Unit 11
Wire Tables and
Conductor Sizes
Unit 11
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.
Unit 11
Wire Tables and Conductor Sizes
Objectives:
• Determine proper wire sizes.
• List parallel conductor requirements.
• Discuss testing insulation with a
MEGGER®
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).
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
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.
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.
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.
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
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
.
Unit 11
Wire Tables and Conductor Sizes
.
Unit 11
Wire 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 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 = d2
Unit 11
Wire Tables and Conductor Sizes
Mil Foot
• A wire 1 foot long and 1 mil in diameter.
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
.
Unit 11
Wire Tables and Conductor Sizes
Computing Voltage Drop
• E = I x R (Ohm’s Law)
• E (volt drop) = I x R (wire)
• I = current
.
Unit 11
Wire 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.”
.
Unit 11
Wire 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
.
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.
.
Unit 11
Wire Tables and Conductor Sizes
Parallel Conductors
.
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.
Unit 11
Wire Tables and Conductor Sizes
Testing for shorts with a MEGGER®.
Unit 11
Wire Tables and Conductor Sizes
Testing for grounds with a MEGGER®.
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.
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.
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.