Non-potable water lines shall be labeled with __________ labels and ________ letters.

The correct answer is that non-potable water lines must be labeled with yellow labels and black letters. This standard is established to ensure clarity and immediate recognition of non-potable water systems, which are not safe for drinking. The use of yellow for the labels serves as a visual warning, while black letters provide a high contrast that enhances readability. This color-coding system is designed to prevent contamination and promote safety in plumbing systems, ensuring that individuals can quickly identify and understand the nature of the water lines they encounter. By adhering to this labeling system, professionals uphold safety standards and minimize the risk of misuse.

The other options represent different color schemes that are generally associated with other piping systems or safety indications. For example, black labels with yellow letters might be used for certain types of hazardous materials, while red labels typically signal fire protection equipment or hazards. The specific combination of yellow labels and black letters is standardized for non-potable water to distinctly differentiate it from other utility lines.