Electrician's Licensing Practice Exam

Why do dryers require a four-wire system?

1. To operate the motor neutral

2. To supply power to the timer

3. Two hots operate the heater neutral

4. For additional grounding protection

The correct answer is: Two hots operate the heater neutral

Dryers require a four-wire system primarily to ensure that both the heating element and the motor operate safely and efficiently. In this setup, two hot wires supply the necessary 240 volts for the heating element, which is essential for the drying process. The presence of a neutral wire allows for the proper functioning of 120-volt components, such as the timer and motor controls, providing a complete circuit for their operation. This configuration enhances safety by separating the grounding path from the current-carrying conductors. An important aspect of this four-wire system is that it helps prevent potential shock hazards. The fourth wire, dedicated to grounding, provides an additional layer of protection. In older configurations with only three wires, the grounding and neutral conductors were combined, which could lead to situations where a fault could energize parts of the appliance, presenting safety risks. By utilizing two hot wires for the heater with a separate neutral for control circuits and a distinct grounding wire, the system becomes safer, addressing both operational requirements and electrical safety standards. This configuration is now mostly mandated in modern installations to comply with the requirements of the National Electrical Code (NEC).