Article 210 in the National Electrical Code (NEC) covers branch circuits, except when they supply only motor loads, which Article 430 covers. When sizing conductors for branch circuits, it is necessary to comply with the provisions in Article 210. It may also be necessary to check for applicable provisions in other articles. As 210.2 states, branch circuits shall comply with Article 210 and with the applicable provisions of other articles of this Code. The first table in Article 210 is helpful in finding branch-circuit provisions in other parts of the Code. The provisions for branch circuits supplying equipment listed in Table 210.2—Specific-Purpose Branch Circuits—amend, or supplement, the provisions in Article 210. While the list in Table 210.2 is helpful in determining if it is necessary to look in other parts of the Code, it is not all-inclusive. Just because a certain specific-purpose branch circuit is not covered in Table 210.2 does not mean it is not covered. Unlike some equipment—such as appliances in Article 422, which is not listed—fixed electric space-heating equipment is listed in Table 210.2 and covered in Article 424. 


For the purpose of Article 424, heating equipment shall include heating cable, unit heaters, boilers, central systems, or other approved fixed electric space-heating equipment, but it shall not apply to process heating and room air conditioning. The section referenced in Table 210.2 that covers branch circuits is 424.3. In accordance with 424.3(B), fixed electric space-heating equipment and motors shall be considered continuous-load. As defined in Article 100, a continuous load is a load where the maximum current is expected to continue for three hours or more. Because 424.3(B) specifies that fixed electric space-heating equipment and motors shall be considered continuous-load, it is not necessary to determine if the maximum current will be expected to continue for three hours or more.


For example, a wall-mounted heater is to be installed. The nameplate on the heater shows 1,500 watts (W) at 120 volts (V). The branch-circuit conductors supplying power to this electric heater will be THHN copper, and the conductors will be installed in a raceway. 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. What is the minimum size conductor required to supply this heater and what minimum ampere rating is required for the overcurrent device, which will be a breaker?


First, calculate this heater’s current draw. Since watts and voltage are given, divide the nameplate watts by the nameplate volts to find the amperes (A). For the purpose of this article, assume heating watts equivalent to volt-amperes (VA). This heater has a current draw of 12.5A (1,500 ÷ 120 = 12.5). Be careful at this point, and don’t select a 15A branch circuit, because, as 424.3(B) states, fixed electric space-heating equipment shall be considered a continuous load. When sizing branch-circuit conductors, the minimum branch-circuit conductor size shall have an allowable ampacity not less than 125 percent of the continuous load [210.19(A)(1)]. When sizing overcurrent protection, the overcurrent device rating shall not be less than 125 percent of the continuous load [210.20(A)]. The branch-circuit rating for this heater shall be at least 16A (12.5 × 125% = 15.6 = 16). Therefore, the rating of the breaker must be at least 16A. In accordance with 240.6(A), this is not a standard rating. The next standard rating higher than 16A is 20A. The conductor must also have a rating of at least 16A. In accordance with the 75°C column of Table 310.15(B)(16), a 14 AWG conductor is good for 20A. However, a 14 AWG conductor is one of the conductors with a double asterisk, which references a footnote at the bottom of Table 310.15(B)(16) that states to see 240.4(D) for conductor overcurrent protection limitations. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed 15A for 14 AWG copper [240.4(D)(3)]. Because the rating of the breaker is 20A, the minimum size conductor is 12 AWG (see Figure 1).


In accordance with 424.3(A), individual branch circuits shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which they are rated. For example, a wall-mounted heater will be installed in a commercial occupancy. The nameplate on the heater shows 4,000W at 240V, single-phase. The branch-circuit conductors supplying power to this electric heater will be THHN copper, and the conductors will be installed in a raceway. 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. What is the minimum size conductor required to supply this wall-mounted heater, and what minimum ampere rating is required for the overcurrent device which will be a breaker?


This heater has a current draw of 16.67A (4,000 ÷ 240 = 16.67). After multiplying the load by 125 percent, the branch-circuit rating for this heater shall be at least 21A (16.67 × 125% = 20.8 = 21). Therefore, the breaker’s rating must be at least 21A. In accordance with 240.6(A), the next standard rating higher than 21A is 25A. The conductor must also have a rating of at least 21A. In accordance with the 75°C column of Table 310.15(B)(16), a 12 AWG conductor is good for 25A. However, a double asterisk is also next to 12 AWG. The overcurrent protection shall not exceed 20A for 12 AWG copper [240.4(D)(5)]. Because the rating of the breaker is 25A, the minimum size conductor is 10 AWG (see Figure 2). 


Since the minimum size conductor is 10 AWG, it would be permissible to install a breaker rated 30A, unless the instructions included in the listing or labeling of the heater specify that the maximum rating shall be 25A.


In accordance with 424.3(A), branch circuits supplying two or more outlets for fixed electric space-heating equipment shall be rated 15, 20, 25 or 30A. Unless the instructions included in the listing or labeling of the heater state to install the heater on a dedicated circuit, it shall be permissible to install more than one heater on a circuit.


For example, a number of wall-mounted heaters will be installed. Each heater will have a rating of 1,025W at 240V, single-phase. The branch-circuit overcurrent device will be rated 30A, and the branch-circuit conductors will be 10 AWG THHN copper conductors. How many of these wall-mounted heaters can be installed on one circuit? Each has a current draw of 4.27A (1,025 ÷ 240 = 4.27). When more than one heater is on one branch circuit, the calculation is not the same as when there is more than one motor on one branch circuit. Where several motors are on one branch circuit, the motor with the highest full-load current rating is multiplied by 125 percent, and all of the other motors in the group are added at 100 percent of the full-load current rating. Where more than one heater is on one branch circuit, the load of each heater shall be multiplied by 125 percent, not just the heater with the highest rating. After multiplying by 125 percent, the current needed for each heater is 5.34A (4.27 × 125% = 5.34). To find the maximum number of heaters that can be installed on a 30A circuit, divide 30 by 5.34. The maximum number of heaters in this example that can be installed on a 30A circuit is 5.6 (30 ÷ 5.34 = 5.6). Do not round up. Since 60 percent of a heater cannot be installed, the maximum number of heaters is five (see Figure 3). Note, the calculation shown for finding the maximum number of heaters is one of many different calculation methods. Other calculation methods can be used to provide the same result.


Next month’s column continues the discussion of sizing conductors.