In the same scenario, what is the maximum secondary OCPD rating percentage?

In the same scenario, what is the maximum secondary OCPD rating percentage?

• A. 125%
• B. 100%
• C. 150%
• D. 200%

Answer: (A)

When sizing an overcurrent protection device on the secondary of a transformer, you balance two needs: allowing for brief surge currents (inrush) when the transformer energizes and protecting the windings from overheating during sustained overload. The transformer’s secondary has a rated continuous current, and the protective device should be just large enough to tolerate short surges but still prevent long-term overheating. 125% is the standard ceiling because it accommodates the typical inrush and startup currents without tripping, while still providing protection if the current remains elevated. A 100% rating would trip during energization inrush, causing nuisance trips. Ratings of 150% or 200% would let more current flow for longer before tripping, increasing the risk of overheating under fault conditions. Thus, the maximum practical secondary OCPD rating percentage is 125%.

When sizing an overcurrent protection device on the secondary of a transformer, you balance two needs: allowing for brief surge currents (inrush) when the transformer energizes and protecting the windings from overheating during sustained overload. The transformer’s secondary has a rated continuous current, and the protective device should be just large enough to tolerate short surges but still prevent long-term overheating.

125% is the standard ceiling because it accommodates the typical inrush and startup currents without tripping, while still providing protection if the current remains elevated. A 100% rating would trip during energization inrush, causing nuisance trips. Ratings of 150% or 200% would let more current flow for longer before tripping, increasing the risk of overheating under fault conditions. Thus, the maximum practical secondary OCPD rating percentage is 125%.