The first seven parts of Article 240 in the National Electrical Code (NEC) provide the general requirements for overcurrent protection and overcurrent protective devices not more than 1,000 volts (V), nominal. The scope of Article 240 (located in 240.1) describes the items covered in all nine parts. Overcurrent protection requirements for supervised industrial installations operating at voltages of not more than 1,000V, nominal, are in Part VIII. Part IX covers overcurrent protection over 1,000V, nominal. As a general rule, overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply.
There are alternative provisions that permit the overcurrent protection ahead of the conductor to exceed the value permitted for similar conductors that are protected as described elsewhere in 240.4. As defined in 240.2, these conductors are tap conductors. While service conductors cannot be tap conductors, branch-circuit conductors and feeder conductors can be tap conductors.
The section in the Code that usually comes to mind when we think of tap rules is 240.21. The majority of the rules in this section pertain to feeder taps and transformer secondary conductors. Transformer secondary conductors are similar to tap conductors because overcurrent protection is not located where transformer secondary conductors receive their supply. Rules for transformer secondary conductors are located in 240.21(C)(1) through (6).
The third transformer secondary conductor rule in 240.21(C) applies to a very limited type of installation and can only be used in an industrial facility. This rule can be used if the length of the secondary conductors does not exceed 25 feet and, more important, the transformer supplies power to either switchgear or a switchboard. As defined in Article 100, switchgear is an assembly completely enclosed on all sides and top with sheet metal (except for ventilating openings and inspection windows) and containing primary power circuit switching, interrupting devices, or both, with buses and connections. The definition states the assembly may include control and auxiliary devices. Doors, removable covers, or both provide access to the interior of the enclosure. Before the 2014 edition of the NEC, “switchgear” was referred to as “metal-enclosed power switchgear.” The term “switchboard” also is defined in Article 100 as a large single panel, frame or assembly of panels on which switches, overcurrent and other protective devices, buses, and usually instruments are mounted on the face, back or both. The definition also states these assemblies are generally accessible from the rear as well as from the front and are not intended to be installed in cabinets (see Figure 1).
There was a change to 240.21(C)(3) in the 2014 NEC. Before the 2014 edition, this section started with, “For industrial installations only … .” Now, this section starts with, “For the supply of switchgear or switchboards in industrial installations only ... .” This was not really a change, but a clarification.
Although not specifically stated before, this transformer secondary conductor rule can only be used for the supply of switchgear and switchboards. This rule is not permitted for the supply of motor control centers because a motor control center shall be protected by a single overcurrent protective device in accordance with 430.94. This rule is also not permitted for the supply of panelboards because a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard, in accordance with 408.36.
Besides the provision that limits the transformer secondary conductors to a maximum length of 25 feet, there are four additional requirements that must be met before using this rule. In accordance with 240.21(C)(3)(1), conditions of maintenance and supervision shall ensure that only qualified people service the systems. The second provision pertains to the secondary conductors. The ampacity of the secondary conductors shall not be less than the secondary current rating of the transformer, and the sum of the ratings of the overcurrent devices shall not exceed the ampacity of the secondary conductors. The third provision simply states that all overcurrent devices shall be grouped. The fourth provision states that the secondary conductors shall be protected from physical damage by being enclosed in an approved raceway or by other approved means (see Figure 2).
Most of the time, transformers supply panelboards or safety switches (disconnects), but there are times when the transformer secondary conductor rule in 240.21(C)(3) is used. For example, a 500-kilovolt-ampere (kVA), three-phase transformer will be installed in an industrial facility. In accordance with this company’s electrical safety policy, only qualified people are permitted to service this system. This transformer will have a 480V primary and a 208Y/120V secondary. This transformer will supply a switchboard. The transformer secondary conductors will terminate at main lugs and not at a single overcurrent device. Only four overcurrent protective devices will be in this switchboard: two three-phase, 400-ampere (A) overcurrent devices and two three-phase, 200A overcurrent devices. The length of the transformer secondary conductors will not exceed 25 feet. The transformer secondary conductors will be enclosed in raceways that extend from the transformer to the switchboard. All terminations will be rated 75°C. What minimum size secondary side conductors are required to supply the switchboard?
Although there are many possible combinations, use four sets of copper THHN conductors in parallel for this example. Before sizing the secondary-side conductors, ensure this installation will meet all the other conditions in 240.21(C)(3). The provisions in the main text will be met because the transformer secondary conductors will supply a switchboard in an industrial installation, and the length of the transformer secondary conductors will not exceed 25 feet. The first condition will be met because only qualified people are permitted to service this system. Since all four overcurrent devices are in one switchboard, the overcurrent devices will be grouped. The last provision will be met because the transformer secondary conductors will be enclosed in raceways that extend from the transformer to the switchboard. The last part of the second provision states that the sum of the ratings of the overcurrent devices shall not exceed the ampacity of the secondary conductors. In other words, the ampacity of the secondary conductors must equal or exceed the sum of the ratings of the overcurrent devices. The sum of the ratings of the overcurrent devices in this switchboard is 1,200A (400 + 400 + 200 + 200 = 1,200). Since there will be four sets of conductors in parallel, divide 1,200 by four to find the ampacity. Each conductor must have an ampacity of at least 300A (1,200 ÷ 4 = 300). In accordance with the 75°C column of Table 310.15(B)(16), the ampacity of a 350 kcmil copper conductor is 310A. Based only on the sum of the ratings of the overcurrent devices in this switchboard, the size of the transformer secondary conductors can be 350 kcmil when four sets are installed in parallel (see Figure 3).
Up to this point, the installation in Figure 3 has met all of the provisions except one. In accordance with the first part of the second provision in 240.21(C)(3), the ampacity of the transformer secondary conductors shall not be less than the secondary current rating of the transformer. This transformer is rated 500,000 VA (500 kVA × 1,000 = 500,000). The secondary-side current rating will be 1,389A (500,000 ÷ 360V = 1,389). Since there will be four sets of conductors in parallel, divide 1,389 by four to find the ampacity. Each conductor must have an ampacity of at least 347A (1,389 ÷ 4 = 347). In accordance with the 75°C column of Table 310.15(B)(16), the ampacity of a 500 kcmil copper conductor is 380A. Because of the first part of the second condition in 240.21(C)(3), larger conductors are required. The minimum size conductors, when installing four sets in parallel, are 500 kcmil copper conductors (see Figure 4).
Next month’s column continues the discussion of sizing conductors.