Conductor Selection 291
This class describes the different characteristics of conductors and the considerations involved in selecting conductors for electrical wiring applications. Electricians use the National Electrical Code® (NEC®) to determine the ideal conductor type, insulation, and size. Copper is the most popular conductor material due to its affordability and conductivity. When selecting conductors, electricians must consider correction factors, which are variables that affect ampacity. These factors include ambient temperature, number of conductors, and conductor length.
After taking this class, learners will have an understanding of conductor features and correction factors. Selecting appropriate conductors to carry power essential to the safety and efficiency of electrical systems. Electricians who can properly select conductors reduce corrections, accidents, and unexpected downtime.
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Difficulty Intermediate
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Format Online
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Number of Lessons 16
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Language English
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- Conductors
- Conductor Selection and The Code
- Conductor Size
- Conductor Materials
- Conductor Basics Review
- Conductor Insulation
- Insulation Materials
- Wire Protection
- Conductor Selection Factors Review
- Ampacity and Correction Factors
- Ambient Temperature
- Voltage Drop
- Cross-Sectional Area
- Number of Conductors
- Fill Limits
- Correction Factors and Multiple Conductors 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.
- Identify conductor insulation materials.
- Identify types of wire protection.
- List the factors that affect ampacity.
- Explain how temperature affects ampacity.
- Describe voltage drop.
- Calculate minimum cross-sectional area.
- Explain how the number of conductors used affects ampacity.
- Explain how to determine fill limits.
Aluminum
A highly conductive, lightweight, silvery metal. Aluminum is often used in long-distance electrical distribution.
ambient temperature
The temperature of the air surrounding an area or object. Ambient temperature can increase conductor resistance and therefore decrease ampacity.
American Wire Gauge
AWG. A measurement system that uses whole numbers to express conductor thickness. The American Wire Gauge system assigns numbers to conductors that decrease in value as the thickness of conductors increases.
Ampacity
The allowable current-carrying capacity of a conductor. Ampacity is affected by factors like temperature and the size of the conductor.
amperage
I. The strength or intensity of current flowing through a circuit. Amperage is measured in amperes (A), or amps.
Amperes
A. The unit used to measure current strength. One ampere, or amp, is equal to one volt divided by one ohm.
Amps
A. The unit used to measure current strength. One amp, or ampere, is equal to one volt divided by one ohm.
Annex C
A supplemental section at the end of the NEC® that lists fill limits for installations that use the same type of conductor. Annex C includes several tables for different types of conduits and enclosures.
Annexes
A section of a book or document that provides readers with supplemental material. Annexes A-K in the NEC® provide readers with additional helpful information that is not part of the official requirements.
Area
The amount of space, or number of square units, inside a closed figure. The area of a cross section of a wire is expressed in circular mils.
Article 310
An article in Chapter 3 of the NEC® that lists requirements for conductors used for general wiring. Article 310 includes tables of information about ampacities and correction factors.
Article 314
A section of the NEC® that details the ways conductors can be run through outlets, devices, pull boxes, and junction boxes. Article 314 includes tables that indicate the fill limits of junction boxes.
Article 315
An article in Chapter 3 of the NEC® that lists requirements for medium voltage conductors and cables. Article 315 includes tables of information about ampacities and correction factors.
AWG 0000
A wire gauge with a diameter of 460 mils. AWG 0000, also called AWG 4/0 or four-aught, is the largest wire gauge in the American Wire Gauge system and has a cross-sectional area of 211.6 kcmils.
AWG 36
A wire gauge in the American Wire Gauge system. AWG 36 wire has a cross-sectional area of 25 cmils.
AWG 4/0
A wire gauge with a diameter of 460 mils. AWG 4/0, also called AWG 0000 or four-aught, is the largest wire gauge in the American Wire Gauge system and has a cross-sectional area of 211.6 kcmils.
AWG 40
The smallest wire gauge in the American Wire Gauge system. AWG 40 wire has a cross-sectional area of 9.61 cmils.
Cable
A single large conductor or bundle of smaller wires covered by a jacket. Cables carry electricity from its source to the load.
Ceramic
A hard, brittle, heat- and corrosion-resistant material that consists of metallic and non-metallic atoms held together by strong bonds. Ceramic materials may be used as insulators on power transmission stations, circuit boards, and other components.
Chapter 3
A chapter of the NEC® that covers wiring methods and materials. Chapter 3 includes Articles 310 and 315, which are very helpful for selecting conductors.
Chapter 9
A chapter of the NEC® that consists of several tables. Chapter 9 includes tables that list fill limits, conductor dimensions, conductor properties, and other information.
circuits
A controlled path for electricity. Circuits usually include a source, path, load, and control.
Circular mils
cmils. A unit of measurement that is equal to the area of a circle with a diameter of 1 mil, or 0.001 of an inch (0.0254 mm). Circular mils are used to measure the cross-sectional area of conductors.
Conductive
Able to act as a path for the flow of electricity. Conductive materials have low resistivity.
