Wire Tables and Conductor Sizes
Objectives: Discuss factors that determine conductor ampacity. Discuss resistance of wire. Determine insulation characteristics. Use temperature correction factors. Determine conductor resistance.
Objectives: Determine proper wire sizes. List parallel conductor requirements. Discuss testing insulation with a MEGGER
NEC Tables and Charts Chapter 3 of the NEC provides general information about Wiring Methods and Materials. Tables 310.16 through 310.19 are the most often referenced tables in the Code. These tables provide critical information about conductor ampacity (current-carrying ability).
Factors That Determine Ampacity Conductor Material Copper Aluminum Copper-clad aluminum Insulation Type Table 310.13 Insulation information 60 C, 75 C, or 90 C columns
Factors That Determine Ampacity Correction Factors are used to adjust ampacity based on ambient air temperature. High ambient air temperatures reduce conductor ampacity. Low ambient air temperatures increase conductor ampacity.
Factors That Determine Ampacity Temperature correction factors are found at the bottom of the tables. When more than three current-carrying conductors are installed in the same raceway, their respective ampacities must be reduced. NEC Table 310.15(B)(2)(a) provides the percentages used to adjust ampacity.
Resistance of Wire Four factors determine the resistance of a length of wire: 1. The type of wire material. 2. The cross sectional area of a conductor as measured in circular mils (CM). 3. The length of the conductor. 4. The temperature of the conductor.
Resistivity (K) of Materials As a general rule, the resistance of most conductive materials increases with rising temperature. Their resistance decreases with falling temperatures. These materials have a positive coefficient of temperature (K). Examples: Copper, Aluminum, Tungsten
Resistivity (K) of Materials A few conductive materials have decreasing resistivity with rising temperature and their resistance increases with falling temperatures. These materials have a negative coefficient of temperature (K). Examples: Carbon, Silicon, Germanium.
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Circular Mil (CM) A mil is one thousandth of an inch (0.001 ). A circular mil is a circle with a diameter of one mil.
Circular Mil (CM) Area The diameter of a wire is converted to mils. This number in mils is squared. CM = d 2
Mil Foot A wire 1 foot long and 1 mil in diameter.
Computing Resistance R = (K x L) / CM R = resistance of the wire K = ohms per mil foot (Copper = 10.4) L = length of wire in feet CM = circular mil area of wire.
Computing Voltage Drop E = I x R (Ohm s Law) E (volt drop) = I x R (wire) I = current.
Computing Single-Phase Voltage Drop E D = (2 x K x I x L) / CM E D = voltage drop K = ohms per mil foot I = current L = length of conductor in feet CM = circular mil area of the conductor Also known as the Two Kil Formula..
Computing Three-Phase Voltage Drop E D = ( 3 x K x I x L) / CM E D = voltage drop K = ohms per mil foot I = current L = length of conductor in feet CM = circular mil area of the conductor.
Parallel Conductor Rules Identical length of conductors. Identical conductor material. Identical conductor circular mil area. Identical conductor insulation type. Identical terminations of conductors. Each raceway contains all phase legs..
Parallel Conductors.
Installations are often checked with a MEGGER before a system is energized. In order to test the quality of the insulation, a very high voltage but low current is impressed on the system. Thus wiring defects can be determined and corrected safely.
Testing for shorts with a MEGGER.
Testing for grounds with a MEGGER.
Review: 1. The NEC tables are used for wire sizing. 2. Four factors determine wire resistance: a. the type of conductor material. b. the conductor length. c. the conductor circular mil area. d. the conductor temperature.
Review: 3. Conductor ampacity must be reduced in high-temperature locations. 4. More than three conductors in a raceway requires ampacity reduction. 5. Conductor ampacity is affected by the type of wire insulation.
Review: 6. The English system uses the mil foot as a standard reference. 7. Conductors should be checked with a MEGGER after installation.