The rating of the overcurrent device must be considered when sizing a conductor. In accordance with 240.4 in the National Electrical Code (NEC), 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).

The rules in 240.4(A) through (G) are alternatives. They pertain to power-loss hazards, overcurrent devices rated 800 amperes (A) or less, overcurrent devices rated over 800A, tap conductors, transformer secondary conductors, and overcurrent protection for specific conductor applications. Another alternative provision pertains to small conductors (and is sometimes referred to as the small-conductor rule). Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed the requirements of (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied [240.4(D)]. Conductor sizes covered by this section include 18 through 10 AWG copper and 12 through 10 AWG aluminum and copper-clad aluminum. While this small-conductor rule has been a requirement for more than 30 years, it has not always been in Article 240. From the 1978 edition of the NEC to the 1996 edition, this rule was a footnote of Table 310.16. Now, the footnote for 14–10 AWG conductors refers the reader to 240.4(D) for overcurrent protection limitations.

Prior to the 2008 edition, this provision only pertained to 14, 12 and 10 AWG conductors. Size 18 and 16 AWG copper conductors were added to the 2008 edition of the NEC. The overcurrent device shall not exceed 7A for an 18 AWG copper conductor and 10A for a 16 AWG copper conductor. Besides the provisions for the overcurrent device’s maximum ampacity, 18 and 16 AWG conductors have additional stipulations. The first stipulation for 18 AWG conductors states that continuous loads shall not exceed 5.6A. Likewise, the first stipulation for 16 AWG conductors states that continuous loads shall not exceed 8A. This is equivalent to the branch-circuit conductor requirement in 210.19(A)(1) for continuous loads because continuous loads are multiplied by 125 percent. A continuous load of 5.6A multiplied by 125 percent is 7A (5.6 × 125% = 7), and a continuous load of 8A multiplied by 125 percent is 10A (8 × 125% = 10).

The second stipulation pertains to overcurrent protection. In accordance with 240.4(D)(1)(2), overcurrent protection shall be provided by one of the following: 1. branch-circuit-rated circuit breakers listed and marked for use with 18 AWG copper wire; 2. branch-circuit-rated fuses listed and marked for use with 18 AWG copper wire; or 3. Class CC, Class J, or Class T fuses. In accordance with 240.4(D)(2)(2), overcurrent protection shall be provided by one of the following: 1. branch-circuit-rated circuit breakers listed and marked for use with 16 AWG copper wire; 2. branch-circuit-rated fuses listed and marked for use with 16 AWG copper wire; or 3. Class CC, Class J, or Class T fuses. There are installations where the overcurrent protection could be more than 7A for an 18 AWG copper conductor and 10A for a 16 AWG copper conductor. One example is fixture wires tapped to branch-circuit conductors. In accordance with 240.5(B)(2), 18 AWG fixture wires are permitted on 20A circuits as long as the run length is not more than 50 feet. Likewise, 16 AWG fixture wires are permitted on 20A circuits as long as the run length is not more than 100 feet.

It is important to keep the small-conductor rule in mind when sizing 14, 12 and 10 AWG conductors because the maximum rating for the overcurrent device may be less than the maximum ampacity of the conductor. For example, what size branch-circuit overcurrent protection is required for 12 AWG THHN copper conductors under the following conditions? The load will be 20A, noncontinuous. This 120-volt (V) branch circuit will consist of one ungrounded conductor, one grounded conductor and one equipment grounding conductor. These branch-circuit conductors will be in a raceway. The voltage drop will not exceed the NEC recommendation. All of the terminations will be rated 75°C. The maximum ambient temperature will be 30°C. Because there are only two current-carrying conductors and the ambient temperature will not be above 30°C, it is not necessary to apply correction and adjustment factors. Although the THHN conductors are rated 90°C, the allowable ampacity shall not exceed the 75°C column because of the termination provision in 110.14(C)(1)(a). The ampacity of a 12 AWG conductor, from the 75°C column of Table 310.15(B)(16), is 25A. Therefore, the maximum ampacity for these conductors in this installation is 25A. In accordance with 240.4(D)(5), the maximum overcurrent protection for 12 AWG copper conductors is 20A. Although these conductors have an allowable ampacity of 25A, and 25A is a standard rating for an overcurrent device, the maximum overcurrent protection for the conductors in this installation is 20A (see Figure 1).

