Overcurrent protection location for panelboards fed through transformers?

Overcurrent protection location for panelboards fed through transformers?

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

Answer: (B)

When a panelboard is fed through a transformer, the overcurrent protection should be placed on the transformer's secondary side. The secondary side is the supply to the panelboard, so putting the protective device there directly guards the panelboard conductors from overcurrents and ensures proper coordination with the transformer's impedance. If protection were on the primary side, you’d be protecting the transformer windings instead of the downstream panelboard, and fault current on the secondary could reach the panelboard before any protective action occurs. Thus, protecting the secondary side best safeguards the panelboard and its feeders.

When a panelboard is fed through a transformer, the overcurrent protection should be placed on the transformer's secondary side. The secondary side is the supply to the panelboard, so putting the protective device there directly guards the panelboard conductors from overcurrents and ensures proper coordination with the transformer's impedance. If protection were on the primary side, you’d be protecting the transformer windings instead of the downstream panelboard, and fault current on the secondary could reach the panelboard before any protective action occurs. Thus, protecting the secondary side best safeguards the panelboard and its feeders.