Article 310 in the National Electrical Code (NEC) covers general requirements for conductors and their type designations, insulations, markings, mechanical strengths, ampacity ratings and uses. Ampacities for conductors rated 0 to 2,000 volts (V) shall be as specified in the Allowable Ampacity Table 310.15(B)(16) through Table 310.15(B)(19) and Ampacity Table 310.15(B)(20) and Table 310.15(B)(21) as modified by 310.15(B)(1) through (B)(7) [310.15(B)]. Section 310.15(B) also states that the temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the conductor temperature rating if the corrected and adjusted ampacity does not exceed the ampacity for the termination temperature rating in accordance with the provisions of 110.14(C).


While there are many ampacity tables, Table 310.15(B)(16) is used most often. The ampacities shown in this table are based on no more than three current-carrying conductors in a raceway, cable or earth (directly buried). 


The ampacities listed in Table 310.15(B)(16) are based on an ambient temperature of 30°C. When there are more than three current-carrying conductors, the conductor’s ampacity must be adjusted. Likewise, if the ambient temperature is something other than 30°C, the table ampacities shall be corrected. It may even be necessary to apply both a correction factor and an adjustment factor.


Before selecting a conductor, the load must be determined. Article 220 provides requirements for calculating branch circuit, feeder and service loads. As stated in 220.18, the total load shall not exceed the branch circuit rating, and it shall not exceed the maximum loads specified in 220.18(A) through (C) under the conditions specified therein. The first specifications in this section pertain to motor-operated and combination loads. The second sentence in 220.18(A) pertains to air conditioning and refrigeration equipment. Where a circuit supplies only air conditioning equipment, refrigerating equipment or both, Article 440 shall apply. Article 440 applies to electric motor-driven air conditioning and refrigerating equipment and to the branch circuits and controllers for such equipment. Selecting conductors and overcurrent protective devices for air conditioning equipment is not the same as it is for motors. With a hermetic refrigerant motor-compressor, the nameplate on the equipment usually provides the information necessary for selecting the conductor size and the overcurrent protective device rating. In accordance with 440.6(A), it is permissible to use the nameplate information in determining the rating or ampacity of the disconnecting means; the branch-circuit conductors; the controller; the branch-circuit short-circuit and ground-fault protection; and the separate motor overload protection. Some of the electrical information on air conditioning equipment nameplates includes the rating in volts, frequency and number of phases, minimum supply circuit conductor ampacity, and the maximum rating of the branch-circuit short-circuit and ground-fault protective device. See 440.4 for all of the required nameplate information.


Because the minimum supply circuit conductor ampacity is on the nameplate, calculating the equipment ampacity is not necessary. For example, an outdoor air conditioner is to be installed. The minimum circuit ampacity shown on the nameplate is 35.8 amperes (A), and the maximum rating for the overcurrent device is 50A. The branch-circuit conductors supplying power to this air conditioner will be THWN copper, and the conductors will be installed in a raceway. What is the minimum size conductor required to supply this air conditioner, and what is the maximum rating for the overcurrent device, which will be a circuit breaker?


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, and there will not be more than three current-carrying conductors in the raceway. Because the minimum circuit ampacity shown on the nameplate is 35.8A, the conductor must have an ampacity of at least 35.8A. In accordance with the 75°C column of Table 310.15(B)(16), an 8 AWG conductor is good for 50A. A 10 AWG conductor is close, but it is only good for 35A. As shown on the nameplate, the maximum rating for the overcurrent device is 50A. Therefore, the maximum rating for the circuit breaker is 50A (see Figure 1).


Correction and/or adjustment factors may have to be applied when selecting a conductor for air conditioning equipment. For example, a rooftop air conditioner will be installed. The minimum circuit ampacity shown on the nameplate is 43.2A, and the maximum rating for the overcurrent device is 70A. The branch-circuit conductors supplying power to this air conditioner will be THWN-2 copper, and the conductors will be installed in a raceway that will be exposed to direct sunlight. The distance above the roof to the bottom of the raceway will be 4 inches. What is the minimum size conductor required to supply this air conditioner?


The voltage drop will not exceed the NEC recommendation. All of the terminations will be rated 75°C. The maximum ambient temperature will be 40°C, and no more than three current-carrying conductors will be in the raceway. As stated on the nameplate, the conductor must have an ampacity of at least 43.2A. 


In accordance with the 75°C column of Table 310.15(B)(16), an 8 AWG conductor is good for 50A, which is more than the minimum circuit ampacity. The ambient temperature is more than 30°C, and, therefore, the allowable ampacity must be corrected. Because the raceway is exposed to direct sunlight above a roof, a temperature adjustment must be added to the ambient temperature. In Table 310.15(B)(3)(c), the temperature adder for a raceway located 4 inches above the roof is 17. With the temperature adder, the new ambient temperature is 57°C (40 + 17 = 57). The Table 310.15(B)(16) ampacity for an 8 AWG THWN-2 conductor in the 90°C column is 55A. The Table 310.15(B)(2)(a) correction factor, in the 90°C column for an ambient temperature of 57°C is 0.71. After applying the correction factor (often referred to as derating), an 8 AWG THWN-2 conductor has a maximum ampacity of only 39A (55 × 0.71 = 39). Since the minimum circuit ampacity is 43.2A, this 8 AWG THWN-2 conductor will not be permitted. Therefore, select the next larger size conductor, which is 6 AWG, and perform the calculation again. The Table 310.15(B)(16) ampacity for a 6 AWG THWN-2 conductor, in the 90°C column, is 75A. After applying the correction factor, a 6 AWG THWN-2 conductor has an ampacity of 53A (75 × 0.71 = 53.25 = 53). Because the minimum circuit ampacity is 43.2, the minimum size THWN-2 conductor required to supply this air conditioner is 6 AWG (see Figure 2).


If a certain type of insulated conductor is installed, it will not be necessary to include the temperature adder for raceways or cables exposed to sunlight on or above rooftops. In the 2014 NEC, a new exception was added to 310.15(B)(3)(c): Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment. For example, the same roof-top air conditioner from Figure 2 will be installed. Everything will be the same except the type of conductor. Instead of THWN-2, the branch-circuit conductors supplying power to this air conditioner will be XHHW-2. What minimum size conductor is now required to supply this air conditioner? As stated on the nameplate, the conductor must have an ampacity of at least 43.2A. Because the conductors will be XHHW-2, it is not necessary to include the temperature adder. The ambient temperature is 40°C; therefore, the allowable ampacity must be corrected. The Table 310.15(B)(16) ampacity for an 8 AWG XHHW-2 conductor in the 90°C column is 55A. The Table 310.15(B)(2)(a) correction factor, in the 90°C column for an ambient temperature of 40°C, is 0.91. After applying the correction factor, an 8 AWG XHHW-2 conductor has a ampacity of 50A (55 × 0.91 = 50). Because the minimum circuit ampacity is 43.2, the minimum size XHHW-2 conductor required to supply this air conditioner is 8 AWG (see Figure 3).


Next month, the discussion of sizing conductors continues.