Premises electrical distribution systems in outside buildings, structures or facilities are often delivered at high- or medium-voltage levels by the serving utility, depending on the area. The purpose of transformers is to reduce voltages to useful levels for all equipment that must be connected and operate on the property.
Low-voltage as used in this article refers to a system voltage level that is established in ANSI C84, American National Standard for Electric Power Systems and Equipment—Voltage Ratings (60 Hertz). This standard establishes system voltage classes, with low-voltage being classified as 1,000V or less.
ANSI C84.1-2020 provides a better understanding of voltages associated with power systems and utilization equipment by establishing nominal voltage ratings and operating tolerances for 60-Hz electric power systems above 100V.
The National Electrical Code generally addresses this by referencing either 1,000V or less, or greater than 1,000V, without system voltage classifications. This gap and an inconsistency between electrical codes and standards will eventually have to be addressed for consistency and promotion of common understanding and application, but that’s a story for another time.
The service voltage on any premises is usually limited to those voltages that the serving utility provides or by the voltage output of a source(s) for standalone systems, as covered in Article 710.
But back to the primary focus of this article. Transformers are categorized as equipment for general use by the NEC , and the requirements for transformers are found in Article 450. Low-voltage power and lighting transformers are required to be protected in accordance with sections 450.3(A), (B) or (C). This article provides specific information about the overcurrent protection rules in Section 450.3(B), which apply to transformers rated at 1,000V and less. This section refers users to Table 450.3(B), which addresses protection methods, including protection through the primary overcurrent protection only, or by primary and secondary overcurrent protection.
At this point, it is essential to focus on just the overcurrent protection for the transformer, although the sizes required for conductors on the primary and secondary are influenced by the rating of the transformer overcurrent protection as the percentages in the table are applied. This is the reason primary and secondary conductor sizes for a transformer need to be considered together with the sizing of the protection for the transformer. Protection for the transformer and the conductors are obviously necessary. Protect the transformer first, and then size the conductors so they are protected by the transformer overcurrent protection.
Primary overcurrent protection for transformers responds to short circuits, ground faults and overload protection on the primary side. However, it is usually less responsive to the same events on the secondary side, especially if there is an unbalanced condition caused by a single-phase, 3-wire system or a three-phase, 4-wire system. The secondary conductors could be protected based on a direct ratio across a single-phase, 2-wire primary to 2-wire secondary or a three-phase, delta-delta-connected transformer having a 3-wire secondary, which could be protected by properly rated overcurrent protection on the primary side, as indicated in 240.21(C)(1).
Now, in regards to applying Section 450.3(B): if primary-only protection is used, the overcurrent protective device must be rated at not more than 125% of the rated primary current of the transformer. Note 1 to Table 450.3(B) indicates that the next-higher rating of overcurrent protective device that does not exceed the next-higher standard overcurrent device rating shall be permitted. The example used here is a 15 kilovolt-amp (kVA), 3-phase, 480V transformer using primary-only protection.
First, obtain the rated primary current. It is usually found on the transformer nameplate, but a calculation can also be used. The primary current is the result of the kVA x 1,000, divided by the voltage x 1.73. For our example, this results in 15,000 / 830 = 18.07A. In accordance with Table 450.3(B) involving transformers with a current of 9A or more, 18.07A is multiplied by 125%, resulting in 22.58A.
In accordance with Section 240.6(A) and Table 240.6(A), the next-higher rated overcurrent protective device is 25A. This is the maximum rated overcurrent protective device for this transformer when using the primary-only protection method. The next step is to size the primary conductors supplying the transformer using Table 310.16 and 240.4(D), which results in minimum 10 AWG copper conductors.