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Note: All Code references are based on the 2014 NEC.
The NEC has separate sections for transformer feeder protection and transformer protection. Article 240 lists requirements for transformer feeder protection, while Art. 450 provides requirements for transformer protection. The Fine Print Note of Sec. 450.3 states the relationship between the two different Articles: "the overcurrent protection required by Art. 450 may also satisfy the requirements in Art. 240 for conductor protection, and vice versa, but it is also possible that they do not." So although the two Articles are related, meeting one set of requirements does not necessarily mean it meets the other when it comes to overcurrent protection.
With Exceptions listed in Sec. 240.4(A) through (G), overcurrent protection is required for conductors.
"Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent ..." [240.4].
Furthermore, overcurrent protection is required for electrical conductors at the point where power is received.
"Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in Sec. 240.21(A) through (H)" [240.21].
However, there are a few Exceptions associated with these requirements. Let's review those now.
Protection at the primary side of the transformer
Transformer primary protection - Overcurrent protective devices (OCPDs) are required as primary protection for transformers when the primary voltage is greater than 1,000V [Tables 450.3(A)] and for transformers with primary voltage of 1,000V or less [Table 450.3(B)], with no Exceptions. However, a fire pump has a special requirement for its transformer primary and secondary protection, which is considered a modification to Art. 450. If a transformer serves a fire pump (Art. 695), its primary OCPD shall be sized per the locked-rotor current of the fire pump, and transformer secondary protection shall not be permitted [695.5(B)].
Related Art. 450: Transformers and Transformer Vaults
Small Dry-Type Transformers and Appropriate OCPD Sizing
Wiring Multiple Buildings from One Transformer
Transformer primary feeder protection - The NEC does not list any specific Exception for transformer primary feeder protection. An OCPD is required for the transformer primary conductor. The only Exception is found in Sec. 240.4(A). If overload protection of the OCPD would create a power loss hazard, such as fire...