Illinois Plumbing Code Practice

In Illinois Plumbing Code Practice, a circular wash sink with a specific rim measurement is considered equivalent to one lavatory based on the plumbing fixtures' sizing requirements. For this question, 18 inches of rim on a circular wash sink aligns with the standard dimensional requirements for a single lavatory.

This equivalence is important in ensuring that the fixtures meet flow rates and user accessibility needs, as well as determining the overall capacity and functionality of the plumbing system. As a result, 18 inches serves as a standard benchmark for assessing wash sinks within the context of lavatory requirements, making it the correct answer.

Other options provide dimensions that do not meet the established criteria for equivalency, emphasizing the importance of adhering to these standards for effective plumbing design and operation.