Selecting the correct size conductor is not difficult, but it involves more than just selecting a conductor based on the ampacity shown in Table 310.15(B)(16) in the National Electrical Code (NEC). Many factors must be considered to select the correct size conductor. Some of these include 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. With certain loads, it is necessary to look at the article in which the load is referenced. For example, when selecting a branch circuit conductor for a fixed-storage-type water heater, look in Article 422, Appliances, which covers electrical appliances used in any occupancy. The specification pertaining to storage-type water heaters is in 422.13. A fixed storage-type water heater that has a capacity of 450 liters (120 gallons) or less shall be considered a continuous load for the purposes of sizing branch circuits. Although this section does not state conductors that supply these types of water heaters shall have an ampacity of not less than 125 percent of the load, that is what it means when it states these types of water heaters shall be considered a continuous load for the purposes of sizing branch circuits. Look in 210.19(A)(1)(a) to find this requirement.
Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load. Article 210 provides general requirements for branch circuits. Section 210.3 and Table 210.3 are helpful when looking for other articles for specific-purpose branch circuits. The reference in this table for motors, motor circuits and controllers is Article 430.
Part II in Article 430 specifies ampacities of conductors that are capable of carrying the motor current without overheating under the conditions specified. Conductors that supply a single motor must be sized in accordance with 430.22. When the branch circuit conductors will supply power to more than just a single motor, the conductors must be sized in accordance with 430.24. Conductors supplying several motors, or motor(s) and other load(s), shall have an ampacity not less than the sum of each of the following: 1) 125 percent of the full-load current (FLC) rating of the highest rated motor, as determined by 430.6(A); 2) sum of the FLC ratings of all the other motors in the group, as determined by 430.6(A); 3) 100 percent of the noncontinuous non-motor load, and 4) 125 percent of the continuous non-motor load.
For example, a calculation is needed to determine the minimum size THHN copper conductors that will be supplying power to three 208-volt (V), three-phase motors. One branch circuit will be used to supply power to all three of these motors. One three-phase motor is rated 2 horsepower (hp), and the nameplate shows this motor will draw 5.7 amperes (A) at 208V. One three-phase motor is rated 1.5 hp, and the nameplate shows this motor will draw 5.1A at 208V. One three-phase motor is rated 1 hp, and the nameplate shows this motor will draw 3.6A at 208V. All conductor terminations will be 75°C.
When sizing conductors in accordance with 430.24, the FLC rating shall be determined by 430.6(A). Section 430.6(A)(1) states not to use the actual current marked on the motor’s nameplate when determining the ampacity needed for sizing conductors unless the motor is built for speeds less than 1,200 RPM or high torques, or unless the motor is a multispeed motor. Instead of using the amperes on the motor’s nameplate, use the FLC values given in tables 430.247, 430.248, 430.249 and 430.250. Since the motors in this example are three-phase, use Table 430.250. Under induction-type, squirrel-cage, and wound-rotor motors, find the 208V column. Follow down the 208V column, and find the rows with 1-hp, 1.5-hp and 2-hp motors. As shown in Table 430.250, the FLC for a 1-hp motor is 4.6A. The FLC for a 1.5-hp motor is 6.6A, and the FLC for a 2-hp motor is 7.5A (see Figure 1).
The next step after finding the FLC of each motor is to multiply the FLC rating of the highest rated motor by 125 percent. Since all three motors are three-phase motors, the highest rated motor has an FLC rating of 7.5A. After multiplying by 125 percent, this motor has a rating of 9.4A (7.5 × 125% = 9.375 = 9.4). In accordance with 430.24(2), add the FLC ratings of all the other motors in the group, as determined by 430.6(A). The minimum ampacity for sizing the conductors supplying power to these motor is 21A (9.4 + 6.6 + 4.6 = 20.6 = 21). Finally, select a copper conductor from Table 310.15(B)(16).
Size 14 THHN copper conductors have an allowable ampacity of 25A as shown in the 90°C column in Table 310.15(B)(16), but the conductor’s ampacity shall not be selected from the 90°C column. Because of the provision in 110.14(C), the conductor’s ampacity shall not exceed the temperature rating of the terminations. Since the terminations in this example will be 75°C, the conductor’s ampacity shall not exceed the rating in the 75°C column of Table 310.15(B)(16). A 14 AWG conductor has an allowable ampacity of only 20A as shown in the 75°C column. The minimum size THHN copper conductors supplying power to these three motors is 12 AWG (see Figure 2).
There are three exceptions under 430.24. The first exception pertains to motors used for short-time, intermittent, periodic or varying duty. The second exception pertains to motor-operated fixed electric space-heating equipment. The third exception under 430.24 pertains to motors or other loads on the same circuit that cannot be energized at the same time. Where the circuitry is interlocked so as to prevent simultaneous operation of selected motors or other loads, the conductor ampacity shall be permitted to be based on the summation of the currents of the motors and other loads to be operated simultaneously that results in the highest total current [240.24 Exception No. 3].
In Article 220, this type of load is called a noncoincident load. Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service [220.60].
If two motors are on the same branch circuit but both motors cannot operate at the same time, it is permissible to size the branch circuit conductors as if there is only one motor. For example, what minimum size THHN copper conductors are required to supply power to two 5-hp, 230V, single-phase pump motors? Both of these motors will be used in a continuous-duty application. All conductor terminations will be 75°C. The motors will be wired so only one motor is in operation at any time. The lead pump will change every time the system is stopped. Although there are two motors, they will never operate at the same time. Because the circuitry is interlocked so as to prevent simultaneous operation of these two motors, the conductor ampacity can be sized as if there is only one motor. In accordance with Table 430.248, the FLC for a 5-hp, 230V, single-phase motor is 28A. Because there is only one motor, the conductor can be sized in accordance with 430.22.
The next step is to multiply the FLC rating of the highest rated motor by 125 percent. Since both motors are the same, the highest rated motor has an FLC rating of 28A. After multiplying by 125 percent, this motor has a rating of 35A (28 × 125% = 35). Now select a copper conductor from Table 310.15(B)(16). Since the terminations will be 75°C, the conductor’s ampacity shall not exceed the rating in the 75°C column of Table 310.15(B)(16). The minimum size THHN copper conductors supplying power to the motors in this example is 10 AWG (see Figure 3).
Next month’s column continues the discussion of requirements for motors, motor circuits and controllers.