Conductor Selection 240

This course describes different conductor and insulation types and explains proper wire sizing according to NEC standards and calculations. Includes an Interactive Lab.

Class Details

Class Name:
Conductor Selection 240
This course describes different conductor and insulation types and explains proper wire sizing according to NEC standards and calculations. Includes an Interactive Lab.
Number of Lessons:
Additional Language:
Related 2.0 Class:
Conductor Selection 291

Class Outline

  • Objectives
  • What Is a Conductor?
  • Units of Conductor Size
  • National Electrical Code
  • Conductor Materials
  • Insulation Materials
  • Correction Factors
  • Number of Conductors
  • Conductor Fill
  • Length of Conductors
  • Voltage Drop Calculations
  • Additional Conductor Protection
  • Understanding the Code Tables
  • Using the NEC Tables: An Example
  • Summary


  • Define basic wiring terms.
  • Distinguish between common units for measuring conductor size.
  • Explain the NEC’s role in conductor selection.
  • Distinguish between common conductor materials.
  • Describe insulation materials.
  • Calculate a corrected ampacity based on ambient temperature.
  • Calculate a corrected ampacity based on number of conductors.
  • Describe how to properly size raceways.
  • Describe how conductor length affects wire selection.
  • Calculate a voltage drop to select correct conductor size.
  • Distinguish between outer coverings used for wire protection.
  • Explain how to use Code tables.
  • Identify the proper wire based on NEC tables.


  • MSSC Maintenance Awareness


Vocabulary Term Definition
ambient temperature The nominal temperature of the air that surrounds devices and systems. Ambient temperature must be accounted for when selecting a conductor.
American Wire Gage A system of units used to express conductor sizes in the NEC, required in Article 110.6. AWG is a convenient system that uses whole numbers with clear relationships to each other.
ampacity The allowable current-carrying capacity of a conductor measured in amps. NEC tables list maximum safe ampacities for each wire gage.
Annex C A series of tables in the back of the NEC that provides easy reference for conductor fill.
area The amount of space, or number of square units, inside a closed figure. The cross-sectional area of a wire is expressed in circular mils.
cable A term generally used to describe larger conductor sizes. Cable is used to mean various things in the NEC, but its meaning is always clearly defined.
circular mil The standard unit of a wire's cross-sectional area. Circular mils are expressed in mils squared.
circular mil foot A standard unit of conductor volume used to make comparisons between different metals. One circular mil foot is equal to a wire 1 foot in length with an area of 1 circular mil.
conductor A material or element that allows free movement of electrons and therefore allows easy flow of electricity. Most conductors are metals.
conduit A pipe or liner used as a raceway to carry and protect conductors.
correction factor Any variable condition that must be accounted for when selecting a wire size. Correction factors such as temperature, number of conductors, and conductor length influence allowable ampacity.
derating A reduction in the ampacity of a conductor due to correction factors. Conductors are rated for a specific set of conditions, and when those conditions change, ampacity must be derated.
dielectric The insulating material that separates conductors and the outer protective covering. Dielectrics are non-metallic.
dielectric strength The voltage at which the insulating qualities of a material break down.
fibrous braid A nonmetallic outer covering used to protect a conductor’s insulating material. Fibrous braid is the most common NM covering.
impedance The resistance to current flow in an AC circuit.
insulation A material that isolates conductive materials and helps to correctly channel electricity. Most wires are covered by insulation.
insulation resistance The resistance to current leakage through and over the surface of the material. Insulation resistance is one of two fundamental wire insulator properties.
kcmil Abbreviation for a thousand circular mils. Kcmil is a unit used to express large conductor sizes.
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 from any moisture damage.
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.
mil A linear unit of diameter equal to 0.001 of an inch. The mil is used because wire diameters are often very small.
National Electrical Code The standard for minimum safe electrical installations. The NEC is adopted in some form as law in all 50 states.
National Fire Protection Association The organization that produces the National Electric Code. The NFPA is primarily a volunteer organization.
raceway An enclosed channel of metal or nonmetallic materials designed expressly for holding wires or cables.
resistance The opposition to current flow. Resistance is measured in ohms.
resistivity Also known as specific resistance, the measure of a material's natural resistance to current flow. Resistivity is the opposite of conductivity, so it follows that good conductors have low resistivity per circular mil foot.
specific resistance Another term for resistivity. Every material has a set specific resistance per circular mil foot at a specific temperature.
Table 310.15(B)(2)(a) An NEC table that lists the correction factors for more than three current-carrying conductors in a raceway or cable.
Table 310.16 The frequently used NEC table to find "Allowable Ampacities of Insulated Conductors." Table 310.16 provides the minimum safe wire gage under specific listed conditions.
temperature coefficient A ratio of increased conductor resistance per degree Celsius rise in temperature. Most metals increase in resistance as temperature increases, giving them a positive temperature coefficient.
tensile strength The maximum stress, or tension, a material can endure before it breaks. Tensile strength is usually expressed in pounds per square inch.
thermoplastic A plastic that may be repeatedly heated, shaped, and cooled without damage. Thermoplastic is commonly used for wire insulation.
voltage drop The reduction in voltage level from the source to the load caused by conductor resistance.
volume The amount of space contained within a three-dimensional shape. The circular mil foot is a unit of conductor volume used to compare things such as specific resistance.
wire A generic term, sometimes used interchangeably with conductor or cable, meaning any slender rope of drawn metal. The term "wire" is often used to include the insulation.