Unit 11Wire Tables and Conductor Sizes

Unit 11Wire Tables and Conductor SizesObjectives:

• Discuss factors that determine conductor ampacity.

• Discuss resistance of wire.• Determine insulation characteristics.• Use temperature correction factors.• Determine conductor resistance.

Unit 11Wire Tables and Conductor Sizes

Objectives:

• Determine proper wire sizes.

• List parallel conductor requirements.

• Discuss testing insulation with a MEGGER®

Unit 11Wire Tables and Conductor Sizes

NEC® Tables and ChartsChapter 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).

Unit 11Wire Tables and Conductor Sizes

Factors That Determine AmpacityConductor Material

• Copper• Aluminum• Copper-clad aluminum

Insulation Type• Table 310.13 Insulation information• 60°C, 75°C, or 90°C columns

Unit 11Wire 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.

Unit 11Wire 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.

Unit 11Wire Tables and Conductor Sizes

Resistance of WireFour 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.

Unit 11Wire 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

Unit 11Wire 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 11Wire Tables and Conductor Sizes

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Unit 11Wire Tables and Conductor Sizes

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.

Unit 11Wire Tables and Conductor Sizes

Circular Mil (CM) Area

• The diameter of a wire is converted to mils.• This number in mils is squared.• CM = d2

Unit 11Wire Tables and Conductor Sizes

Mil Foot• A wire 1 foot long and 1 mil in diameter.

Unit 11Wire 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 11Wire Tables and Conductor Sizes

Computing Voltage Drop

• E = I x R (Ohm’s Law)• E (volt drop) = I x R (wire) • I = current

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Unit 11Wire Tables and Conductor Sizes

Computing Single-Phase Voltage Drop• ED = (2 x K x I x L) / CM

• ED = 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 11Wire Tables and Conductor Sizes

Computing Three-Phase Voltage Drop

• ED = (√3 x K x I x L) / CM

• ED = 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 11Wire 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 11Wire Tables and Conductor Sizes

Parallel Conductors

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Unit 11Wire 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.

Unit 11Wire Tables and Conductor Sizes

Testing for shorts with a MEGGER®.

Unit 11Wire Tables and Conductor Sizes

Testing for grounds with a MEGGER®.

Unit 11Wire 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.

Unit 11Wire 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.

Unit 11Wire 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.