Rules for transformer secondary conductors are in Section 240.21 of the National Electrical Code (NEC). Many electricians and electrical engineers are familiar with 240.21 because it contains tap rules. Rules for transformer secondary conductors follow the feeder tap rules because transformer secondary conductors are similar to feeder taps. As stated in the definition of tap conductors in 240.2, a tap conductor is a conductor, other than a service conductor, that has overcurrent protection ahead of its supply point that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.

Conductors connected to the secondary side of transformers are not protected with an overcurrent device and, therefore, are not protected against overcurrent in accordance with their ampacities as specified in 310.15. Because the secondary conductors are not protected in accordance with their ampacities, use the transformer secondary rules in 240.21(C)(1) through (C)(6) when installing a transformer. 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)].

The rule in 240.21(C)(2) applies when the transformer secondary conductor does not exceed 10 feet in length. Use the last transformer secondary conductor rule, 240.21(C)(6), if the length of the transformer secondary conductor is more than 10 feet but not more than 25 feet.

When using the transformer secondary conductor rule in 240.21(C)(6), the primary transformer conductors shall be protected in accordance with their ampacities as specified in 310.15. If the primary transformer conductors will not be protected as such, use the transformer secondary conductor rule in 240.21(C)(5), which references the feeder tap rule in 240.21(B)(3). Where the primary transformer conductors will be protected in accordance with their ampacities and the length of the secondary conductor does not exceed 25 feet, all the requirements or provisions in 240.21(C)(6) shall be met.

The first provision pertains to the ampacity of the secondary conductors, which is not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device protecting the transformer’s primary [240.21(C)(6)(1)]. In accordance with the second provision in 240.21(C)(6), the secondary conductors must terminate in a single circuit breaker or set of fuses that limit the load current to not more than the conductor ampacity that 310.15 permits. In accordance with the third condition, the secondary conductors must be enclosed in an approved raceway or protected from damage by other approved means (see Figure 1).

The second provision not only states that the secondary conductors must terminate in a single circuit breaker or set of fuses but also that the overcurrent rating shall not exceed the ampacity of the secondary conductors. For example, transformer secondary conductors will terminate in a main breaker panelboard. The length of the transformer secondary conductor will be more than 10 feet but not more than 25 feet. The transformer will be a three-phase, 45-kilovolt-ampere (kVA) transformer, and its primary-side voltage will be 480 volts (V) and the secondary side will be 208Y/120V. The load on the transformer and panelboard will not be more than 90 amperes (A) and will not be a continuous load. The main breaker in this panelboard will have a rating of 125A. All terminations will be rated 75°C. If all the other conditions in 240.21(C)(6) will be met, what minimum size THHN secondary-side conductors are required to supply the panelboard?

In accordance with the 75°C column of Table 310.15(B)(16), 3 AWG conductors have an allowable ampacity of 100A, which is more than the 90A load. As 240.21(C)(6)(2) states, the overcurrent device shall limit the load current to not more than the conductor ampacity that 310.15 permits.

Because the overcurrent device’s rating is 125A, the ampacity of the transformer secondary conductors must be at least 125A. It may be tempting to use 2 AWG conductors because, after rounding up from 115A, the next standard rating is 125A. Besides violating 240.21(C)(6)(2), it also violates the last sentence in 240.21(C). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors. In other words, it is not permissible to use the round-up rule when sizing transformer secondary conductors. In accordance with the 75°C column of Table 310.15(B)(16), 1 AWG conductors have an allowable ampacity of 130A. Therefore, the minimum size secondary-side conductors required to supply the panelboard in this example is 1 AWG (see Figure 2).

Quite often, when sizing transformer secondary conductors, the first provision in 240.21(C)(6) is overlooked. The secondary conductor ampacity shall not be less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device protecting the primary of the transformer [240.21(C)(6)(1)].

For example, a 75-kVA, three-phase transformer will be installed in a small industrial plant. This transformer will have a 480V primary and a 208Y/120V secondary. The length of the transformer secondary conductor will be more than 10 feet but not more than 25 feet. The overcurrent device on the primary side will be rated 125A, and the primary-side conductors will be 2 AWG copper conductors. The transformer secondary conductors will be 4/0 AWG copper conductors and will terminate in a 225A main breaker panelboard. The combined calculated loads on this transformer will not exceed 175A. The transformer secondary conductors will be enclosed in a raceway that extends from the transformer to the panelboard. Will this installation be permitted?

To see if the first provision in 240.21(C)(6) will be met, a calculation is needed. Start by multiplying the rating of the primary overcurrent protective device, which is 125, by the primary-to-secondary transformer voltage ratio. Note that the first provision pertains to the primary-to-secondary voltage ratio, which is 2.31 (480 ÷ 208 = 2.31). After multiplying the rating of the primary overcurrent protective device by 2.31, the result is 289 (125 × 2.31 = 288.75 = 289). The ampacity of the secondary conductors shall be at least one-third of 289A. The minimum ampacity for the transformer secondary conductors in this example is 96A (289 ÷ 3 = 96). In accordance with the 75°C column of Table 310.15(B)(16), 4/0 AWG conductors have an allowable ampacity of 230A, which is more than the 96A minimum. Therefore, the first provision will be met. The second provision will be met because the ampacity rating of the secondary conductors will not be less than the rating of the main circuit breaker in the panelboard. As previously mentioned, 4/0 AWG conductors have an allowable ampacity of 230A, and it is more than the rating of the main circuit breaker, which will have a rating of 225A. The third provision will be met because the transformer secondary conductors will be enclosed in a raceway. Since all the provisions will be met, this installation will be permitted (see Figure 3).

The first provision in 240.21(C)(6) is usually not a factor when there is only one set of secondary conductors that terminates in a single circuit breaker or set of fuses. In the last example, the secondary conductors were required to have an ampacity of 96A to meet the first provision. Since a 75-kVA, three-phase transformer has a full-load current of 208A at 208Y/120V (75,000 ÷ 208 ÷ 1.732 = 208), it is unlikely that this size transformer would be used to supply a load that is less than 100A.

This first provision is very important if multiple sets of transformer secondary conductors supply separate loads. For example, instead of one 225A main-breaker panelboard, three sets of conductors will be installed on the secondary side of the transformer in Figure 3. Each of the three sets of secondary conductors will terminate in a fused disconnect with 70A fuses. The load on each fused disconnect will not exceed 56A. What minimum size THHN secondary-side conductors are required to supply each 70A fused disconnect?

As previously mentioned, the secondary conductors were required to have an ampacity of at least 96A to meet the first provision. Although each disconnect will be rated 70A, the conductors supplying each disconnect must have an ampacity of at least 96A. In accordance with the 75°C column of Table 310.15(B)(16), a 3 AWG conductor has an allowable ampacity of 100A. Therefore, the minimum size secondary-side conductors required to supply each of the three disconnects in this example is 3 AWG (see Figure 4).

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