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2. The number of amperes of current to be carried 

3. The amount of voltage drop permitted 

4. Whether the current to be carried is intermittent or continuous 

5. The estimated or measured temperature of the conductor 

6. Whether the wire to be installed is in a conduit or in a bundle 

7. Whether it is a single conductor in free air 

Suppose that you want to install a 50-foot conductor from the aircraft bus to the equipment in a 28-volt system. For this length, a 1-volt drop is permissible for continuous operation with a conductor temperature of 20 ºC or less. By referring to the chart in Figure 4-79 , the maximum number of feet a conductor may be run carrying a specified current with a 1-volt drop can be determined. In this example, the number 50 is selected. 

Assuming the current required by the equipment is 20 

amperes, the line indicating the value of 20 amperes should be selected from the diagonal lines. Follow this diagonal line downward until it intersects the horizontal line number 50. From this point, drop straight down to the bottom of the graph to find that a conductor between size No. 8 and No. 10 is required to prevent a greater drop than 1 volt. Since the indicated value is between two numbers, the larger size, No. 8, should be selected. This is the smallest size that should be used to avoid an excessive voltage drop. 

If the installation is for equipment having only an intermittent (maximum 2 minutes) requirement for power, the graph in Figure 4-80  is used in the same manner. 

## Conductor Insulation 

Two fundamental properties of insulation materials (e.g., rubber, glass, asbestos, and plastic) are insulation resistance and dielectric strength. These are entirely different and distinct properties. 

Insulation resistance is the resistance to current leakage 

