Mixed Wire Sizes

Correct Answer

A — smaller size wire

Concept and Reasoning

When conductors of different sizes are joined in one protected circuit, the overcurrent device must protect the weakest conductor. Therefore its rating cannot exceed the allowable ampacity of the smallest conductor unless a specific tap-rule exception applies.

Calculation / Key Relation

OCPD rating ≤ ampacity of the smallest protected conductor

Exam Takeaway

Protection is governed by the lowest ampacity in the series path.

Common Mistake

Do not size the protective device from the largest conductor in the run.