Table 310.15(B)(16) is one of the most referenced tables in the National Electrical Code (NEC). It contains allowable (or maximum) ampacities for insulated conductors rated up to and including 2,000 volts (V). The ampacities listed in this table are based on specific conditions.


One condition is the number of ­current-carrying conductors. The ampacities shown in this table are based on no more than three current-carrying conductors in a raceway, cable or earth (directly buried). Another condition is the ambient temperature. The ampacities listed in Table 310.15(B)(16) are based on an ambient temperature of 30°C.


When there are more than three current-carrying conductors and/or the ambient temperature is something other than 30°C, the table ampacities must be corrected and/or adjusted. After the ampacities of the conductors have been corrected and/or adjusted, the conductors must be protected against overcurrent. As a general rule, conductors must be protected at their ampacities. In accordance with 240.4, conductors (other than flexible cords, flexible cables and fixture wires) shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).


Section in 240.4(E) permits overcurrent device ratings to be higher than the ampacity of the conductor. It references various sections containing rules for tap conductors, one of which is 240.21. It is the section we usually think of in regard to tap rules. In accordance with 240.21(B), conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5).


The first two tap rules pertain to feeder taps supplying overcurrent devices such as fuse disconnect switches, circuit-breaker disconnect switches and main-breaker panelboards. The third tap rule in 240.21 pertains to feeder taps supplying transformers. With most installations, conductors supplying the transformer (or primary side conductors) are protected at the conductor’s ampacity. Even if they aren’t, they are usually protected with the next higher standard overcurrent device rating above the ampacity of the conductor.


For example, a 75-kilovolt-ampere (kVA), three-phase transformer with an input voltage of 480V will be installed in a small industrial plant. This transformer is rated 75,000 VA (75 kVA × 1,000 = 75,000). The primary side current will be 90 amperes (A) (75,000 ÷ 480 ÷ 1.732 = 90.2 = 90). This transformer will be sized for continuous loads; therefore, the conductors must have an ampacity of at least 113A (90 × 125% = 112.5 = 113). In accordance with Table 310.15(B)(16), 2 AWG copper conductors have an allowable ampacity of 115A. Therefore, 2 AWG copper conductors will be installed to supply the transformer. The primary side overcurrent protection device will have a rating of 125A, which is the next higher standard overcurrent device rating above 115A. Because of the roundup rule in 240.4(B), an overcurrent device with a rating of 125A shall be permitted to protect a conductor with an ampacity of 115A.


Using the roundup rule may seem like a violation because of the last sentence in 240.21(B), which states, “the provisions of 240.4(B) shall not be permitted for tap conductors.” The primary side conductors in this installation are acceptable because they are not tap conductors. Because the conductors supplying this transformer are protected with the next higher standard overcurrent device rating above the conductor’s ampacity, the primary side conductors are not feeder tap conductors (see Figure 1).


Where the primary side conductors are feeder tap conductors and are not protected at the conductor’s ampacity, the transformer can be installed in accordance with the third tap rule. The tap rule in 240.21(B)(3) pertains to taps supplying a transformer, but this tap rule is not just for the primary side conductors. Use this transformer tap rule 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 [240.21(B)(3)(3)].


Besides the conductor (primary plus secondary) length, four other conditions must be met when using this tap rule. The primary side conductors must 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 primary and secondary conductors must be protected from physical damage by being enclosed in an approved raceway or by other approved means [240.21(B)(3)(4)]. 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 is permitted by 310.15 [240.21(B)(3)(5)] (see Figure 2).


When using the tap rule for taps supplying a transformer (primary plus secondary not longer than 25 feet), the conditions for both primary side conductors and secondary side conductors must be met. For example, a three-phase, 112.5-kVA transformer will be installed. The primary side voltage will be 480V, and the secondary side voltage will be 208Y/120V. A three-phase, fused disconnect with 300A fuses will be installed on the primary side of this transformer. Size 1/0 AWG copper conductors will be installed from the fused disconnect to the primary side of the transformer. 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 secondary side conductors are required to supply the panelboard?


Before sizing the secondary side conductors, ensure this installation will meet all the other conditions in 240.21(B)(3). This transformer is rated 112,500 VA (112.5 × 1,000 = 112,500). The primary side current will be 135A (112,500 ÷ 480 ÷ 1.732 = 135.3 = 135). In accordance with Table 310.15(B)(16), 1/0 AWG copper conductors have an allowable ampacity of 150A. These conductors are not protected at their ampacity because of the 300A fuses in the disconnect supplying the transformer. In accordance with 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).


The condition in 240.21(B)(3)(1) will be met because the conductors supplying the transformer will have an ampacity of 150A. 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 protected from physical damage by being enclosed in an approved raceway.


In accordance with 240.21(B)(3)(5), 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 is permitted by 310.15. Because the overcurrent device must limit the load current to not more than the ampacity of the conductor, the ampacity of these secondary side conductors 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. Before finalizing the selection of the secondary side conductors, ensure the size selected meets the condition in 240.21(B)(3)(2). The conductors supplied by the secondary of the transformer must 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 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 (see Figure 3).