Electrical Systems

Conductor Selection 291

Conductor Selection 291 describes the different features of conductors and the considerations involved selecting conductors for electrical wiring applications. Electricians use the National Electrical Code(R) (NEC) to guide the selection process. Different materials are used to construct conductors. Copper is the most popular choice due to its affordability, low resistance, and good conductivity. Insulation is used to protect conductors from damage and electricians from injury. Wire protection is added to conductors to prevent damage from environmental factors.

Correction factors must be considered when selecting a conductor, including ambient temperature, number of conductors, and conductor length. When electricians use the NEC and have an understanding of conductor features and correction factors, selecting appropriate conductors to safely carry power is ensured and essential when working with electrical systems.

  • Difficulty Intermediate

  • Format Online

  • Number of Lessons 16

  • Language English


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Course Outline
  • Conductors
  • Conductor Selection and the Code
  • Conductor Size
  • Conductor Materials
  • Conductor Basics Review
  • Conductor Insulation
  • Wire Protection
  • Using Code Tables
  • Correction Factors and Ampacity
  • Conductor Selection Factors Review
  • Correction Factors: Temperature
  • Correction Factors: Conductor Length
  • Voltage Drop
  • Correction Factors: Number of Conductors
  • Fill Limits
  • Correction Factors, Voltage Drop, and Fill Limits Review
  • Describe electrical conductors.
  • Explain the role of the NEC® in conductor selection.
  • Describe the systems used to measure conductors.
  • Describe conductor materials.
  • Describe conductor insulation.
  • Describe wire protection.
  • Explain how to use Code tables.
  • List the factors that affect ampacity.
  • Explain how temperature affects ampacity.
  • Explain how conductor length affects ampacity.
  • Describe voltage drop.
  • Explain how the number of conductors used affects ampacity.
  • Describe fill limits.
Vocabulary Term


A highly conductive, lightweight, silvery metal. Aluminum is often used in long-distance power distribution.

ambient temperature

The temperature of the air that surrounds devices and systems. Ambient temperature, when high, contributes to the heat and decreases the amount of current that conductors carry.

ambient temperature tables

The tables in Article 310 of the NEC that rate conductor ampacity based on the temperature of the environment. The ambient temperature tables include Table 310.15(A)(3) IN No. 1, 310.15(B)(2)(a), 310.15(B)(2)(b), 310.15(3)(c) through Table 310.15(B)(21), 310.60(B)(4), and 310.60(C)(4).

American Wire Gauge

AWG. A measurement system that expresses conductor thickness. The American Wire Gauge system assigns whole numbers to conductors that decrease in value as the thickness of conductors increases.


The allowable current-carrying capacity of a conductor measured in amps. The maximum safe ampacity of each type of wire gage are listed in NEC tables.

ampacity tables

Specific informational charts that guide electrical installations based on ampacity levels. Ampacity tables used by electricians to consider ampacity levels include Table 310.15, 310.15(B)(16) through Table 310.15(B)(21), Table 310.60, Table 310.60(C)(67) through Table 310.60(C)(86), and Table B.310.15(B)(2)(1) through Table B.310.15(B)(2)(10).

Annex C

A series of tables in the back of the NEC. Annex C provides reference information for conductor fill.

area formula

A = ρ (L/R), where A is area in cmil, ρ is resistivity in circular mil feet, L is length in feet, and R is resistance in ohms. The area formula is used to find the area of a conductor. The area formula can assist electricians in limiting ampacity loss in conductors.


The amount of space, or number of square units, inside a closed figure. Area of a cross-section of a wire is expressed in circular mils.

Article 310

A section in the NEC that includes tables that electricians use to guide them in using conductors for general wiring purposes. Article 310 does not account for increased resistance as a result of conductor length.

Chapter 9

A section of the NEC that is made up of informational tables. Chapter 9 includes tables that identify sizing limits for raceways and conduits.

circular mil

cmil. The standard unit of a wire's cross-sectional area. Circular mils are the square of a conductor's diameter.

circular mil foot

A standard unit of conductor or wire length. One circular mil foot is equal to a wire 1 foot in length and 1 circular mil in area.


A common, naturally-occurring ceramic material that is easily shaped and solidifies when dried. Clay can be used as an insulating material.


A measure of a material's ability to conduct current and act as a path for its movement. Conductivity is a positive feature of most metals.


A material or element that allows free movement of electrons and therefore allows easy flow of electricity. Conductors require insulation to protect them from damage.


A tubular enclosure for wires or cables. Conduits are heat resistant.


A metal that is ductile, conductive, and corrosion resistant. Copper is one of the most commonly used conductive metals in the electrical field.

correction factors

Variable conditions that must be accounted for when selecting wires. Correction factors such as temperature, number of conductors, and conductor length influence allowable ampacity.


The process by which a metal degrades from a reaction with a chemical such as an acid. Corrosion does not occur in plastics, making them a common choice for wire protection.


The flat interior space of a round conductor. The cross-sectional area of a round conductor is expressed using cmils.


The flow of electricity into a circuit. Current can be carried by conductors.


A reduction in the ampacity of a conductor due to correction factors. Derating must occur when the conditions of a conductor are rated at one level but change.


The distance between the edges of a circle. Diameters are one of the units used to calculate conductor area.

dielectric strength

A material's ability to withstand stress caused by voltage. Dielectric strength is one of two fundamental insulator properties.

digital multimeter

DMM. A device that can measure voltage, current, or resistance. Digital multimeters can be used to check for voltage drop within a circuit.


