While selecting the correct size conductor is not difficult, more is involved than selecting a conductor based solely on the ampacity shown in Table 310.15(B)(16) in the National Electrical Code (NEC). Many factors are involved, such as the temperature ratings of terminations, conductors and devices; the connected load or the load calculated in accordance with Article 220; continuous and noncontinuous loads; ambient temperature; and the number of current-carrying conductors. Other factors, such as overcurrent protection provisions in Article 240, must also be considered when sizing a conductor.


As stated in 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), which presents alternative rules. Without these alternative rules, it would always be necessary to ensure the ampacity of the conductor is at least equal to or greater than the ampere (A) rating of the overcurrent device. For example, 500 kcmil copper conductors could not be installed to supply a 400A feeder or service because the allowable ampacity is less than 400A. The ampacity of a 500 kcmil conductor, from the 75°C column of Table 310.15(B)(16), is only 380A. These alternative rules pertain to power-loss hazards, overcurrent devices rated 800A or less, overcurrent devices rated more than 800A, small conductors, transformer secondary conductors, and overcurrent protection for specific conductor applications.


Another alternative provision pertains to tap conductors. Section 240.4(E) does not provide specific provisions for tap conductors; this section provides references for tap conductor provisions that are located in other parts of the Code. Instead of tap conductors being protected against overcurrent in accordance with their ampacities, tap conductors can be protected against overcurrent in accordance with specific provisions depending on the type of tap. Types of taps in this section include taps for household ranges and other cooking appliances, fixture wires tapped to branch-circuit conductors, taps for busways, and taps for single motors. Another reference in this section is for the tap rules that are located in 240.21.

Before covering the tap rules, it is essential to understand the term “tap conductor.” As used in Article 240, a tap conductor is 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 240.4 [240.2]. The first thing mentioned in this definition is that service conductors are not tap conductors. Unlike feeder and branch-circuit conductors, service conductors do not have short-circuit and ground-fault protection. Each ungrounded service conductor shall have overload protection [230.90].

In the field, it is common to say that one conductor is “tapped” to another conductor, but this phrasing may be incorrect. For example, a service using the six-disconnect rule has been installed. This service consists of four 100A service disconnects. One set of 500 kcmil copper conductors enter a wireway through a raceway. Size 3 AWG service conductors from each 100A service disconnect will also enter the junction box and terminate to 500 kcmil conductors. This installation has been placed in accordance with all applicable rules in Article 230 that pertain to services. Smaller service conductors that connect to larger service conductors are quite often called tap conductors. But as stated in the definition, service conductors are not tap conductors (see Figure 1).

As also stated in the definition, a tap conductor has overcurrent protection ahead of its point of supply, but the tap conductor is not protected against overcurrent in accordance with its ampacity. In other words, the rating of the fuse or breaker is higher than the rating that is normally required or permitted. For example, a 3 AWG THW copper conductor is connected to a 250 kcmil THW copper conductor in a junction box. This installation has been placed in accordance with one of the tap rules in 240.21. The other end of the 250 kcmil conductor terminates to an overcurrent device with a rating of 250A. Since the 3 AWG conductor is connected to the 250 kcmil conductor, and the 250 kcmil conductor is protected by a 250A overcurrent device, the 3 AWG conductor is also protected by a 250A overcurrent device. In accordance with the 75°C column of Table 310.15(B)(16), the maximum ampacity of a 3 AWG conductor is 100A. While this 3 AWG conductor has overcurrent protection ahead of it, this 3 AWG conductor is not protected in accordance with its ampacity. Therefore, this 3 AWG conductor is a tap conductor (see Figure 2).

Just because a smaller conductor is connected to a larger conductor does not automatically mean that the smaller conductor is a tap conductor or that the smaller conductor is tapped to the larger conductor. For example, a 1 AWG THW copper conductor is connected to a 1/0 AWG THW copper conductor in a junction box. The other end of the 1/0 AWG conductor terminates to an overcurrent device with a rating of 150A. These conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads. Because of the connection to the 1/0 AWG conductor, the 150A overcurrent device also protects the 1 AWG THW copper conductor. In accordance with the 75°C column of Table 310.15(B)(16), the maximum ampacity of a 1 AWG conductor is 130A. Since these conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads, and 130A is not a standard ampere rating for an overcurrent device, rounding up to the next standard size overcurrent device shall be permitted [240.4(B)]. In accordance with 240.6(A), the next standard rating above 130A is 150A. Since this 1 AWG conductor has overcurrent protection ahead of it that is in accordance with its ampacity, this 1 AWG conductor is not a tap conductor (see Figure 3).


Sometimes conductors are increased in size to compensate for voltage drop. In this type of installation, a conductor might be called a tap conductor when it is not. For example, a 200A branch circuit has been installed and, because of the length, 250 kcmil THW copper conductors have been installed to compensate for voltage drop. The 250 kcmil conductors end in a junction box. A 3/0 AWG THW copper conductor is connected to each 250 kcmil conductor in that junction box. The other end of the 250 kcmil conductor terminates to an overcurrent device with a rating of 200A. Because of the connection to the 250 kcmil conductor, the 200A overcurrent device also protects the 3/0 AWG THW copper conductor. In accordance with the 75°C column of Table 310.15(B)(16), the maximum ampacity of a 3/0 AWG conductor is 200A.

In the field, this 3/0 AWG conductor might be called a tap conductor. But this 3/0 AWG conductor has overcurrent protection ahead of it, and it is in accordance with its ampacity. Therefore, this 3/0 AWG conductor is not a tap conductor (see Figure 4).


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