Conductors%20and%20Insulators

1
Conductors and Insulators
Chapter
11

• Topics Covered in Chapter 11
• 11-2 Standard Wire Gage Sizes
• 11-8 Wire Resistance
• 11-9 Temperature Coefficient of Resistance

2
11-2 Standard Wire Gage Sizes

• Sizes are specified by the American Wire Gage
  (AWG) system (p. 317, Table 11-1).
• Higher gage numbers mean thinner wire.
• Typical sizes are 22 AWG for electronic hookup
  wire and 12 AWG for home electrical wiring.
• The cross-sectional area of round wire is
  measured in circular mils.
• The higher the gage number and the thinner the
  wire, the greater its resistance for any length.

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11-2 Standard Wire Gage Sizes

• Wire Size

p. 317 Table 11-1
Circular mil area Diameter2 642.4 R / 1000ft
of copper wire _at_ 25?C 16.46 ?
4
11-2 Standard Wire Gage Sizes

• Wire Size
• The circular area of the wire doubles for every
  three gage sizes.
• 19 is three gages larger than 22 and has
  approximately twice the circular mil area. This
  is always the case when the gage number is
  decreased by 3.

Gage CMA
17 2048
18 1624
19 1288
20 1022
21 810
22 643
5
11-2 Standard Wire Gage Sizes

• Determine the resistance of a 30 feet section of
  30 wire.
• Determine the maximum length of a 25 cable if
  the wire resistance must be below 5 ?.
• The resistance of a 200ft section cable must be
  less than 2 W, determine the required wire gage
  .

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11-8 Wire Resistance

• Wire Resistance
• Resistance is proportional to the length of the
  wire.
• The resistance of a conductor can be found by the
  formula

? specific resistance of the conductor
R ?(l / A)
cross-section of the wire
length of the wire
7
11-8 Wire Resistance

• Specific Resistance
• Specific resistance ? CMA circular mil
  area O/ft
• Resistance of a conductor R ? (length/CMA)

Material Gage CMA r
Aluminum 17 2048 17
Copper 18 1624 10.4
Iron 19 1288 58
Nichrome 20 1022 676
Silver 21 810 9.8
Tungsten 22 643 33.8
Find R for 1000 ft. of 18 cu R ?
(length/CMA) R 10.4 (1000/1624) R 6.4 O
8
11-9 Temperature Coefficient of Resistance

• Temperature coefficient of resistance indicates
  how much the resistance changes for a change in
  temperature. It is indicated by the alpha symbol
  (a).
• A positive a value means R increases with
  temperature.
• A negative a value means R decreases with
  temperature.
• A value of 0 means R stays constant.

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11-9 Temperature Coefficient of Resistance

• a is generally positive for pure metals.
• a is generally negative for semiconductors
  (silicon, germanium) and electrolyte solutions
  (sulfuric acid, water).
• The increase in resistance may be calculated
  using the formula
• Rt R0 R0(a?t)
• R0 the resistance at 20 C.
• Rt the resistance at the higher temperature
• ?t the temperature rise over 20 C.

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11-9 Temperature Coefficient of Resistance

• Positive Temperature Coefficient (?)
• Some devices show a large increase in resistance
  when energized.
• Whats the lamps resistance at 2020 C?
• Rt R0 R0(??t)
• Rt 2 O 2 O 0.005 2000 22 O