When sizing conductors, more is involved than just selecting a copper or aluminum conductor with the right ampacity from Table 310.15(B)(16) (formerly Table 310.16) in the National Electrical Code (NEC). Conductors must be selected and installed in accordance with all applicable provisions pertaining to conductors.

One provision states that the ampacity of a conductor shall not exceed the terminal connection temperature limitations in 110.14(C). A provision in 310.15(A)(3) states that conductors shall not be associated in such a way (with respect to type of circuit, the wiring method employed or the number of conductors) that any conductor exceeds its limiting temperature. Factors that can affect the temperature rating of a conductor include ambient temperature, heat generated internally in the conductor as the result of load current flow, the rate at which generated heat dissipates into the ambient medium, and adjacent load-carrying conductors. These factors are sometimes referred to as the conditions of use.

Other provisions, such as overcurrent protection requirements in Article 240, must also be considered when sizing conductors. For example, 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). Section 240.4(B) covers overcurrent devices rated 800A or less. This section 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.

The first condition pertains to a receptacle branch circuit. In accordance with 240.4(B)(1), if the conductors being protected are not part of a branch circuit that supplies more than one receptacle for cord-and-plug-connected portable loads, it shall be permissible to round up to the next higher standard overcurrent device rating (above the ampacity of the conductors being protected). As long as the conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads, the first condition in 240.4(A) will be met.

For example, what size THWN/THHN copper conductors are required to supply a single-phase, 208-volt (V), 20-ampere (A) branch circuit under the following conditions? This branch circuit will supply parking-lot lighting for an office building. The load will be 15.2A and will be a 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 six 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 38°C. Because these conductors will be in a wet location, Type THWN must be used in the calculation. Since the load is continuous, multiply the entire load by 125 percent. The minimum ampacity after multiplying by 125 percent is 19A (15.2 125% = 19). In accordance with 210.20(A), a 20A fuse or breaker must protect this branch circuit. Based only on the temperature ratings of the terminations and on the load being a continuous load, 14 AWG conductors are permitted (see Figure 1).

Because the ambient temperature will be higher than 30°C and there will be more than three current-carrying conductors in the raceway, correction and adjustment factors must be applied to the conductors. Since 14 AWG conductors were selected to satisfy the requirements for continuous loads and for the terminations, check if the ampacity of these conductors will equal or exceed the load after applying correction and adjustment factors. The conductor in this example is dual-rated.

Type THHN conductors are only permitted in dry and damp locations. Type THWN conductors are permitted in dry and wet locations. Since this branch circuit will be in a wet location, do not use the 90°C ampacity for a THHN conductor. While a THWN/THHN conductor is permitted in wet locations, the maximum operating temperature is only 75°C. A 14 AWG copper conductor, listed in the 75°C column of Table 310.15(B)(16), shows an allowable ampacity of 20A. The maximum ambient temperature in this example will be 38°C. The Table 310.15(B)(2)(a) correction factor, in the 75°C column (because of the THWN conductor), for an ambient temperature of 38°C is 0.88. The Table 310.15(B)(3)(a) adjustment factor for six 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 14A (20 0.88 0.80 = 14.08 = 14). Size 14 AWG conductors are not permitted because the load is 15.2A. Therefore, select the next larger size conductor to verify that the conductor can carry the load after applying the adjustment and correction factors. A 12 AWG copper conductor, listed in the 75°C column of Table 310.15(B)(16), shows an allowable ampacity of 25A. After derating because of ambient temperature and adjacent load-carrying conductors, this conductor has a maximum ampacity of 18A (25 0.88 0.80 = 17.6 = 18).

Because this will be a 20A branch circuit, it is necessary to use the round-up rule. Because the conductors protected in this example are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads, it is permissible to round up to the next higher standard size overcurrent device. The next higher standard size above 18A is 20A. Therefore, 12 AWG THWN/THHN copper conductors are permitted (see Figure 2).

The second condition pertains to standard ampere ratings of overcurrent devices. In accordance with 240.4(B)(2), if the conductor’s ampacity does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments), rounding up shall be permitted. Conversely, if the conductor’s ampacity does correspond with the standard ampere rating of a fuse or a circuit breaker, rounding up is not permitted.

For example, what size THHN copper conductors are required to supply a three-phase, 208V, 50A branch circuit under the following conditions? The 50A load will be noncontinuous. 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 three 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 30°C. This branch circuit will be installed entirely in a dry location. Since the load is not continuous, it is not necessary to multiply the load by 125 percent. There are no correction or adjustment factors because the ambient temperature will not be higher than 30°C and there will not be more than three current-carrying conductors in the raceway. Because of the terminations, select a conductor from the 75°C column of Table 310.15(B)(16). Since the load will be 50A, select a conductor with an ampacity of at least 50A. An 8 AWG copper conductor, listed in the 75°C column, has an allowable ampacity of 50A. In accordance with 240.6(A), 50A is a standard ampere rating for fuses and inverse-time circuit breakers. Since this is a standard ampere rating, it is not permissible to round up to the next size (see Figure 3).

It is not always necessary to use the round-up rule. There are times when the conductor’s ampacity is above the rating of the fuse or circuit breaker, even after applying the correction and adjustment factors.

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

MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and “The Electrician’s Exam Prep Manual.” He can be reached at 615.333.3336, [email protected] and www.charlesRmiller.com.