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How to select the proper Overcurrent Circuit Protection for a Transformer?

Issue:
How to select protection and is both primary and secondary protection required?

Product Line:
LV Transformers

Environment:
Applies to Low Voltage Transformer products by SquareD/Schneider Electric

Cause:
Overcurrents and Short Circuit events can damage Transformers and related equipment and wiring

Resolution:
NEC Section 450.3(B) requires for Transformers rated 600 volts or less, that overcurrent protection be provided in accordance with Table 450.3(B).

Table 450.3(B) does not require overcurrent protection for the secondary of a Transformer if the primary current is 9 amperes or more and the primary overcurrent protection is limited to 125% of the Transformer rated current.

Table 450.3(B) permits primary overcurrent protection to be 250% of the Transformer rated current if the secondary current is 9 amperes or more and secondary overcurrent protection limited to 125% is provided.

The requirement for overcurrent protection for the secondary winding of a Transformer depends on the protection provided for the primary winding.

The overcurrent protection of the feeder conductors for the Transformer, if not larger than 250% of the Transformer primary current, may serve as the overcurrent protection for the Transformer primary winding.

Protection of the Transformer secondary conductors must be provided. Section 240.21(C)(1) through (6) provides the requirements for this protection.

The requirements for overcurrent protection for a panelboard supplied by the secondary of a Transformer are shown in 408.16(D).

Section: 450.3(B), Table 450.3(B), 240.21(C)(1) - (6), 408.