To spread, separate, and disappear. Heat dissipates in raceways filled with the appropriate number of conductors.

electric shock

The flow of electricity through the body. Electric shock can be fatal.

electrical circuits

A completely enclosed path of devices that contain an electrical current. Electrical circuits are made up of many components including conductors.


A form of energy created by the movement of electrons. Electricity can be carried by conductors.


A particle that orbits the nucleus of an atom. Electrons have negative charges.

fibrous braid

A nonmetallic covering used to protect a conductor's insulating material. Fibrous braids are made of dielectric materials.

fill limits

The maximum number of conductors that can be safely housed in a raceway or conduit. Fill limits leave sufficient space for heat to dissipate and not become a fire hazard.


A standard of measurement used to determine the diameter of a wire. Gauge numbers increase as wire diameters get smaller.


A brittle, hard material that is often transparent. Glass can be used as an insulating material.


Material that does not allow for the easy flow of electricity. Insulation is used to prevent electrical shock.


Material that does not allow for the easy flow of electricity. Insulators are used to protect conductors from excess heat and protect employees from electric shock.


Abbreviation for a thousand circular mils. Kcmils are used to express conductor sizes that are large.

lead sheath

A watertight metallic outer covering used to protect a conductor's insulating material. A lead sheath is a continuous jacket molded around the wire to seal it against moisture.

metal jacket

A molded metal covering surrounding a conductor. A continuous metal jacket can be used to waterproof a conductor.

metallic armor

A heavy duty metallic outer covering used to protect a conductor's insulating material. Metallic armor is used in situations where a wire is exposed to physical wear and tear.


A hard, strong material that conducts electricity and heat and can be bent and formed. Metals are the most common material used to make conductors.


A linear unit of diameter that is equal to 0.001 of an inch. The mil is often used to express wire diameters that are very small.

National Electrical Code

NEC. The standard for minimum safe electrical installations in both residential and commercial settings. The National Electrical Code is adopted in some form as law in all 50 states.

National Fire Protection Association

NFPA. The organization that produces the National Electrical Code. The National Fire Protection Association is made up of experienced industry experts.


National Electrical Code. The standard for minimum safe electrical installations in both residential and commercial settings. The NEC is adopted in some form as law in all 50 states.


A unit of measurement of electrical resistance. Ohms are used to find the area of a conductor.


A state in which the level of current exceeds the recommended level for a device or circuit. Overloading can cause overheating and equipment damage.


A lightweight material that is corrosion resistant and easily shaped. Plastics are the most common materials used to insulate conductors and protect wires.


A hard, non-porous material that is translucent and a type of ceramic. Porcelain can be used as an insulating material.


A hard, crystalline material made of silica. Quartz can be used as an insulating material.


An enclosed channel of metal or nonmetallic materials designed expressly for holding wires or cables. Raceways must be heat resistant.


The opposition to current flow. Resistance in conductors can cause excess heat and damage.


The measure of a material's natural resistance to current flow. Resistivity, also known as specific resistance, is the opposite of conductivity, so good conductors have low resistivity.


An elastic material made from the latex sap of a tree. Some rubbers can be used as insulating materials.


A soft metal that has the highest electrical conductivity of all metals. Silver has the lowest specific resistance of all metals as well.

specific resistance

The measure of a material's natural resistance to current flow. Specific resistance, also known as resistivity, is the opposite of conductivity, so good conductors have low specific resistance.


The product of multiplying a number by itself one time. The square of a wire's diameter, or circular mil, is the unit of measurement for conductors.


A force that attempts to deform an object. Stress at a high level requires materials, like copper, that can withstand its forces.

Table 1

A table in Chapter 9 of the NEC. Table 1 lists raceway fill limits.

Table 310.15(B)(16)

A table in the NEC that lists the allowable ampacities of insulated conductors. Table 310.15(B)(16) identifies how much conductors must be derated based on the number of additional wires.

Table 310.15(B)(3)(a)

A table in the NEC that lists adjustment factors for conductors. Table 310.15(B)(3)(a) is used when an electrician needs to work with more than three conductors at once.

Table 4

A table in Chapter 9 of the NEC. Table 4 lists raceway fill limits.

Table 5

A table in Chapter 9 of the NEC. Table 5 lists the area of different types of conductors.


Informational charts used to guide electrical installations. Tables appear throughout the NEC.

temperature coefficient

A ratio of increased conductor resistance per degree Celsius rise in temperature. The temperature coefficient of most metals is positive because most of them increase in resistance as temperature increases.

tensile strength

A material's ability to resist forces that attempt to pull it apart or stretch it. When a material's tensile strength is exceeded, the material will break, bend, or suffer, in some manner.


A group of plastics that can be softened by heat, hardened by cooling, and then softened again by heat. Thermoplastic is often used as conductor insulation.


The delivery of electricity. The transmission of electricity is carried out by conductors.

transmission lines

A power conductor that sends electricity over long distances. Transmission lines do not have insulation.


A measure of electrical pressure or potential. Voltage is measured in volts (V).

voltage drop

The amount of energy used by an electrical device in a circuit. Voltage drop increases as resistance increases and decreases as resistance decreases.

voltage drop formula

D = 2pIL/CM, where D is voltage drop, p is resistivity, I is current, L is wire length, and CM is circular mil area. A formula used to calculate acceptable voltage loss for unlisted conductor combinations. The voltage drop formula is not regularly used by electricians.


A unit used to measure electromotive force or pressure, which is called voltage. Volts cause stress for conductors and insulators.


The amount of space contained within a three-dimensional shape. Volume, as expressed in the circular mil foot, is needed when dealing with wire length.


A generic term, often used interchangeably with the term conductor, that refers to any slender rope of drawn metal. Wires include the insulation of a conductor.