Using Wire Tables and Determining Conductor Sizes--part 1

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  • Intro; The American Wire Gauge (AWG)
  • Using the NEC Charts
  • Factors that Determine Ampacity
  • Correction Factors
  • Calculating Conductor Sizes and Resistance
  • Calculating Voltage Drop
  • Parallel Conductors
  • Testing Wire Installations
  • Summary/Quiz


  • Ambient air temperature
  • American Wire
  • Gauge (AWG)
  • Ampacity (current-carrying ability)
  • Circular mil
  • Correction factor
  • Damp locations
  • Dry locations
  • Insulation
  • Maximum operating temperature
  • Mil-foot
  • National Electrical Code (NEC)
  • Parallel conductors
  • Wet locations

INTRO: Being able to determine the amount of current a conductor is permit ted to carry or the size wire need for an installation is essential to any electrician, whether he or she works as an installation electrician or as a maintenance electrician. This guide:

  • explains how the amount of current a conductor is permitted to carry is not the same as selecting the proper wire for an installation and describes the differences.
  • differentiates the different types of wire insulation and the appropriate use of each based on ambient temperatures.
  • explains the method for using tools such as a MEGGER when deter mining the resistance of wire insulation.
  • discusses how conductor length and size impact resistance and determine the required conductor size.
  • provides the tools for determining ampacity rating of conductors when applying correction factors for wiring in a raceway.
  • explains that, as a general rule, electricians select wire sizes from the NEC.

However, there are instances where the wire run is too long or some special type of wire is being employed. In those cases, wire size and type are chosen by determining the maximum voltage drop and calculating the resistance of the wire.

This guide explains how to determine wire size using the NEC and calculating wire resistance.

Learning goals:

  • select a conductor from the proper wire table.
  • discuss the different types of wire insulation.
  • determine insulation characteristics.
  • use correction factors to determine the proper ampacity rating of conductors.
  • determine the resistance of long lengths of conductors.
  • determine the proper wire size for loads located long distances from the power source.
  • list the requirements for using parallel conductors.
  • discuss the use of a MEGGER for testing insulation.


The size of the conductor needed for a particular application can be determined by several methods. The National Electrical Code (NEC) is used throughout industry to determine the conductor size for most applications. It’s imperative that an electrician be familiar with Code tables and correction factors.

In some circumstances, however, wire tables cannot be used, as in the case of extremely long wire runs or for windings of a transformer or motor. In these instances, the electrician should know how to determine the conductor size needed by calculating maximum voltage drop and resistance of the conductor.

The American Wire Gauge (AWG)

The American Wire Gauge was standardized in 1857 and is used mainly in the United States for the diameters of round, solid, nonferrous electrical wire. The gauge size is important for determining the current-carrying capacity of a conductor. Gauge sizes are determined by the number of draws necessary to produce a given diameter or wire. Electrical wire is made by drawing it through a succession of dies.

+++++One side of the wire gauge is marked with the AWG size.

+++++The other side of the wire gauge lists the diameter of the wire in thousandths of an inch.

+++++The slot, not the hole, determines the wire size.

+++++Wire is drawn through a succession of dies to produce the desired diameter. Wire Die Draw block

+++++Wire gauge

Each time a wire passes through a die, it’s wrapped around a draw block several times. The draw block provides the pulling force necessary to draw the wire through the die. A 24 AWG wire would be drawn through 24 dies, each having a smaller diameter. In the field, wire size can be determined with a wire gauge. One side of the wire gauge lists the AWG size of the wire. The opposite side of the wire gauge indicates the diameter of the wire in thousandths of an inch. When determining wire size, first remove the insulation from around the conductor. The slots in the wire gauge, not the holes behind the slots, are used to determine the size.

The largest AWG size is 4/0, which has an area of 211,600 circular mills (CM). Conductors with a larger area are measured in thousand circular mills.

The next largest conductor past 4/0 is 250 thousand circular mills (250 kcmil). Conductors can be obtained up to 2000 kcmil. In practice, large conductors are difficult to pull through conduit. It’s sometimes desirable to use parallel conductors instead of extremely large conductors.

Using the NEC Charts

NEC 310 deals with conductors for general wiring. Table 310.15(B)(16) through Table 310.15(B)(19) are generally used to select a wire size according to the requirements of the circuit. Each of these tables lists different conditions. The table used is determined by the wiring conditions. Table 310.15(B)(16) lists ampacities (current-carrying ability) of not more than three single

insulated conductors in raceway or cable or buried in the earth based on an ambient (surrounding) air temperature of 30 degree C (868F). Table 310.15(B)(17) lists ampacities of single insulated conductors in free air based on an ambient temperature of 3 C. Table 310.15(B)(18) lists the ampacities of three single insulated conductors in raceway or cable based on an ambient temperature of 40 degree C (1048F). The conductors listed in Table 310.15(B)(18) and Table 310.15(B)(19) are generally used for high-temperature locations. The heading at the top of each table lists a different set of conditions.

+++++ Insulation around conductor. Insulation Conductor

Factors That Determine Ampacity

Conductor Material: One of the factors that determines the resistivity of wire is the material from which the wire is made. The wire tables list the current-carrying capacity of both copper and aluminum or copper-clad aluminum conductors. The currents listed in the left-hand half of Table 310.15(B)(16), e.g., are for copper wire. The currents listed in the right-hand half of the table are for aluminum or copper-clad aluminum. The table indicates that a copper conductor is permit ted to carry more current than an aluminum conductor of the same size and insulation type. An 8 American Wire Gauge (AWG) copper conductor with Type TW insulation is rated to carry a maximum of 40 amperes. An 8 AWG aluminum conductor with Type TW insulation is rated to carry only 35 amperes.

One of the columns of Table 310.15(B)(18) and Table 310.15(B)(19) gives the ampacity rating of nickel or nickel-coated copper conductors.


EXAMPLE: Find the maximum operating temperature of Type RHW insulation. (Note: Refer to the NEC.)

Solution: Find Type RHW in the second column of Table 310.104(A). The third column lists a maximum operating temperature of 758C, or 1678F.

Can Type THHN insulation be used in wet locations?


Locate Type THHN insulation in the second column. The fourth column indicates that this insulation can be used in dry and damp locations. This type of insulation cannot be used in wet locations. For an explanation of the difference between damp and wet locations, consult "locations" in Article 100 of the NEC.


Insulation Type: Another factor that determines the amount of current a conductor is permitted to carry is the type of insulation used. This is due to the fact that different types of insulation can withstand more heat than others. The insulation is the non conductive covering around the wire). The voltage rating of the conductor is also determined by the type of insulation. The amount of voltage a particular type of insulation can withstand without breaking down is determined by the type of material it’s made of and its thickness. NEC Table 310.104(A) lists information concerning different types of insulation. The table is divided into columns that list the trade name; identification letters; maximum operating temperature; whether the insulation can be used in a wet, damp, or dry location; material; thickness; and outer covering.

A good thing to remember is that insulation materials that contain the letter W, such as RHW, THW, THWN, and so on may be used in wet locations.

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Sunday, May 3, 2020 7:28