Transformer secondary conductor rules are similar to the rules for tap conductors. Rules for transformer secondary conductors and tap conductors are even in the same section of the National Electrical Code (NEC). A tap conductor has overcurrent protection ahead of its point of supply, but the rating of that overcurrent protective device exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4. Likewise, most transformer secondary conductors are not protected in accordance with their ampacities as specified in 310.15. 


Where transformer secondary conductors are not protected in accordance with their ampacities, the secondary conductors shall be sized and installed in accordance with 240.21(C). As stated in 240.21(C), a set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). 


The fifth transformer secondary conductor rule in 240.21(C) applies to transformer secondary conductors coming from a transformer that is supplied by feeder tap conductors. This rule is a little different from the others because it applies to an installation with both feeder tap conductors and transformer secondary conductors; the rule for this type of installation is in 240.21(B)(3). Section 240.21(C)(5) even states that transformer secondary conductors installed in accordance with 240.21(B)(3) shall be permitted to have overcurrent protection. 


If the primary transformer conductors will be protected in accordance with their ampacities as specified in 310.15, use one of the transformer secondary conductor rules in 240.21(C). If the primary transformer conductors will not be protected in accordance with their ampacities as specified in 310.15, use the feeder tap rule in 240.21(B)(3). Also, use the feeder tap rule in 240.21(B)(3) when the total length of one primary plus one secondary conductor, excluding any portion of the primary conductor that is protected at its ampacity, is not more than 25 feet. 


The maximum length of one primary plus one secondary conductor is just one of five conditions that shall be followed to use this tap rule. The conductors supplying the primary side of the transformer shall have an ampacity at least one-third the rating of the overcurrent device protecting the feeder conductors [240.21(B)(3)(1)]. The conductors supplied by the secondary of the transformer shall have an ampacity that 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 feeder conductors [240.21(B)(3)(2)]. 


The third condition is the 25-foot maximum length of one primary plus one secondary conductor. 


The fourth condition states that the primary and secondary conductors shall be protected from physical damage by being enclosed in an approved raceway or other approved means. 


In accordance with the fifth condition, the secondary conductors must terminate in a single circuit breaker or set of fuses that limits the load current to not more than the conductor ampacity that is permitted by 310.15 (see Figure 1).


Where installing a feeder tap to a transformer, ensure the installation meets all the conditions in 240.21(B)(3). For example, 350 kcmil THHN feeder conductors will terminate in a metal wireway (trough). A 300-ampere (A) overcurrent device will protect these feeder conductors. Two sets of feeder tap conductors will be connected to the 350 kcmil conductors. One set of tap conductors will supply power to a 100A fused disconnect. The other set of tap conductors will supply power to a three-phase, 112.5 kilovolt-ampere (kVA) transformer. The transformer’s primary side voltage will be 480 volts (V), and the secondary side voltage will be 208Y/120V. The load on the transformer will not be continuous. The transformer will supply power to a main-breaker panelboard. The main breaker in this panelboard will have a rating of 350A. The total length of one primary plus one secondary conductor will not exceed 25 feet. The primary and secondary conductors will be protected from physical damage by being enclosed in an approved raceway. All terminations will be rated 75°C. What minimum size primary side conductors are required to supply the transformer, and what minimum size secondary side conductors are required to supply the panelboard? 


Before sizing primary and secondary side conductors, ensure this installation will meet the other conditions in 240.21(B)(3). Because the total length of one primary plus one secondary conductor will not exceed 25 feet, the condition in 240.21(B)(3)(3) will be met. The condition in 240.21(B)(3)(4) will be met because the primary and secondary conductors will be enclosed in an approved raceway and protected from physical damage. 


As stated in 240.21(B)(3)(1), the primary side conductors must have an ampacity at least one-third the rating of the overcurrent device protecting the feeder conductors. In this installation, the rating of the overcurrent device protecting the feeder conductors is 300A; therefore, the minimum ampacity for the primary side conductors is 100A (300 ÷ 3 = 100). In accordance with the 75°C column of Table 310.15(B)(16), 3 AWG conductors could be installed to satisfy the requirement in 240.21(B)(3)(1). 


Although 3 AWG conductors are permitted as feeder tap conductors to supply the transformer, are these conductors sized correctly for the load of the transformer?


To size the primary side conductors, calculate the current draw of a 480V, three-phase, 112½-kVA transformer. This transformer is rated 112,500 volt-amperes (VA) (112.5 × 1,000 = 112,500). The primary side current will be 135A (112,500 ÷ 480 ÷ 1.732 = 135.3 = 135). Since 3 AWG conductors have an allowable ampacity of only 100A, these conductors would not be the correct size for the load. In accordance with Table 310.15(B)(16), 1/0 AWG copper conductors have an allowable ampacity of 150A. Since 1/0 AWG conductors are larger than 3 AWG conductors, the condition in 240.21(B)(3)(1) will be met. Size 1/0 AWG conductors are also large enough for the load of the transformer (see Figure 2).


In accordance with 240.21(B)(3)(5), the secondary conductors must terminate in a single circuit breaker or set of fuses that limits the load current to not more than the conductor ampacity that is permitted by 310.15. Since the secondary conductors of the transformer will terminate in a panelboard with a main breaker rated 350A, the ampacity of the conductor must be at least 350A. In accordance with the 75°C column of Table 310.15(B)(16), 500 kcmil conductors have an ampacity of 380A. 


Now ensure these conductors can carry the load on the secondary side of the transformer. The secondary side current will be 312A (112,500 ÷ 208 ÷ 1.732 = 312.3 = 312). Since 500 kcmil conductors have an ampacity of 380A, the conductors can carry the load. At this point, only one condition has not been verified. The second condition in 240.21(B)(3) states that the conductors supplied by the transformer secondary shall have an ampacity that 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 feeder conductors. The primary-to-secondary voltage ratio in this installation is 2.31 (480 ÷ 208 = 2.31). One-third of the rating of the overcurrent device protecting the feeder conductors is 100A (300 ÷ 3 = 100). The conductors supplied by the secondary of the transformer in this installation must have an ampacity of at least 231A (2.31 × 100 = 231). Since 500 kcmil conductors have an ampacity that is more than 231A, the condition in 240.21(B)(3)(2) will be met. The minimum size secondary side conductors required to supply the panel­board in this installation is 500 kcmil copper conductors (see Figure 3).


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