Tapping In: Understanding Feeder Taps, Transformer Secondary Rules

Over the years, there always seemed to be a lack of understanding of the requirements for feeder taps as well as transformer primary and secondary overcurrent protection. For example, in the 1990 National Electrical Code (NEC), the rules covering feeder taps and transformer secondary conductors were all exceptions to the general rule of providing protection at the point where the conductor to be protected receives its supply. As exceptions, these requirements were difficult to understand and apply. Code-Making-Panel 10 rewrote 240-21 for the 1993 NEC by translating the exceptions into positive text and subdividing the section into multiple subsections, each covering a particular type of feeder tap or transformer primary and secondary conductors.

A further rewrite to 240-21 occurred during the 1999 edition of the NEC, placing feeder taps in 240-21(b) and transformer secondary conductors in 240-21(c). Reorganization of this section did much to improve the layout and make it easier to understand.

The definition for tap conductors was added as a last paragraph in 240-3(e)—later, it was moved to its present location in 240.2—and read as follows: “As used in this article, a tap conductor is defined as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in this section.” The modification of this section over a nine-year period resulted in a section that is much easier to use and understand (for more on taps, see Code in Focus, page 50).

Feeders can have taps that are not more than 10 feet long, taps not more than 25 feet long, taps more than 25 feet and not longer than 100 feet, and outside feeder taps of unlimited length. There are feeder taps that supply the primary of a transformer, where the conductor length on the primary and secondary does not exceed 25 feet, but where the conductor on the primary side is not protected at its ampacity (making it a feeder tap). Let’s look at two of these applications: the 25-foot feeder tap and the 25-foot transformer feeder tap. Once everyone understands these two basic feeder taps, we can go into the transformer secondary rules in 240.21(C).

The main requirement for any of these feeder taps is that the length of the conductors is fixed to a maximum length, and the provisions of 240.4(B) for rounding up to the next standard overcurrent protective device where the ampacity of the conductor does not correspond to a standard size device is not permitted. So 240.21(B)(2) permits a 25-foot tap where the ampacity of the tap conductor is not less than one-third of the rating of the overcurrent device protecting the feeder conductors. The tap conductors must terminate in a single circuit breaker or single set of fuses that will limit the load to the ampacity of the tap conductors. Finally, the tap conductors must be protected from physical damage by being enclosed in an approved raceway or by other approved means. An example of this application could be a 400-ampere (A) overcurrent device supplying a set of 500 kcmil copper XHHW conductors rated at 380A. The smallest tap conductor must be at least one-third the size of the 400A device or at least 133A or a 1/0 XHHW copper conductor.

A 75-kilovolt-ampere transformer, rated at 480 volts (90A) on the primary side and 120/208 volts (208A) on the secondary side can be protected with [90A times 250 percent = 225A, based on Table 450.3(B)] a 225A circuit breaker. Using no less than one-third the size of the overcurrent protective device, a 1 AWG XHHW copper conductor rated at 130A could be used on the primary.

Since the primary conductor is not protected at its ampacity, the primary conductor length plus the secondary conductor length together cannot exceed 25 feet. The secondary conductor 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 on the primary side of the transformer.

Since the primary side of the transformer is protected at 250 percent, the secondary side of the transformer cannot be protected at greater than 125 percent (can be rounded up to next standard size device as provided in Table 450.3(B)) so 208A times 125 percent equals 260A or a 300A overcurrent device. Since the secondary conductors must be protected at their ampacity based on the last sentence of 240.21(B)(3), parallel 1/0 XHHW copper conductors could be used [150A × 2 = 300A]. Just remember the primary plus the secondary conductors must not be longer than 25 feet, and the secondary conductors must be terminated in a single circuit breaker or set of fuses. Understanding these simple calculations can make installation a breeze.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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