Article 240 in the National Electrical Code (NEC) provides general requirements for overcurrent protection and overcurrent protective devices. When sizing conductors, the rating of the overcurrent device must be considered. In accordance with 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). Without these provisions in (A) through (G), it would be a violation for the ampacity of the conductor to be less than the ampacity rating of the overcurrent device.

For example, after correction and adjustment factors have been applied, if the ampacity of a 10 AWG conductor is 24 amperes (A), protecting this conductor with a 30A breaker (or fuse) would not be permitted. The rating of the overcurrent device would not be able to exceed 24A. Although this main rule states that

conductors must be protected in accordance with their ampacities, there are some alternative provisions. One such provision is for overcurrent devices rated 800A or less. This rule, in 240.4(B), is often referred to as the round-up rule.

In accordance with 240.4(B), the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the conditions in 240.4(B)(1) through (3) are met. This provision is used quite often. For example, what size THWN/THHN copper conductors are required to supply a 400A, three-phase, 4-wire, 208-volt (V) service under the following conditions? The total calculated load is less than 275A. The neutral conductor carries only the unbalanced load from the other conductors and, therefore, will not be required to be counted as a current-carrying conductor. There also will be an equipment grounding conductor in this raceway. The service will be installed outdoors in a wet location. The voltage drop will not exceed the recommendation in 210.19(A)(1) Informational Note No. 4. All of the terminations will be rated 75°C. The maximum ambient temperature will be 30°C.

Because these conductors will be in a wet location, Type THWN must be used in the calculation. Because there are only three current-carrying conductors and the ambient temperature will not be above 30°C, it is not necessary to apply correction and adjustment factors. The ampacity of a 500 kcmil, THWN conductor, from Table 310.15(B)(16) is 380A. While the ampacity of this conductor is more than the calculated load, it is not more than the 400A overcurrent device at the service. If this installation meets the three conditions in 240.4(B), 500 kcmil conductors may be installed. Since, 1. the conductors being protected are not part of a branch circuit supplying more than one receptacle, 2. the ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker, and 3. the next higher standard rating does not exceed 800A, 500 kcmil conductors are permitted (see Figure 1).

When using the provision in 240.4(B), don’t forget to select and install the conductors in accordance with all of the provisions that are applicable. For example, what size XHHW-2 copper conductors are required to supply an 800A, three-phase, 4-wire, 208V service under the following conditions? Paralleled conductors will be installed to supply the service. The total calculated load is less than 600A. The neutral conductors carry only the unbalanced load from the other conductors and, therefore, will not be required to be counted as current-carrying conductors. There also will be an equipment grounding conductor in this raceway. The service will be installed outdoors in a wet location. 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 three current-carrying conductors and the ambient temperature will not be above 30°C, it is not necessary to apply correction and adjustment factors. The conductors are type XHHW-2 and, in accordance with Table 310.104(A), can be installed in a wet location. Although the XHHW-2 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)(b). The ampacity of a 500 kcmil conductor, from the 75°C column of Table 310.15(B)(16), is 380A. Because there will be a parallel set of conductors, the total ampacity will be 760A (380 × 2 = 760). While the ampacity of the parallel set of conductors is more than the calculated load, it is not more than the 800A overcurrent device at the service. Like the first example, the conductors being protected are not part of a branch circuit supplying more than one receptacle and the ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker. This example also meets the third condition in 240.4(B) because the overcurrent device will not be rated over 800A. In this example, a parallel set of 500 kcmil XHHW-2 conductors is permitted to supply this 800A service (see Figure 2).

Since, as a general rule, 500 kcmil conductors can be installed for 400A and a parallel set of 500 kcmil conductors can be installed for 800A, it would seem that three sets of 500 kcmil conductors paralleled could be installed for 1,200A. However, 240.4(C) does not permit such an installation. Where the overcurrent device is rated over 800A, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 240.6 [240.4(C)].

When the overcurrent device is rated more than 800A, the ampacity of the conductors must be equal to or greater than the rating of the overcurrent device. For example, what size XHHW copper conductors are required to supply a 1,200A, three-phase, 4-wire, 208V feeder under the following conditions? The feeder conductors will be paralleled. The load will be 1,120A, noncontinuous. The neutral conductors carry only the unbalanced load from the other conductors and, therefore, will not be required to be counted as current-carrying conductors. There also will be an equipment grounding conductor in this raceway. The feeder will be installed indoors in a dry location. 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 three 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 XHHW 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)(b). The ampacity of a 500 kcmil conductor, from the 75°C column of Table 310.15(B)(16), is 380A. Three sets of conductors in parallel have an ampacity of 1,140A (380 × 3 = 1,140). The ampacity of the paralleled conductors is more than the 1,120A load, but it is not more than the 1,200A overcurrent device at the feeder.

Unlike the first two examples, the rating of this overcurrent device is more than 800A. Therefore, the ampacity of the conductors must be equal to or greater than 1,200A. Since the ampacity of three sets of 500 kcmil, XHHW conductors is only 1,140A, this installation is not permitted (see Figure 3).

If only three sets of paralleled conductors will be installed for the 1,200A feeder in Figure 3, the ampacity of each conductor must be at least 400A (1,200 ÷ 3 = 400). The ampacity of a 600 kcmil conductor, from the 75°C column of Table 310.15(B)(16), is 420A. Three sets of conductors in parallel have an ampacity of 1,260A (420 × 3 = 1,260). Since the ampacity of the paralleled 600 kcmil conductors is more than the 1,200A overcurrent device at the feeder, this installation is permitted.

Another solution would be to install four parallel sets of conductors. If four sets of paralleled conductors will be installed for the 1,200A feeder, the ampacity of each conductor must be at least 300A (1,200 ÷ 4 = 300). The ampacity of a 350 kcmil conductor, from the 75°C column of Table 310.15(B)(16), is 310A. Four sets of conductors in parallel have an ampacity of 1,240A (310 × 4 = 1,240). Since the ampacity of the paralleled 350 kcmil conductors is more than the 1,200A overcurrent device at the feeder, this installation also is permitted (see Figure 4).

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