With a lot of work from many dedicated individuals, the 2014 edition of the National Electrical Code (NEC) became available at the end of August 2013. The Code is revised every three years, but the revision cycle has not always been three years. Revision cycles have ranged from one to four years.
The NEC has been in existence for almost 117 years. The first edition of the Code book was published in 1897. The National Fire Protection Association (NFPA) has sponsored the NEC since 1911. The 2014 edition is the 53rd edition of the NEC. Look on the first page for a complete list of all 53 editions. A lot has changed since the first edition.
There were more than 3,500 proposals submitted to revise the 2011 edition to the 2014 edition. The number of proposals is actually down from the last three editions, which averaged more than 4,500 proposals for each of those. In this new edition, there are global changes, new articles, new sections, relocated sections and revisions to existing sections.
Changing the voltage threshold from 600 to 1,000 volts (V) in many locations throughout the Code was one of the global changes. This change was because of voltage levels used in wind generation and photovoltaic systems.
Four new articles were added: Article 393 Low-Voltage Suspended Ceiling Power Distribution Systems, Article 646 Modular Data Centers, Article 728 Fire-Resistive Cable Systems, and Article 750 Energy Management Systems.
Section 110.21(B) is a new section that has been added. It contains provisions for field-applied hazard markings, such as caution, warning, or danger signs or labels.
An example of a relocated section is the definition of an effective ground-fault current path. In the 2011 edition, this term was defined in 250.2. This definition was moved to Article 100 because—in accordance with the scope of Article 100 and with the NEC Style Manual—only those terms that are used in two or more articles are defined in Article 100. Besides being used in Article 250, the term “effective ground-fault current path” is also used in 404.9(B) and 517.13(A). Expanding the requirements for ground-fault circuit-interrupter (GFCI) protection for personnel and arc-fault circuit-interrupter protection (AFCI) are examples of revisions to existing sections. See 210.8 for the expanded GFCI requirements and 210.12 for the expanded AFCI requirements.
There is a significant change that pertains to sizing branch-circuit, feeder and service conductors. This change is more of a clarification. The clarification is in 210.19(A)(1) for branch-circuit conductors, 215.2(A)(1) for feeder conductors and 230.42(A) for service conductors. There was no change to the first sentence in 210.19(A)(1), which states branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Regardless of anything else, the ampacity of the conductors shall not be less than the load.
Before the 2014 NEC, the second sentence in this section was often misinterpreted. It stated: “where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.” This appeared to be saying to multiply continuous loads by 125 percent before, or in addition to, the application of any adjustment or correction factors. The revised wording in the 2014 edition clearly states that these are two separate calculations (see Figure 1).
In accordance with the second sentence of 210.19(A)(1), conductors shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b). These sections, (a) and (b), contain two calculation procedures that are to be performed separately. The larger of the two sizes calculated is, therefore, the minimum size conductor.
For example, what size THHN copper conductors are required to supply a branch circuit under the following conditions? The load will be a 39 amperes (A), nonmotor, continuous load. These branch-circuit conductors will be in a raceway. There will be a total of eight current-carrying conductors and an equipment grounding conductor in this raceway. All the terminations in this branch circuit are rated 75°C. The maximum ambient temperature will be 40°C.
In accordance with 210.19(A)(1)(a), the minimum size conductors for a branch circuit that supplies continuous loads or any combination of continuous and noncontinuous loads shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load. Since this entire load is continuous, multiply the entire load by 125 percent. The minimum ampacity after multiplying by 125 percent is 49A (39 × 125% = 48.75 = 49). Although the conductors are rated 90°C, the allowable ampacity shall not exceed the 75°C column because of the terminations [see 110.14(C)(1)(a)]. An 8 AWG copper conductor, in the 75°C column of Table 310.15(B)(16), has an allowable ampacity of 50A. Based only on the temperature ratings of the terminations and on the load being a continuous load, the minimum size conductors are 8 AWG copper conductors (see Figure 2).
After performing the first of two calculation procedures in 210.19(A)(1), an 8 AWG conductor is required for the example in Figure 2. The second calculation procedure is used when there are more than three current-carrying conductors and/or when the ambient temperature is something other than 30°C. In the example, the ambient temperature will be higher than 30°C, and there will be more than three current-carrying conductors in the raceway. In accordance with 210.19(A)(1)(b), the minimum branch-circuit conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors. Use the exact load with this calculation, even if there are continuous loads.
There is more than one way to perform this calculation. One way is to divide the actual load of 39A by the correction and adjustment factors and then select a conductor. But, since there is already a minimum size that has been selected to satisfy the requirements for continuous loads and for the terminations, check to see if the ampacity of those conductors will equal or exceed the load after applying correction and adjustment factors. The Table 310.15(B)(16) ampacity for an 8 AWG THHN conductor, in the 90°C column, is 55A.
A good question usually comes up at this point. Since the terminations are only rated 75°C, why was the ampacity of a 90°C conductor selected? In accordance with the last sentence of 110.14(C), conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both. Although the terminations limit the ampacity to the 75°C column, it is permissible to use the ampacity in the 90°C column for correction and adjustment. Be careful because—while it is permissible to start with the ampacity in the 90°C column—it is not permissible to exceed the temperature rating, which, in this example, is the 75°C column. The ambient temperature in this example will be 40°C. The Table 310.15(B)(2)(a) correction factor, in the 90°C column, for an ambient temperature of 40°C is 0.91. The Table 310.15(B)(3)(a) adjustment factor for eight current-carrying conductors in the raceway is 70 percent (or 0.70). After applying the correction and adjustment factors (which is often referred to as derating), 8 AWG THHN conductors have a maximum ampacity of only 35A (55 × 0.91 × 0.70 = 35). Since the load is 39A, this 8 AWG THHN conductor will not be permitted because the ampacity is only 35A after derating. Therefore, select the next larger size conductor, and perform the calculation again. The next larger size conductor is 6 AWG THHN. The Table 310.15(B)(16) ampacity for a 6 AWG THHN conductor, in the 90°C column, is 75A. After applying the correction and adjustment factors, 6 AWG THHN conductors have an ampacity of 48A (75 × 0.91 × 0.70 = 47.775 = 48). Although the continuous load is 49A, because of 210.19(A)(1)(b), the conductors are only required to have a rating of the actual load of 39A (see Figure 3).
There is a similar change for sizing feeder conductors and service conductors. See 215.2(A)(1) for feeder conductors and 230.42(A) for service conductors.
Next month’s column continues the discussion of sizing conductors.
THHN (90°C) conductor
Motor marked with a design Letter D
THWN conductor (larger than 1 AWG)
1/0 AWG THHN conductor
Do not exceed the 75°C for this conductor.
THHN (90°C) conductor
Motor marked with a design Letter D