transformer secondary conductor rules are similar to tap conductor rules. Article 240.21 of the National Electrical Code (NEC) covers the location and sizing requirements of overcurrent protection for tap conductors and transformer secondary conductors. Conductors connected to the secondary side of a transformer are not protected with an overcurrent device and, therefore, are not protected against overcurrent in accordance with their ampacities specified in 310.15. When installing a transformer, use the transformer secondary rules in 240.21(C)(1) through (C)(6).

Where installed in accordance with one of these six rules, a set of conductors feeding a single load, or each set of conductors feeding separate loads, can be connected to a transformer secondary without overcurrent protection at the secondary [240.21(C)]. There is an important informational note under this section that references 450.3. Section 450.3 covering overcurrent protection of transformers, and tables 450.3(A) and (B) provide the maximum rating or setting of overcurrent protection for transformers. Table 450.3(A) is for transformers over 600 volts (V), and Table 450.3(B) is for transformers 600V and less.

Sometimes there is confusion when looking at these tables. For example, a 75-kilovolt-ampere (kVA), three-phase transformer will be installed in a small industrial plant. The primary side voltage will be 480V, and the secondary side voltage will be 208Y/120V. A three-phase, fused disconnect with 125-ampere (A) fuses will be installed on the primary side of this transformer. Is secondary overcurrent protection required for this transformer?

This transformer is rated 75,000 volt-amperes (75 kVA × 1,000 = 75,000). The primary side current will be 90A (75,000 ÷ 480 ÷ 1.732 = 90.2 = 90). In accordance with the top row or Table 450.3(B), the maximum rating for the primary overcurrent protection is 125 percent. After multiplying the primary current by 125 percent, the ampacity is 113A (90 × 125% = 112.5 = 113). Note 1 under Table 450.3(B) states, where 125 percent of this 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. Therefore, this installation does not exceed the 125 percent primary protection shown on the top row of Table 450.3(B). In accordance with Table 450.3(B), secondary overcurrent protection is not required if the primary protection does not exceed 125 percent.

While this is true, Article 450 covers the installation of transformers. Tables 450.3(A) and (B) pertain to overcurrent protection for transformers and not transformer conductors. While secondary overcurrent protection is not required for the transformer itself, overcurrent protection is required for the conductors. The last paragraph in 240.21(C)(1) states single-phase (other than two-wire) and multiphase (other than delta-delta, three-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device. It’s important to note that overcurrent protection provisions for conductors, including transformer primary side and secondary side conductors, are in Article 240 (see Figure 1).

The first rule for transformer secondary conductors pertains to only two types of transformers; the first is a single-phase transformer having a two-wire (single-voltage) secondary. There are many types of transformers that match this description, some of which include doorbell transformers, landscape-lighting transformers, buck-boost transformers and motor-control transformers. The second type of transformer covered in 240.21(C)(1) is a three-phase, delta-delta connected transformer having a three-wire (single-voltage) secondary. Because the secondary side of this transformer is three-wire and single-voltage, this type of transformer does not have a neutral conductor. Provided the provisions in 240.21(C)(1) are met, the overcurrent device protecting either of these types of transformers can also serve as the protection for the secondary conductors (see Figure 2).

In all of the transformer secondary conductor rules except the first rule in 240.21(C)(1), the transformer conductors are not considered to be protected by the primary overcurrent protective device; therefore, secondary protection is required. When using the first transformer secondary conductor rule, the transformer must be either a single-phase transformer having a two-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a three-wire (single-voltage) secondary, and two conditions must be met. First, overcurrent protection must be in accordance with 450.3. Second, the rating of the transformer’s primary overcurrent protection shall not be more than the secondary conductor ampacity multiplied by the secondary-to-primary transformer voltage ratio.

If the transformer installation does not meet both conditions, secondary protection will be required. For example, a 75-kVA, three-phase, delta-delta connected transformer will be installed in an industrial facility. The primary or input voltage will be 480V, and the primary conductors will be 1 AWG THHN conductors. The secondary for this transformer will be three-wire, 240V (single-voltage), and the conductors will be 3/0 AWG THHN. The primary overcurrent protection for this transformer will have a rating of 125A. Is secondary overcurrent protection required for this transformer? While this transformer is one of the two transformers shown in 240.21(C)(1), both conditions in this section must be met, or secondary protection will be required. The primary side current will be 90A (75,000 ÷ 480 ÷ 1.732 = 90.2 = 90). In accordance with the top row or Table 450.3(B), the maximum rating for the primary overcurrent protection is 125 percent. After multiplying the primary current by 125 percent, the ampacity is 113A (90 × 125% = 112.5 = 113). Because of Note 1 under Table 450.3(B), this installation does not exceed 125 percent for the primary overcurrent protection.

Next, the rating of the transformer’s primary overcurrent protection shall not be more than the secondary conductor ampacity multiplied by the secondary-to-primary transformer voltage ratio. The secondary conductor ampacity is 200A. To find the secondary-to-primary transformer voltage ratio, divide the secondary voltage by the primary voltage. The secondary-to-primary transformer voltage ratio is 0.5 (240 ÷ 480 = 0.5). After multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio, the result is 100 (200 × 0.5 = 100). With this installation, the rating of the primary overcurrent protection must not be more than 100A. Because this installation does not meet both conditions in 240.21(C)(1), secondary protection is required (see Figure 3).

The transformer installation as shown in Figure 3 requires secondary protection, but, by lowering the rating of the primary overcurrent protection or by increasing the conductor size, secondary protection will not be required. For example, the primary overcurrent protection in Figure 3 was originally going to be rated at 125A. If the primary overcurrent protection is lowered to 100A, the installation would meet all the conditions in 240.21(C)(1). By lowering the rating of the primary overcurrent protection, this installation still does not exceed 125 percent for the primary overcurrent protection. The result of the secondary conductor ampacity multiplied by the secondary-to-primary transformer voltage ratio would still be 100 (200 × 0.5 = 100). But now, the rating of the primary overcurrent protection is not more than 100A. The new rating of 100A does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

It would also be permissible to increase size of the secondary conductors instead of lowering the primary overcurrent protection. The size of the secondary conductors in Figure 3 was originally going to be 3/0 AWG. To determine if the last condition in 240.21(C) will be met, recalculate with the next larger size conductor. In accordance with Table 310.15(B)(16), the next larger size above 3/0 is 4/0. The ampacity of 4/0, in the 75°C column, is 230A. After recalculating, the result is 115 (230 × 0.5 = 115). Since this conductor is still not large enough, select the next larger size conductor and recalculate. The next larger size is 250 kcmil and the ampacity of this conductor is 255A. After recalculating, the result is 128 (255 × 0.5 = 127.5 = 128). Now, the primary overcurrent device rating of 125A does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio. With either of these two changes, secondary protection is not required.

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