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.