Table 310.15(B)(16) is one of the most referenced tables in the National Electrical Code (NEC). It provides allowable ampacities for insulated conductors rated up to and including 2,000 volts (V).

The ampacities listed in the table are based on an ambient temperature of 30°C and on no more than three current--carrying conductors in a raceway, cable or earth (directly buried). When the ambient temperature is other than 30°C and/or there are more than three current-carrying conductors, the table ampacities must be corrected and/or adjusted. After the correction and/or adjustment factors have been applied, the conductors must be protected against overcurrent.

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). If it were not for the provisions in (A) through (G), the rating or ampacity of the overcurrent protection would not be permitted to be more than the ampacity of the conductor. For example, the allowable ampacity for a 500 kcmil THW copper conductor, listed in the 75°C column of Table 310.15(B)(16), is 380 amperes (A). Standard ampere ratings for fuses and inverse-time circuit breakers are listed in 240.6(A), and 380 is not a standard ampere rating. Without the provisions in 240.4(A) through (G), the maximum rating for the overcurrent device protecting this conductor would be 350A.

The first provision pertains to hazards due to loss of power. In accordance with 240.4(A), conductor overload protection shall not be required where the interruption of the circuit would create a hazard. This section continues by providing a couple of examples of equipment that would create a hazard with an interruption of power. A material-handling magnet (electromagnet) circuit is the first example, and a fire pump circuit is the second example. While overload protection for these types of circuits is not required, this section states that it is required to provide short-circuit protection.

The next section in 240.4 pertains to overcurrent devices rated 800A or less. 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 is often referred to as the round-up rule.

The first condition in using the round-up rule pertains to a receptacle branch circuit. Rounding up to the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall not be permitted if the conductors being protected are part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads [240.4(B)(1)].

For example, what size THWN/THHN copper conductors are required to supply a 120V, 20A branch circuit under the following conditions? This branch circuit will supply 11 general-purpose duplex receptacles. The receptacles will be installed outdoors, in locations exposed to weather. The type of occupancy will be a store. 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.

Start by calculating the branch-circuit load. In accordance with 220.14(I), receptacle outlets shall be calculated at not less than 180 volt-amperes (VA) for each single or multiple receptacle on one yoke. The receptacle load is 1,980 VA (11 180 = 1,980). By dividing volt-amperes by 120V, the amperage is 17A (1,980 ÷ 120 = 16.5 = 17). Since general-purpose receptacles do not meet the definition of a continuous load, this 20A branch circuit is 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 12 AWG copper conductors are usually installed on 20A circuits, apply the correction and adjustment factors to this size conductor. Because the conductor is a dual-rated conductor, look in Table 310.104(A) for the conductor applications. Type THHN conductors are permitted in dry and damp locations. Since this branch circuit will be in a wet location, do not use the ampacity for a THHN conductor. Type THWN conductors are permitted in dry and wet locations. Therefore, Type THWN must be used in the calculation.

While this conductor is permitted in wet locations, the maximum operating temperature is only 75°C. A 12 AWG copper conductor, listed in the 75°C column of Table 310.15(B)(16), shows an allowable ampacity of 25A. 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 current-carrying conductors, this conductor has a maximum ampacity of 18A (25 0.88 0.80 = 17.6 = 18). Since the load is only 17A, it might seem that 12 AWG conductors would be permitted for this installation. Size 12 AWG conductors are not permitted because of 240.4(B)(1). Because the conductors being protected in this example are part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads, it is not permissible to round up to the next higher standard size overcurrent device. After derating for the conditions in this example, 12 AWG THWN conductors have a maximum ampacity of only 18A. Since 18 is not a standard size and it is not permitted to round up to 20, these 12 AWG THWN conductors must be protected by a 15A breaker or fuse. A 15A overcurrent device is not rated high enough for this installation (see Figure 2).

Because 12 AWG conductors are not permitted for this 20A branch circuit, select the next larger size conductor and apply the adjustment and correction factors. The next larger size conductor is 10 AWG THWN/THHN. A 10 AWG copper conductor, listed in the 75°C column of Table 310.15(B)(16), shows an allowable ampacity of 35A. The Table 310.15(B)(2)(a) correction factor, in the 75°C column, 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 applying the correction and adjustment factors, this 10 AWG conductor has an ampacity of 25A (35 0.88 0.80 = 24.64 = 25). Because the round-up rule is not permitted with this installation, 10 AWG THWN/THHN copper conductors are required (see Figure 3).

Another popular calculation method for sizing conductors is to divide the load by the ambient temperature correction factor and the adjustment factor for more than three current-carrying conductors. After dividing, select the conductor from Table 310.15(B)(16). Use caution with this method because it could lead to a wrong answer.

In the above example, the load is 16.5A. The Table 310.15(B)(2)(a) correction factor 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 dividing the load by the correction and adjustment factors, the ampacity is 23A (16.5 ÷ 0.88 ÷ 0.80 = 23.4 = 23). A 12 AWG conductor, in the in the 75°C column of Table 310.15(B)(16), shows an ampacity of 25A. It would be easy to select a 12 AWG conductor because 23 is less than 25. But, as previously determined, this conductor is not permitted because of the round-up rule. It is acceptable to use this method, but perform the calculation again by multiplying the conductor’s table ampacity by the correction and adjustment factors. Then make sure the conductors are protected against overcurrent in accordance with their ampacities.

Next month’s column continues 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, charles@charlesRmiller.com and www.charlesRmiller.com.