1-12
instance, rubber insulation will begin deteriorating at relatively low temperatures, whereas varnished cloth
insulation retains its insulating properties at higher temperatures. Other types of insulation are fluorinated
ethylene propylene (FEP), silicone rubber, or extruded polytetrafluoroethylene. They are effective at still
higher temperatures.
SAFE CURRENT RATINGS
The National Board of Fire Underwriters prepares tables showing the safe current ratings for sizes
and types of conductors covered with various types of insulation. The allowable current-carrying
capacities of single copper conductors in free air at a maximum room temperature of 30º C (86º F) are
given in table 1-3. At ambient temperatures greater than 30º C, these conductors would have less current-
carrying capacity.
Table 1-3.—Temperature Ratings and Current-Carrying Capacities (in Amperes) of Some Single Copper Conductors at
Ambient Temperatures of 30ºC
Size
Moisture Resistant
Rubber or
Thermoplastic
Varnished Cambric
or Heat Resistant
Thermoplastic
Silicone Rubber or
Fluorinated
Ethylene Propylene
(FEP)
Polytetra-
Fluoroethylene
0000
300
385
510
850
000
260
330
430
725
00
225
285
370
605
0
195
245
325
545
1
165
210
280
450
2
140
180
240
390
3
120
155
210
335
4
105
135
180
285
6
80
100
135
210
8
55
70
100
115
10
40
55
75
110
12
25
40
55
80
14
20
30
45
60
Q11.
List the four factors you should use to select wire for a specified current rating.
Q12.
What are three types of nonmetallic insulating materials that can be used in a high-temperature
environments?
Q13.
State why it is important for you to consider the ambient (surrounding) temperature of a
conductor when selecting wire size.
COPPER-VERSUS-ALUMINUM CONDUCTORS
Although silver is the best conductor, its cost limits its use to special circuits. Silver is used where a
substance with high conductivity or low resistivity is needed.
The two most commonly used conductors are copper and aluminum. Each has positive and negative
characteristics that affect its use under varying circumstances. A comparison of some of the
characteristics of copper and aluminum is given in table 1-4.
