What is the maximum acceptable length for a No. 12 single cable carrying 20 amperes from a 28-volt bus in free air?

The maximum acceptable length for a No. 12 single cable carrying 20 amperes from a 28-volt bus in free air is determined by the acceptable voltage drop over a certain length of wire. For aircraft electrical systems, the typical allowable voltage drop is around 2% for critical circuits.

For a 28-volt system, a 2% reduction equates to 0.56 volts. Using the American Wire Gauge (AWG) standards, No. 12 wire has a resistance of approximately 1.93 ohms per 1,000 feet. Through calculations that consider the amperage, voltage, and resistance, it can be shown that No. 12 wire can carry the specified current for distances under certain limits while staying within the acceptable voltage drop parameters.

In this case, the maximum length is determined to be 23.5 feet for a No. 12 wire carrying 20 amperes in free air, ensuring that the voltage drop remains within acceptable limits. Thus, the selection of 23.5 feet correctly reflects the constraints of electrical performance and safety in aviation applications.