Non-potable water lines shall be labeled with ___________ labels and ______________ letters.

Non-potable water lines must be clearly marked to prevent any confusion or contamination with potable water supply. The requirement for using yellow labels is specified to ensure high visibility, making it easier for personnel and the general public to identify non-potable water lines. The use of black letters on these yellow labels enhances readability, providing a clear and distinct contrast that is crucial for safety.

This labeling practice is a standard in plumbing codes to promote awareness about non-potable lines, which carry water that is not safe for drinking. By using yellow for the label and black for the lettering, the plumbing industry establishes a recognizable and consistent warning system that helps reduce the potential for accidents. The other options provided do not align with this safety standard, reinforcing the rationale behind choosing yellow labels with black letters for non-potable water lines.