Selecting the correct size conductor is not difficult, but it involves more than selecting a conductor based solely on the ampacity shown in Table 310.15(B)(16) in the National Electrical Code (NEC). Many factors must be considered, including temperature ratings of terminations, conductors and devices; the actual connected load or the load calculated in accordance with Article 220; continuous and noncontinuous loads; ambient temperature; and the number of current-carrying conductors.
Another factor is the load itself. For example, will the conductors supply power to a panelboard, a disconnect (safety switch), lighting, receptacles, one or more motors, heating and air conditioning equipment, an appliance, etc.? Besides referencing general requirements in Article 310 (Conductors for General Wiring), Article 110 (Requirements for Electrical Installations), and Article 240 (Overcurrent Protection), it may be necessary to reference requirements in other articles that pertain to the load itself. For example, say a dishwasher is to be installed in a dwelling unit. The nameplate on the dishwasher shows the current draw will be 10.9 amperes (A) at 120 volts (V). The nameplate does not show a horsepower rating for the motor, but it shows the current draw. The motor will draw 1.8A, and the other loads of the dishwasher will be 9.1A. The branch-circuit conductors supplying power to this appliance will be nonmetallic-sheathed cable. What is the minimum size conductor required to supply this dishwasher?
Article 422 is a good place to start because it covers electrical appliances used in any occupancy. Section 422.10 specifies the ratings of branch circuits capable of carrying appliance current without overheating under the conditions specified. In accordance with 422.10(A), the rating of an individual branch circuit shall not be less than the marked rating of the appliance or the marked rating of an appliance having combined loads as provided in 422.62, which pertains to required markings on appliance nameplates. Since the branch-circuit rating shall not be less than the appliance’s marked rating, the conductor supplying power to this dishwasher shall have an ampacity of at least 10.9A. Since the conductor will be nonmetallic-sheathed cable, it is important to look in Article 334 for the appropriate requirements. Although the conductor insulation for nonmetallic-sheathed cable is rated at 90°C (194°F), the ampacity shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor (334.80 and 334.112). In accordance with the 60°C column of Table 310.15(B)(16), a 14 AWG conductor is good for 15A. In accordance with 240.4(D)(3), the overcurrent protection for a 14 AWG copper conductor shall not exceed 15A. The minimum size conductor required to supply this dishwasher is 14 AWG (see Figure 1).
Although the minimum size conductor required for this dishwasher is 14 AWG, many electricians would install a 12 AWG conductor and protect it with a 20A overcurrent device.
While 422.10(A) covers appliances on individual branch circuits, 422.10(B) covers circuits supplying two or more loads. For branch circuits supplying appliance and other loads, the rating shall be determined in accordance with 210.23. The first sentence states that in no case shall the load exceed the branch-circuit ampere rating. Although it may be worded differently in other locations, this same provision is located in sections throughout the Code. For example, branch-circuit conductors shall have an ampacity not less than the maximum load to be served [210.19(A)(1)]. The second sentence in 210.23 states a branch circuit supplying two or more outlets or receptacles shall supply only the loads specified according to its size as specified in 210.23(A) through (D) and as summarized in 210.24 and Table 210.24. The first provision in this section pertains to 15- and 20A branch circuits. As 210.23(A) states, a 15- or 20A branch circuit shall be permitted to supply lighting units or other utilization equipment, or a combination of both, and shall comply with 210.23(A)(1) and (A)(2).
The first section in 210.23(A) pertains to cord-and-plug-connected utilization equipment that is not fastened in place. The second section in 210.23(A) pertains to fastened-in-place utilization equipment. As stated in 210.23(A)(1), the total rating of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, also are supplied. Note that 210.23(A)(2) does not specifically pertain to cord-and-plug-connected utilization equipment like 210.23(A)(1) but to utilization equipment fastened in place. If a fastened-in-place appliance is on the same circuit as lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, the ampacity of the appliance shall not exceed 50 percent of the branch-circuit ampere rating. Because the dishwasher in Figure 1 has an ampere rating that is more than 50 percent of the 20A branch-circuit, it would be a violation to install lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both on the same circuit. Even with dishwashers that have an ampere rating of 10A or less, be careful about installing anything on the same circuit. Installation instructions included with some dishwashers say to install the dishwasher on an individual or dedicated circuit. If so, the dishwasher must be installed on an individual branch circuit. In accordance with 110.3(B), listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
Some changes in the 2014 NEC are noteworthy when discussing certain appliances, especially dishwashers in dwellings. Before the 2014 NEC, installing ground-fault circuit interrupter (GFCI) protection for the dishwasher was not required, but now it is. In accordance with 210.8(D), GFCI protection shall be provided for outlets that supply dishwashers installed in dwelling-unit locations. The term “outlet” in this section is not just a receptacle outlet. As defined in Article 100, an outlet is a point on the wiring system at which current is taken to supply utilization equipment. Therefore, this new GFCI requirement pertains to hard-wired as well as cord-and-plug-connected dishwashers installed in dwellings. While this is not a new requirement, the GFCI shall be installed in a readily accessible location (210.8).
A significant change for dwelling units pertains to arc-fault circuit interrupter (AFCI) protection. In addition to the rest of the rooms and areas listed in 210.12(A), AFCI protection is now required for all 120V, single-phase, 15- and 20A branch circuits supplying outlets or devices installed in dwelling unit kitchens and laundry areas. This is significant because a dwelling-unit kitchen usually has a number of 15- and 20A branch circuits. Therefore, a dishwasher is now required to be GFCI-and AFCI-protected (see Figure 2).
It is also necessary to reference several articles when sizing a garbage disposer. For example, a ¾-horsepower (hp), in-sink waste disposer will be installed on an individual branch circuit in a dwelling unit. (In the 2014 NEC, the term “kitchen waste disposer” changed to “in-sink waste disposer.”) The nameplate on the disposer shows the current draw will be 8.1A at 120V but does not show a minimum branch-circuit rating. The branch-circuit conductors supplying power to this appliance will be nonmetallic-sheathed cable. What is the minimum size conductor required to supply this in-sink waste disposer?
In accordance with 422.10(A), the rating of an individual branch circuit for motor-operated appliances not having a marked rating shall be in accordance with Part II of Article 430. Not having a marked rating means not having a minimum size branch circuit on the nameplate such as 15 or 20A. In Part II of Article 430, 430.22 states that conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor full-load current (FLC) rating, as determined by 430.6(A)(1) or not less than specified in 430.22(A) through (G). Section 430.6(A)(1) states to use the FLCs values in Table 430.247 through 430.250 when determining the ampacity of conductors. The Table 430.248 FLC for a ¾-hp motor at 115V is 13.8A. Therefore, instead of using the nameplate rating of 8.1A, use 13.8A to determine the ampacity of conductors. As previously mentioned, the ampacity of the conductors shall not be less than 125 percent of the motor FLC rating. Therefore, the conductors must have an ampacity of at least 17A (13.8 × 125% = 17.25 = 17). In accordance with the 60°C column of Table 310.15(B)(16), a 12 AWG conductor is good for 20A. The minimum size conductor required to supply this electrically operated in-sink waste disposer is 12 AWG (see Figure 3).
Next month’s column continues the discussion of sizing conductors.