cords). Rated for 300 volts
  • Type MP: High voltage mining rated cable for 5-35 kV direct burial applications
  • Type W: Heavy duty rubber portable power cable with two to five conductors for outdoor service
  • Type S: Extra hard service (portable cords). Rated for 600 volts.

Conductor Sizing

Beyond the fundamental considerations just listed, the required conductor size to feed a circuit is based principally on three considerations:

  • Net current-carrying capacity (ampacity) of selected conductor (after derates)
  • Short-circuit current
  • Voltage drop.

The current-carrying capacity of a cable is affected primarily by its size and permissible operating temperature of its insulation. Refer to Table 2 for NEC allowable ampacities for the various sizes, insulation types, and applicable temperature ratings.

The higher the operating temperature rating of the insulation, the higher the fundamental current-carrying capacity of a given conductor size. The temperature at which a particular cable will operate is affected by the ability of the surrounding material to conduct away the heat. Therefore, the current-carrying capacity is materially affected by the ambient operating temperature as well as by the installation conditions.

For example, a cable installed in a 40°C (104°F) ambient temperature has an ampacity that is only around 90% of the ampacity in a 30°C (86°F) ambient temperature. In addition, running a single-conductor cable through a magnetic conduit, such as rigid steel and EMT conduit, will increase the apparent resistance of the cable and result in a lower current-carrying capacity due to the additional resistance and magnetic losses.

Similarly, when a cable is run close to or in conjunction with other current-carrying cables, the presence of the other cables effectively increases the ambient temperature, which decreases the ability of the cable to dissipate its heat. In these instances, the current-carrying capacity of all conductors must be derated.

It is apparent from the above that many conditions must be known before an accurate current-carrying capacity can be determined for a particular cable installation. Sizing electrical conductors based on amperage and circuit voltage drop will be outlined in the third column in this series, which will be in the January 2024 issue of Water Well Journal.

Color Coding

A wire’s assigned color generally designates the purpose of the wire. The NEC references a white conductor as the grounded or neutral conductor, a green color or bare conductor as the system and equipment grounding conductor, and all other colors as ungrounded or energized conductors unless otherwise designated.

Any other color than white and green usually denotes an energized (hot) wire that carries an electrical current. Colored tape is frequently used to designate a single insulation color for different uses, such as:

  • White insulation: Typically considered as a neutral wire but can sometimes be used as an energized lead in certain situations, such as switch loops or 230 VAC circuits. In existing wiring, white wires may also be marked with black or red tape to indicate that it’s now an energized wire.
  • Green and bare copper wire: Always used as the system ground or equipment grounding conductor.
  • Black insulation: Energized wire for services, feeders, switches, outlets, motors, and other devices.
  • Red/Purple insulation: Energized wire for switch legs, motors, and other devices. Often used to designate the high voltage or wild leg on three-phase, 240 VAC, Delta power systems.
  • Pink: Often assigned as an energized control conductor for a power or motor control circuit.
  • Blue/Yellow insulation: Energized wire pulled through a conduit for motors and powered devices. Single colors, such as the uniform use of a black color, are frequently used to designate a single- or three-phase power circuit.

Red, yellow, and black colors are also designated as conductors to a single-phase submersible motor as:

  • Red: Auxiliary or Start winding on a single-phase motor
  • Black: Main or Run winding on a single-phase motor
  • Yellow (or sometimes White): Common between Start and Run windings on a single-phase motor.

The wire gauge number (#) indicates the electrical wire size, as defined by the American Wire Gauge (AWG) system. The most common wire gauges in use are #10, #12, or #14.

The gauge and diameter of the wire are inversely related. In other words, as the gauge number becomes higher, the diameter and current capacity area of the wire becomes smaller. For example, a #10-gauge wire is larger and can handle more amperage than a #12-gauge wire with the same insulation rating.

All the information needed to know about the type of cable is printed on its insulation or sheathing. Use the following descriptors to determine if a cable or conductor is right for a specific project:

  • Conductor Type: Shown as CU for copper or AL for aluminum.
  • Insulation Type: Lists the type of insulation such as USE, THW, THHN, or UF.
  • Gauge: The gauge of the individual wires inside the insulation or cable, such as #14, #12, #10, etc.
  • Number of wires: This number follows the gauge and includes the number of current-carrying conductors. For example, a #12/2 designation indicates there are two 12-gauge current-carrying wires within the cable (a ground wire if part of the cable is not included in this description).
  • Grounding: The word GROUND or the abbreviated letters G, GRD. or GRND. indicates the added presence of a ground wire such as a #12-2 w/grnd or 12/2 with ground.
  • Voltage rating: The most common maximum voltage rating for water well work is 600 volts, but this can vary with the motor horsepower and service size. The stamped number on the insulation or sheathing indicates the maximum voltage the cable can safely carry.
  • UL, IEC, or CSA: Indicates that the cable is safety certified and approved for use by Underwriters Laboratories, International Electrotechnical Commission, or the Canadian Standards Association.

Conduit Fill

Unlike the allowable current of conductors in a raceway (conduit) being based on the number of energized conductors, the allowable percentage of conduit fill (conductor fill) is based on the total number or wires including energized and non-energized (ground and neutral) wires within the conduit; wire AWG, MCM, or outer cable size; wire or cable insulation type and thickness; the conduit type, schedule (if applicable), application, and internal diameter; and total number of fittings and degree changes.

This is also an NEC Code stipulation (NEC Article 300.17) and tables are readily available with the allowable number of wires for each wire size, insulation type, metallic and nonmetallic type of conduit, as well as schedule numbers and/or internal diameter.

The basic code states that a single wire can fill only 53% of a raceway, two wires can fill only 31%, and if there are more than two conductors in a raceway, the maximum fill percentage is 40%.

However, the AHJ can lower these fill limits, so verify the intended ratio is permissible before beginning work. Although the temptation may be present to push the boundaries and pull the maximum allowable number of wires through the conduit to save money, this should be resisted for long runs as insulation damage or excessive bundling may occur.

Additionally, the use of a UL-approved pull string and lubricant is always recommended to lower the amount of pull tension needed and protect against undue jamming of the wires. There are similar rules for allowable junction box and conduit and outlet bodies fill based on the boxes volume in NEC Article 314.

______________________________________________

This wraps up this edition of Engineering Your Business. We will cover submersible pump cable next month in the second part of our series, and in the third part in the January 2024 issue will finish things up by outlining procedures for wire sizing based on voltage drop.

Until then, work safe and smart.

Learn How to Engineer Success for Your Business
 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. Click here for more information.

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.

Read the Current Issue

you might also like