What is the minimum ampacity requirement for branch circuit conductors supplying a single motor in continuous duty applications?

The minimum ampacity requirement for branch circuit conductors supplying a single motor in continuous duty applications is correctly identified as 125 percent of the motor's full load current. This guideline is crucial because it ensures that the conductors can handle the continuous load without overheating, particularly during prolonged operation when motors can draw more current than their full load rating due to inrush or starting currents.

In continuous duty applications, motors are expected to run for extended periods, and having a conductor capacity above the full load current provides a safety margin. By using 125 percent, the National Electrical Code (NEC) accounts for the potential for increased heat generation and the need for cooling. This percentage helps mitigate risks associated with voltage drop and the associated heat buildup in circuits, which can lead to failure or fire hazards.

The other percentages listed lack the necessary safety margins for continuous operation. Using lower percentages like 100 percent or 110 percent does not account for potential additional load conditions that could lead to overheating. Additionally, a requirement of 150 percent is excessive for standard applications and could unnecessarily increase material costs and selectivity in breaker settings. Thus, 125 percent ensures both compliance with safety codes and optimal performance in residential electrical systems.