Conductivity
The measure of a material’s intrinsic ability to convey electricity. Materials with high conductivity have low resistivity.
Conductors
A material that allows free movement of electrons and, therefore, allows easy flow of electricity. Conductors include wires and cables.
Conduits
A tubular enclosure used to contain wires or cables. A conduit seals permanently around conductors and protects them from damage.
Copper
A highly conductive, reddish-brown metal that is ductile and corrosion resistant. Copper is the most commonly used conductor material in the electrical field.
correction factors
A value used to account for variable conditions that affect the ampacity of a conductor. Correction factors may also refer to the conditions themselves, which include temperature, number of conductors, and conductor length.
Corrosion
The gradual deterioration of a material due to atmosphere, moisture, chemicals, or other agents. Corrosion often appears as rust.
Cross section
An interior surface that is or would be exposed by making a straight cut perpendicular to an object’s axis. The area of a conductor’s cross section determines the conductor’s size.
Current
The flow of electricity through a circuit. Current strength is called amperage and is measured in amperes (A), or amps.
derating
The process of determining a conductor’s lowered ampacity when accounting for correction factors. Derating must occur whenever a conductor is used under conditions that can affect ampacity.
Diameter
The distance from one edge of a circle to the other edge as measured through the center. The diameter of a conductor is measured in mils and used to determine its area.
Dielectric strength
The amount of voltage a material can withstand before beginning to break down. Dielectric strength is one of two fundamental insulator properties.
Dissipate
To spread, separate, and disappear. Raceways, conduits, and junction boxes must have enough space to allow heat to safely dissipate.
Distribution systems
A network of electrical components that provides electricity from the utility to specific end destinations. Distribution systems often use aluminum transmission lines to transmit electricity over long distances.
Electric shock
The flow of electricity through the body. Electric shock can be fatal.
Electromagnetic interference
EMI. Electrical noise in a circuit that can interrupt or degrade the circuit’s performance. Electromagnetic interference can be reduced by using metallic mesh sheaths as wire protection.
Electrons
An atomic particle with a negative charge. Electrons orbit the nucleus of an atom and can move from one atom to another.
Fill limits
The maximum percentage of a raceway, conduit, or junction box that can be filled with conductors. Fill limits are required to leave sufficient space for heat to dissipate safely.
Four-aught
A wire gauge with a diameter of 460 mils. Four-aught, also called AWG 4/0 or AWG 0000, is the largest wire gauge in the American Wire Gauge system and has a cross-sectional area of 211.6 kcmils.
Gauge
A whole number used to identify conductors that have a specific diameter. Gauge numbers increase as wire diameters get smaller.
Glass
A brittle, hard material that is often transparent. Glass is often used as insulation on transmission lines.
Insulation
Material that does not conduct electricity. Insulation is applied to conductors to prevent current from straying from its intended path.
Insulators
A material that has little electrical conductivity and high resistance to electrical charges. Common insulators include plastics, rubber, and ceramics.
Jacket
A protective outer layer on a cable or bundle of wires. The jacket protects the inner conductor or wires from damage and may prevent interference.
Junction box
A protective enclosure that multiple conductors pass through or are connected within. Junction boxes must have space inside them for heat to dissipate.
mesh sheath
A woven sleeve used to organize and protect conductors. Mesh sheaths can be made from metallic or non-metallic material.
Metal-clad
Encased in a metal covering. Metal-clad conductors are used in applications that expose conductors to constant wear.
Metallic armor
A heavy-duty metallic outer covering used to protect a conductor. Metallic armor is used in situations when conductors are exposed to a great deal of physical wear and tear.
metals
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.
mils
A unit of measurement that is equal to 0.001 of an inch (0.0254 mm). The mil is used to measure the diameter of round conductors.
National Electrical Code
NEC®. The standard for minimum safe electrical installations in the United States. The National Electrical Code, also referred to as 'the Code' or by the technical name NFPA 70®, is adopted in some form as law in all 50 states.
National Fire Protection Association
NFPA. A non-profit organization that maintains standards of public safety and fire prevention. The National Fire Protection Association produces the National Electrical Code®.
NEC
National Electrical Code. The standard for minimum safe electrical installations in the United States. The NEC, also referred to as the Code or by the technical name NFPA 70, is adopted in some form as law in all 50 states.
NFPA 70
The standard for minimum safe electrical installations in the United States. NFPA 70®, also referred to as the National Electrical Code® or 'The Code', is adopted in some form as law in all 50 states.
NM
Non-metallic. A type of plastic wire protection typically made from polyvinyl chloride. NM conductors are often used in residential wiring.
Ohm’s Law
The equation that describes the relationship between amperage, voltage, and resistance. Ohm’s Law states that resistance is equal to voltage divided by amperage.
Ohms
O. The unit used to measure electrical resistance. One ohm is equal to one ampere divided by one volt.
Plastic
A lightweight material that is corrosion resistant and easy to shape. Plastics are the most common materials used to insulate conductors and protect wires.