In accordance with 240.4(D), the maximum overcurrent protection is after the application of any correction and adjustment factors. For example, what size branch-circuit overcurrent protection is required for 14 AWG THHN copper conductors under the following conditions? This circuit will be a single-phase, 240V branch circuit with a noncontinuous 14A load. The voltage drop in this branch circuit will not exceed the recommendation in 210.19(A)(1) Informational Note No. 4. These branch-circuit conductors will be in a raceway. There will be four current-carrying conductors and an equipment grounding conductor in this raceway. The terminations on both ends are rated at least 75°C. The maximum ambient temperature will be 35°C. All of this branch circuit will be installed in a dry location.

Since the load is not continuous, it is not necessary to multiply the load by 125 percent. The ampacity of a 14 AWG conductor, from the 90°C column of Table 310.15(B)(16), is 25A. The Table 310.15(B)(2)(a) correction factor, in the 90°C column, for an ambient temperature of 35°C is 0.96. The Table 310.15(B)(3)(a) adjustment factor for four current-carrying conductors in the raceway is 80 percent (or 0.80). After derating because of ambient temperature and adjacent load-carrying conductors, this conductor has a maximum ampacity of 19A (25 × 0.96 × 0.80 = 19.2 = 19). In accordance with 240.4(D)(3), the maximum overcurrent protection for 14 AWG copper conductors is 15A. Although these conductors have an allowable ampacity of 19A after the application of correction and adjustment factors, the rating of the overcurrent device shall not exceed 15A (see Figure 2).

After applying correction and adjustment factors to 14, 12 or 10 AWG conductors, the ampere rating of the overcurrent device may not be above the maximum rating specified in 240.4(D). For example, what size THHN copper conductors are required to supply a branch circuit under the following conditions? The load will be a 24A, continuous load. The voltage drop in this branch circuit will not exceed the recommendation in 210.19(A)(1) Informational Note No. 4. These branch-circuit conductors will be in a raceway. There will be a total of seven current-carrying conductors and an equipment grounding conductor in this raceway. The terminations on both ends are rated at least 75°C. The maximum ambient temperature will be 35°C.

Because of 210.19(A)(1), multiply the continuous load by 125 percent. The minimum ampacity after multiplying by 125 percent is 30A (24 × 125% = 30). Because of the termination provision in 110.14(C)(1)(a), select a conductor from the 75°C column of Table 310.15(B)(16). Because the load is continuous, the minimum size conductor is 10 AWG copper.

Now ensure these conductors will work with the required overcurrent protection.

In accordance with 210.20(A), the overcurrent protection for a branch circuit supplying a continuous load shall be at least 125 percent of the continuous load. Since the branch-circuit overcurrent protection must be at least 30A (24 × 125% = 30), and 10 AWG THHN conductors have a rating of 35A in the 75°C column of Table 310.15(B)(16), 10 AWG THHN conductors will work with a 30A breaker or fuse. Now see if a 10 AWG THHN conductor can carry 24A after applying the correction and adjustment factors. The Table 310.15(B)(2)(a) correction factor, in the 90°C column, for an ambient temperature of 35°C, is 0.96. The adjustment factor for seven current-carrying conductors is 70 percent or 0.70. Because the conductors are THHN, it is permitted to use the 90°C column (40 × 0.96 × 0.70 = 26.88 = 27). After applying correction and adjustment factors, this conductor has an ampacity of 27A. Although the continuous load is 30A, the conductors are only required to have a rating of the actual load of 24A. Because this installation meets the conditions in 240.4(B), it is permissible to round up to the next standard size overcurrent device rating above 27A. In this installation, the minimum size THHN copper conductors are 10 AWG (see Figure 3).

Next month’s column will continue the discussion of sizing conductors.