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Unit 11 Wire Tables and Conductor Sizes

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Objectives: Discuss factors that determine conductor ampacity. Discuss resistance of wire. Determine insulation characteristics. Use temperature correction factors. Determine conductor resistance.

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Unit 11 Wire Tables and Conductor Sizes Objectives: Determine proper wire sizes. List parallel conductor requirements. Discuss testing insulation with a MEGGER ®

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Unit 11 Wire Tables and Conductor Sizes 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).

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Unit 11 Wire Tables and Conductor Sizes 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

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Unit 11 Wire Tables and Conductor Sizes 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.

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Unit 11 Wire Tables and Conductor Sizes 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.

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Unit 11 Wire Tables and Conductor Sizes 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.

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Unit 11 Wire Tables and Conductor Sizes 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

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Unit 11 Wire Tables and Conductor Sizes 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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Unit 11 Wire Tables and Conductor Sizes.

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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.

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Unit 11 Wire Tables and Conductor Sizes Circular Mil (CM) Area The diameter of a wire is converted to mils. This number in mils is squared. CM = d 2

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Unit 11 Wire Tables and Conductor Sizes Mil Foot A wire 1 foot long and 1 mil in diameter.

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Unit 11 Wire Tables and Conductor Sizes 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.

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Unit 11 Wire Tables and Conductor Sizes Computing Voltage Drop E = I x R (Ohms Law) E (volt drop) = I x R (wire) I = current.

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Unit 11 Wire Tables and Conductor Sizes 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..

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Unit 11 Wire Tables and Conductor Sizes 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.

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Unit 11 Wire Tables and Conductor Sizes 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..

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Unit 11 Wire Tables and Conductor Sizes Parallel Conductors.

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Unit 11 Wire Tables and Conductor Sizes 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.

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Unit 11 Wire Tables and Conductor Sizes Testing for shorts with a MEGGER ®.

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Unit 11 Wire Tables and Conductor Sizes Testing for grounds with a MEGGER ®.

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Unit 11 Wire Tables and Conductor Sizes 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.

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Unit 11 Wire Tables and Conductor Sizes 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.

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Unit 11 Wire Tables and Conductor Sizes Review: 6.The English system uses the mil foot as a standard reference. 7.Conductors should be checked with a MEGGER® after installation.

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