Which of the following is not required to be marked on motors?

In the context of motor labeling, it's essential to understand the key markings that are standard practice and safety requirements. Markings such as model number, rated horsepower, and voltage rating are critical for identifying the motor's performance specifications, compatibility with power sources, and safety compliance.

The model number allows for easy identification of the motor for repairs or replacements, while the rated horsepower indicates the motor's power capacity, which is crucial for ensuring it is suitable for its intended application. The voltage rating is critical for ensuring that the motor is connected to an appropriate power supply, avoiding potential damage or hazards from incorrect voltage applications.

In contrast, while it can be useful to know the date of manufacture for maintenance and warranty purposes, it is not a required marking for safety and operational compliance. This differentiates it from the more essential specifications that must be clearly marked on a motor for safe and effective use. Thus, the absence of the date of manufacture from mandatory motor markings distinguishes it from the other options listed, which are all required.