Article 450 in the National Electrical Code (NEC) covers requirements for transformers. The scope of Article 450, located in 450.1, states that it covers the installation of all transformers, but there are eight exceptions. Some of the transformers not covered include current transformers; transformers that are an integral part of an X-ray, high-frequency or electrostatic-coating apparatus; transformers used with Class 2 and Class 3 circuits that comply with Article 725; transformers for sign and outline lighting that comply with Article 600; and transformers for electric-­discharge lighting that complies with Article 410. In accordance with 450.3, overcurrent protection of transformers shall comply with 450.3(A), (B) or (C). 


Table 450.3(B) shows the maximum rating or setting of overcurrent protection for transformers 1,000 volts (V) and less. The table shows ratings or settings as percentages of transformer-rated current, and it displays percentages in two rows. Use the top row when overcurrent protection will only be installed on the primary side of the transformer. In accordance with the top row, secondary protection is not required so long as the primary protection does not exceed the percentages (and notes) shown in Table 450.3(B). For example, a 75-kilovolt-ampere (kVA), three-phase transformer will be installed. The transformer’s primary-side voltage will be 480V. Secondary protection for this transformer will not be installed. What is the maximum size (rating) fuse that can be installed on the primary side of this transformer so that secondary overcurrent protection will not be required? 


The transformer-rated current on the primary side will be 90 amperes (A) (75,000 ÷ 480 ÷ 1.732 = 90.2 = 90). The primary protection percentage for a transformer having a current draw of 90A or more is 125 percent. After multiplying by the primary protection percentage, 
the current is 113A (90 × 125 percent = 112.5 = 113). 


While the title of this table states that these are the maximum ratings, there is a note beside the primary protection percentage. Below Table 450.3(B), Note 1 states that, where 125 percent of the current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted. Standard ampere ratings for fuses and fixed-trip circuit breakers are in 240.6. The next standard rating above 113 is 125. Therefore, the maximum size fuse that can be installed on the primary side of this transformer is 125A (see Figure 1).


Use the bottom row of Table 450.3(B) when overcurrent protection will be installed on the primary side and the secondary side of the transformer. For example, a 112.5-kVA, three-phase transformer will be installed. The transformer’s primary-side voltage will be 480V, and the secondary-side voltage will be 208Y/120V. This transformer will have primary and secondary overcurrent protection. What is the maximum size (rating) fuse that can be installed on the primary and secondary side of this transformer? 


The transformer-rated current on the primary side will be 135A (112,500 ÷ 480 ÷ 1.732 = 135.3 = 135). After multiplying by the primary protection percentage, the current is 338A (135 × 250 percent = 337.5 = 338). When multiplying by any percentage other than 125, the product is the maximum rating. If the rating is not standard for a fuse or circuit breaker, it is not permissible to round up to the next standard rating. For this transformer, the primary overcurrent device rating shall not exceed 338A. In accordance with 240.6(A), the next standard rating lower than 338 is 300. While the fuse rating for the primary side of this transformer can be less than 300A, the maximum size fuse that can be installed is 300A. The transformer-rated current on the secondary side will be 312A (112,500 ÷ 208 ÷ 1.732 = 312.3 = 312). After multiplying by the secondary protection percentage, the current is 390A (312 × 125 percent = 390). Because there is text to the right of 125 percent that states to see Note 1, it is permissible to round up to the next standard-size fuse. The maximum size fuse that can be installed on the secondary side of this transformer is 400A (see Figure 2).


Sometimes there is confusion when sizing transformer conductors and overcurrent protection, and it is usually because of Tables 450.3(A) and (B). As previously mentioned, transformer overcurrent protection shall comply with 450.3(A), (B) or (C). In accordance with these tables, secondary overcurrent protection may not be required. In the first example, secondary protection was not required because the primary was protected at 125 percent of the transformer rated current. It certainly seems wrong to install a transformer with no overcurrent protection on the secondary side. When looking at the requirements in Article 450, it is important to realize it contains requirements for transformers and for the installation of transformers. Article 450 does not contain requirements for transformer conductors. While secondary overcurrent protection may not be required for the transformer itself, overcurrent protection may be required for the conductors. Requirements for overcurrent protection of conductors are in 240.4. There is even an informational note under 450.3 that refers to 240.4, 240.21, 240.100 and 240.101 for overcurrent protection of conductors. In accordance with the first sentence in 240.4(F) and the last paragraph in 240.21(C)(1), single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device. Because of these two sections, secondary overcurrent protection (for the conductors) is not required if the transformer is a single-phase transformer having a 2-wire (single-voltage) secondary or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary. Although secondary overcurrent protection is not required for these two transformer types, the installation of the conductors and primary overcurrent protection shall comply with 240.21(C)(1) (see Figure 3).


While a disconnecting means is usually installed within sight of the transformer, it is not a requirement. Section 450.14 was added to the Code in the 2011 edition. Transformers, other than Class 2 or Class 3, shall have a disconnecting means located either in sight of the transformer or in a remote location. This section continues by stating, where located in a remote location, the disconnecting means shall be lockable in accordance with 110.25, and its location shall be field-marked on the transformer. The term “in sight” is not just a general term; Article 100 defines it. Where the Code specifies that one piece of equipment shall be “in sight from,” “within sight from,” or “within sight of,” and so forth, another piece of equipment, the specified equipment is to be visible and not more than 15 meters (50 feet) from the other. 


Before the 2014 NEC, this section stated that the disconnect simply had to be lockable. Now this section states that the disconnect shall be lockable in accordance with 110.25, which is a new section in the 2014 edition. Where a disconnecting means is required to be lockable open elsewhere in the NEC, it shall be capable of being locked in the open position. The provisions for locking shall remain in place with or without the lock installed (110.25) (see Figure 4).


Next month’s column continues the discussion of sizing conductors.