Which conduit size has a metric designator of 21?

The correct choice of 3/4 inch corresponds to a metric designator of 21. In electrical wiring, the metric designator system provides a method of identifying conduit sizes based on their nominal inner diameters. For example, the designator "21" indicates that the conduit has an approximate inner diameter of 21 millimeters, which equates closely to 3/4 inch in standard imperial sizing.

This system is particularly useful in applications where ensuring compatibility with international standards is necessary. Understanding the relationship between metric designators and standard conduit sizes helps electrical professionals select the appropriate conduit for their installations effectively.

By aligning the correct conduit size with its metric designator, electricians can maintain compliance with both local codes and international standards, which is essential for safety and functionality in electrical systems.