Panelboards supplied through transformers shall have their overcurrent protection located on which side of the transformer?

Panelboards supplied through transformers shall have their overcurrent protection located on which side of the transformer?

• A. Primary
• B. Secondary
• C. Both
• D. Neither

Answer: (B)

Protection for panelboards fed through a transformer belongs on the secondary side of the transformer. This places the interrupting device right where the panelboard feeders and downstream wiring are connected, ensuring faults on that side are quickly cleared and the conductors are protected from overloads. If the protection were on the primary side, a secondary fault would relate back through the transformer’s impedance and could either fail to trip promptly or not coordinate properly with downstream circuits, risking damage to the panelboard and connected equipment. The primary side is generally protected upstream by the utility or another disconnect, not meant to safeguard the secondary conductors. Therefore, the correct placement is on the secondary side.

Protection for panelboards fed through a transformer belongs on the secondary side of the transformer. This places the interrupting device right where the panelboard feeders and downstream wiring are connected, ensuring faults on that side are quickly cleared and the conductors are protected from overloads. If the protection were on the primary side, a secondary fault would relate back through the transformer’s impedance and could either fail to trip promptly or not coordinate properly with downstream circuits, risking damage to the panelboard and connected equipment. The primary side is generally protected upstream by the utility or another disconnect, not meant to safeguard the secondary conductors. Therefore, the correct placement is on the secondary side.