Which of the following statements about branch-circuit conductors is correct?

The statement that branch-circuit conductors must be counted once if passing through a box is correct because it adheres to the commonly accepted standards and practices for electrical installations. When branch-circuit conductors enter a box, they are considered to maintain their count as a single conductor for purposes of applying NEC (National Electrical Code) fill requirements. This practice ensures that the correct wire fill calculations are performed to avoid overheating and potential safety hazards, aligning with code requirements.

The necessity of correct counting is important in ensuring that appropriate wire sizes are used, and that there is adequate space within junction boxes and conduit systems, which are designed to manage heat and electrical capacity safely. This practice reflects a broader principle in electrical wiring of ensuring safety and compliance with regulatory standards.

While other options might seem plausible, they do not accurately capture the nuances of the code surrounding branch-circuit conductors. Not all conductors must be insulated or made from copper; they can be made from various materials, and there is no specific rule on a minimum length of 100 mm for branch-circuit conductors in this context.