Tap conductor rules are located in Article 240 in the National Electrical Code (NEC). A tap conductor (as used in Article 240) has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4 [240.2]. Also, service conductors are not tap conductors.


Section 240.21 contains tap conductor rules and states overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located where the conductors receive theirsupply except as specified in 240.21(A) through (H), which covers branch-circuit taps, feeder taps, transformer secondary conductors, service conductors, busway taps, motor-circuit taps, conductors from generator terminals and storage battery conductors. 


While 240.21 covers eight types of conductors, most of the rules pertain to feeder taps and transformer secondary conductors. As a general rule, a conductor must be protected at its ampacity, and the overcurrent protection must be located at the point where the conductors receive their supply. Tap conductors feature overcurrent protection at the conductor supply, but the overcurrent device rating is higher than is usually permitted. While transformer secondary conductors are not usually referred to as tap conductors, they meet the definition of tap conductors. Transformer secondary conductors are not protected with an overcurrent device except for the overcurrent device on the transformer’ primary side. Therefore, transformer secondary conductors are not protected against overcurrent in accordance with 310.15. Provisions for transformer secondary conductors are in 240.21(C)(1) through (C)(6).


The second transformer secondary conductor rule can be used if the secondary conductors does not exceed 10 feet. When using this rule, all of the requirements or provisions in 240.21(C)(2)(1) through (4) must be met. These provisions are almost identical to the provisions in 240.21(B)(1). Section 240.21(B)­(1) is a feeder tap rule that applies when the tap conductor length does not exceed 10 feet. The first provision in 240.21(C)(2) pertains to the secondary conductor’s ampacity, which must not be less than the combined calculated loads on the circuits the secondary conductors supply. This is similar to provisions in 210.19(A)(1) and 215.2(A)(1), which state that the conductors’ ampacity shall not be less than the maximum load to be served. The first provision in 240.21(C)(2) also states the secondary conductors’ ampacity shall not be less than the rating of the equipment containing an overcurrent device(s) supplied by the secondary conductors or not less than the rating of the overcurrent protective device at the secondary conductors’ termination [240.21(C)(2)(1)b]. 


The secondary conductors must terminate in an overcurrent device that has an ampere rating equal to or less than the conductor ampacity rating. For example, size 3/0 AWG copper conductors will be installed on the secondary side of a transformer. The combined calculated loads on this transformer will not exceed 160 amperes (A), and the transformer secondary conductors will terminate in a 200A main breaker panelboard. In accordance with the 75°C column of Table 310.15(B)(16), the allowable ampacity of a 3/0 AWG conductor is 200A. 


This meets the first part of 240.21(C)(2)(1) because the ampacity of the secondary conductors will not be less than the combined calculated loads on the transformer. It meets the second part of 240.21(C)(2)(1) because the secondary conductors’ ampacity will not be less than the ampere rating of the main breaker in the panelboard. A 2/0 AWG conductor would not be permitted because a 2/0 AWG conductor does not meet both parts of this provision. Although a 2/0 AWG conductor has an ampacity of 175A, which is more than combined calculated loads on the transformer, the ampacity of a 2/0 AWG conductor is less than the main breaker’s ampere rating. This installation with 3/0 AWG conductors will be permitted provided the rest of the provisions in 240.21(C)(2) will be met (see Figure 1).


The second provision for this rule states that the secondary conductors shall not extend beyond the switchboard, panelboard, disconnecting means or control devices they supply. A single set of transformer secondary conductors shall not be permitted to supply more than one overcurrent device. This does not mean that the transformer itself shall not be permitted to supply more than one overcurrent device. In accordance with 240.21(C) and the second note under tables 450.3(A) and (B), there can be up to six sets of transformer secondary conductors, and each set can supply an overcurrent device such as a single circuit breaker or a single set of fuses.


The third provision states that the secondary conductors must be enclosed in a raceway, and the raceway must extend from the transformer to the enclosure of an enclosed switchboard, panelboard or control devices or to the back of an open switchboard.


The fourth provision pertains to field installations. For field installations where the secondary conductors leave the enclosure or vault in which the supply connection is made, the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary to secondary transformer voltage ratio, shall not exceed 10 times the ampacity of the secondary conductor [240.21(C)­(2)(4)] (see Figure 2).


The transformer ratio in Section 240.21(C)(2)(4) is opposite of the transformer ratio in 240.21(C)(1). The ratio in 240.21(C)(1) is a secondary-to-primary transformer voltage ratio, but the ratio for this 10-foot transformer secondary conductor rule is a primary-to-secondary transformer voltage ratio. For example, a 75-kilovolt-ampere (kVA), three-phase transformer will be installed in a small industrial plant. This transformer will have a 480-volt (V) primary and a 208Y/120V secondary. The overcurrent device on the primary side will be rated 125A, and the primary side conductors will be 3 AWG conductors. The transformer secondary conductors will be 4/0 conductors and will terminate in a 225A main breaker panelboard. The length of transformer secondary conductors will not exceed 10 feet. 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?


This meets the provision’s first part because the secondary conductors’ ampacity will be more than the combined calculated loads on the transformer. This meets the provision’s second part because the secondary conductors’ ampacity rating will not be less than the main circuit breaker’s rating. In accordance with the 75°C column of Table 310.15(B)(16), a 4/0 AWG conductor’s ampacity is 230A, and the main breaker’s rating is 225A. This meets the second provision because the secondary conductors will not extend beyond the panelboard. It meets the third provision because the transformer’s secondary conductors will be enclosed in a raceway. To see if the fourth provision will be met, a calculation is needed.


Start by multiplying the primary overcurrent protective device’s rating, which is 125, by the primary-to-secondary transformer voltage ratio, which is 2.31 (480 ÷ 208 = 2.31). The result is 289 (125 × 2.31 = 288.75 = 289), and this number shall not exceed 10 times the ampacity of the secondary conductor. The ampacity of the secondary conductor is 230A. The rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary-to-secondary transformer voltage ratio, is 1.26 times the ampacity of the secondary conductor (289 ÷ 230 = 1.26). Therefore, the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary-to-secondary transformer voltage ratio, does not exceed 10 times the ampacity rating of the secondary conductor. Since all the provisions in 240.21(C)(2) will be met, this installation will be permitted (see Figure 3).


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