Polyvinyl chloride
PVC. A thermoplastic that can be either rigid or flexible. Polyvinyl chloride is typically used as wire protection on NM conductors.
Quartz
A hard, crystalline material made of silica. Quartz can be used as an insulating material.
Raceways
An enclosed rectangular channel designed to contain wires or cables. A raceway protects conductors from damage and can be opened to access the conductors inside.
Resistance
R. The opposition to current flow. Resistance is measured in ohms (O).
Resistivity
ρ. The measure of a material’s intrinsic resistance to current flow. Materials with high resistivity, also known as specific resistance, have low conductivity.
Rubber
An elastic material made from organic or artificial latex. Organic rubber is extracted as sap from tropical trees.
Silver
A soft, gray-white metal that has the highest electrical conductivity of all metals. Silver is used in some medical electronics and high-end audio equipment.
single-phase
Alternating current power that consists of only one voltage. Single-phase power is used for simple residential applications.
SJO
A type of plastic wire protection made of oil-resistant synthetic rubber and used with lower voltages than SO. SJO conductors are designed for heavy-duty work with up to 300 volts.
SO
A type of plastic wire covering. SO wire is oil resistant and meant for heavy-duty work with up to 600 volts.
Specific resistance
The measure of a material’s intrinsic resistance to current flow. Materials with high specific resistance, also known as resistivity, have low conductivity.
square
The product of multiplying a number by itself one time. The square of a conductor’s diameter is equal to its cross-sectional area.
square root
A mathematical function that indicates which unknown number is multiplied by itself one time. The square root of 3 is approximately 1.73.
Stress
A force that attempts to deform an object. Copper conductors can withstand high levels of stress.
Table 1
A table in Chapter 9 of the NEC® that lists the number of conductors that can placed inside a conduit. Table 1 describes how much of the cross-sectional area of a conduit can be taken up by conductors based on the number of conductors used.
Table 310.15(B)(1)(1)
A table in the NEC>/v>® that lists ambient temperature correction factors based on a temperature of 30°C (86°F). Table 310.15(B)(1)(1) includes correction factors for conductors with temperature ratings of 60, 75, and 90°C.
Table 310.15(B)(1)(2)
A table in theNEC® that lists ambient temperature correction factors based on a temperature of 40°C (104°F). Table 310.15(B)(1)(2) includes correction factors for conductors with temperature ratings of 60, 75, 90, 150, 200, and 250°C.
Table 310.15(C)(1)
A table in the NEC® that lists ambient temperature correction factors when more than 3 conductors are used. Table 310.15(C)(1) includes correction factors for conductors with temperature ratings of 60, 75, and 90°C.
Table 310.4(1)
A table in the NEC® that lists various types of insulation and their properties. Table 310.4(1) includes information about maximum operating temperature, appropriate applications, thickness required for different conductor gauges, and any outer covering requirements for different types of insulation.
Table 4
: A table in Chapter 9 of the NEC® that lists the areas and fill limits for different sizes of conduits. The fill limits in Table 4 are based on the percentages in Table 1.
Table 5
A table in Chapter 9 of the NEC® that lists the approximate areas and diameters of different insulated conductors. Table 5 can be used to determine the percentage of an enclosure conductors will take up.
Table 8
A table in Chapter 9 of the NEC® that lists conductor properties. Table 8 of Chapter 9 lists the resistance of different conductor sizes.
temperature coefficient
A ratio that indicates how conductor resistance changes per one-degree Celsius increase in temperature. Most metals have a positive temperature coefficient, which means their resistance increases as temperature increases.
temperature rating
The temperature at which a conductor has optimal ampacity. Temperature ratings for various conductors are listed in tables in the NEC.
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 or stretch.
The Code
The standard for minimum safe electrical installations in the United States. The Code, also referred to as the National Electrical Code® or by the technical name NFPA 70®, is adopted in some form as law in all 50 states.
Thermoplastic
A plastic that may be repeatedly heated, shaped, and cooled without damage. Thermoplastic is often used as insulation and wire protection.
thousands of circular mills
kcmils. A unit of measurement that is equal to the area of a circle with a diameter of 1,000 mils, or 1 inch (25.4 mm). The thousands of circular mils unit is used to express large conductor sizes.
three-phase
AC power that consists of three overlapping voltages. Three-phase power is used for all large AC motors and is the standard power supply that enters homes and factories.
transmission lines
A conductor that sends electricity over long distances. Transmission lines do not have insulation.
Voltage
E. The electrical pressure or potential that forces current through a circuit. Voltage is measured in volts (V).
voltage drop
A decrease in voltage that occurs as electricity passes through resistance. Voltage drop increases as resistance increases and decreases as resistance decreases.
Volts
V. The unit used to measure electromotive force or pressure, which is called voltage. One volt is equal to one ampere multiplied by one ohm.
Wire
A slender rope of drawn metal used to transmit electrical current or signals. Wires usually consist of the metal conductor itself and a covering of insulation.