Recently, a question came up regarding installing overcurrent protection for dry-type transformers rated less than 1,000 volts (V). The transformer installation is a 150-kilovolt-ampere, three-phase unit with a 480V primary and a 208Y/120V secondary. The transformer is being installed in a mechanical room, and the ambient temperature in the room will normally be kept at or below 86°F. To protect this transformer with an overcurrent device only on the primary side, what is the maximum size overcurrent device required? If I use XHHW copper wire, what is the minimum size required for conductors supplying the 480V primary of this transformer?
To address these two questions, it is important to clarify that the question is seeking the maximum rating of the overcurrent device protecting the transformer and the minimum size conductors required on the primary. This response does not address any secondary conductor sizing or overcurrent protection. An informational note follows the base requirement in 450.3 and refers to the provisions in Article 240—specifically 240.4, 240.21, 240.100 and 240.101—that address overcurrent protection for conductors. With that information, it should be understood that the overcurrent protection requirements for transformers in Article 450 deal with protection for the transformer only. Once the overcurrent protection is determined, appropriately sized conductors can be selected for the installation and must be properly protected against overcurrent.
Article 450 Transformers
The National Electrical Code (NEC) provides installation requirements for transformers in Article 450. Section 450.3(B) specifically addresses overcurrent protection and provides a reference to Table 450.3(B) for transformers rated 1,000V, nominal or less. Reviewing Table 450.3(B) reveals a series of three notes at the bottom of the table. These notes are applicable as requirements as referred to from the table. The column on the left indicates the type of overcurrent protection method used for the transformer. This question seeks the size overcurrent device using primary-only protection, and as we scan the table from left to right, we find that, for a transformer with a primary current of 9 amperes (A) or more, a 125 percent multiplier must be used in calculating the overcurrent device size. By referring to Note 1 of Table 450.3(B), we determine that, where the 125 percent of the primary current of the transformer does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rated overcurrent device that does not exceed the next standard rating shall be permitted. Note that a lower rated overcurrent device could also be used and meet the NEC requirements, but the question seeks the maximum primary overcurrent device.
Primary-only overcurrent protection
It is always a good practice to use a calculation to determine the transformer’s primary amperage. Often the primary and secondary amperage is marked on the transformer nameplate, but it is helpful to know how to calculate this when the information is not available on the nameplate. The primary current for this transformer is calculated as follows: 150,000 volt-amperes (VA) ÷ 480 × 1.73 = 180.6A. Applying the requirements in Table 450.3(B), the value is 180.6 × 125% = 225.75. The standard ratings for fuses and circuit breakers are provided in 240.6, which shows that the next higher standard size from 225 is 250A. This transformer can be protected with a primary overcurrent device that does not exceed 250A.
Primary conductor sizing
Since the transformer primary overcurrent device determined is rated at 250A, the appropriately sized Type XHHW copper conductors can be selected using Table 310.15(B)(16). We must size these conductors to be protected by the 250A overcurrent device as determined for the maximum transformer primary protection. In referencing Table 310.15(B)(16), we find that, for XHHW insulated copper conductors in the 75°C column, 250 kcmil is good for 255A. The size conductors required for supplying the primary of this transformer is 250 kcmil, which will carry the full-load current of the transformer. The 75°C column was used because the terminals for circuits rated larger than 100A have a general temperature rating of 75°C in accordance with 110.14(C)(1)(b)(1). The conductor ampacity is derived directly from Table 310.15(B)(16) and has no ampacity adjustment factor applied because the number of current-carrying conductors feeding the transformer primary does not exceed three. Also, no correction factors are necessary for ambient temperature because the transformer and primary conductors are installed in an environment that does not normally exceed 86°F ambient temperature, based on information provided in the question.
In summary, for transformers rated 1,000V or less, overcurrent protection must be provided in accordance with NEC 450.3(B) and Table 450.3(B). Conductors for supply transformers and supplied by the secondary of transformers must be protected in accordance with the provisions in 240.4 and 240.21. Conductors supplying the primary of the transformer must be sized to carry the load on the transformer primary, so they are protected by the overcurrent device selected to comply with 450.3(B).