Where run vertically, Annex A of NFPA 72 recommends a separation of one foot for the outgoing and return conductors of which circuits?

• A. Class A, N, and X circuits
• B. Class B and C circuits
• C. Class D circuits
• D. All circuits

Answer: (A)

The idea is to minimize interference on long vertical runs by keeping paired conductors apart. Annex A of NFPA 72 says that when these circuits run vertically, the outgoing and return conductors should be separated by at least one foot for certain circuit types. Doing this reduces mutual coupling and noise between the two conductors, which helps ensure reliable signaling and supervision.

This separation applies specifically to Class A, N, and X circuits, because their signaling and supervision characteristics are more sensitive to interference along lengthy vertical runs. Other circuit types, such as Class B, Class C, or Class D, aren’t given this one-foot separation guidance in that section, so the rule isn’t stated for them.