A fuse or the overcurrent trip unit of a circuit breaker must be connected in series with each ungrounded conductor [NEC Sec. 240.15(A)]. Also, the breaker must open all ungrounded conductors of the circuit both manually and automatically [240.15(B)]. There’s a pretty big twist to this, however, if you’re providing protection to motor circuits (see Parts III, IV, V, and XI of Art. 430).
For the ungrounded conductors of a: