1. Conductors and Insulators
Chapter 11
Conductors and Insulators
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.

3. 11-2: Standard Wire Gage Sizes
Wire Size
p. 317
Table 11-1
Diameter in mils
= 25.35
Circular mil area = Diameter2 = 642.4
R / 1000ft of copper 25C = 

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 #.

6. 11-8: Wire Resistance Wire Resistance R = ρ(l / A)
Resistance is proportional to the length of the wire.
The resistance of a conductor can be found by the formula:
ρ = specific resistance of the conductor
R = ρ(l / A)
cross-section of the wire
length of the wire

7. 11-8: Wire Resistance r Specific Resistance
Specific resistance = ρ = CMA [circular mil area] •Ω/ft
Resistance of a conductor = R = ρ (length/CMA)
Material
Gage
CMA r
Aluminum 17
2048
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 = ρ (length/CMA)
R = 10.4 (1000/1624)
R = 6.4 Ω

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 (α).
A positive α value means R increases with temperature.
A negative α value means R decreases with temperature.
A value of 0 means R stays constant.

9. 11-9: Temperature Coefficient of Resistance
α is generally positive for pure metals.
α 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(αΔt)
R0 = the resistance at 20 °C.
Rt = the resistance at the higher temperature
Δt = the temperature rise over 20° C.

10. 11-9: Temperature Coefficient of Resistance
Positive Temperature Coefficient ()
Some devices show a large increase in resistance when energized.
What’s the lamp’s resistance at 2020 °C?
Rt = R0 + R0(t)
Rt = 2 Ω + 2 Ω × × 2000 = 22 Ω