A circuit breaker feeds a 480 to 240/120 V single-phase transformer rated 750 VA. This transformer supplies 240/120 V three-wire to a fused safety switch. Per NEC 450 Article, what is the maximum fuse size allowed to be installed in the safety switch?

A circuit breaker feeds a 480 to 240/120 V single-phase transformer rated 750 VA. This transformer supplies 240/120 V three-wire to a fused safety switch. Per NEC 450 Article, what is the maximum fuse size allowed to be installed in the safety switch?

• A. 3
• B. 2.5
• C. 6
• D. 5

Answer: (D)

Protecting the transformer with a fuse on its secondary is all about limiting overcurrent to safeguard the windings and insulation while still allowing normal operation. For a 750 VA transformer rated 240/120 V, the secondary current is around 3.1 A at 240 V. NEC 450 establishes protection rules for transformers, and in this setup the fused safety switch is listed for a secondary fuse size that provides protection without nuisance trips. With this configuration, the largest fuse size commonly permitted by the listing and practice is five amperes. A smaller fuse could nuisance-trip during normal inrush or load transients, while a larger fuse would defeat the protection intended for the transformer. So the maximum fuse size allowed in the safety switch is five amperes.

Protecting the transformer with a fuse on its secondary is all about limiting overcurrent to safeguard the windings and insulation while still allowing normal operation. For a 750 VA transformer rated 240/120 V, the secondary current is around 3.1 A at 240 V. NEC 450 establishes protection rules for transformers, and in this setup the fused safety switch is listed for a secondary fuse size that provides protection without nuisance trips. With this configuration, the largest fuse size commonly permitted by the listing and practice is five amperes. A smaller fuse could nuisance-trip during normal inrush or load transients, while a larger fuse would defeat the protection intended for the transformer. So the maximum fuse size allowed in the safety switch is five amperes.