A recent change in the 2014 National Electrical Code (NEC) creates a new method for determining the size of service and feeder conductors for 120/240-volt (V), single-phase services for one-family, individual units of two-family dwellings, and individual units for multifamily dwellings. Rather than text and a table, as used in 2011 and previous editions of the NEC, this new method uses text and calculations for determining sizes of feeder and service conductors. The wording and method from the new NEC, as compared to the previous editions, requires careful review and understanding to ensure proper use of this new method.


As mentioned, the 2011 NEC and previous editions used text and a table in 310.15(B)(7) to permit conductors to be sized smaller than would normally be permitted but only for 120/240V, single-phase services and feeders for dwelling units. To use the table, a feeder and/or service calculation was required to determine the load and the required size of the service or feeder. 


The first vertical column of Table 310.15(B)(7) had service and feeder ratings from 100 amperes (A) through 400A. The second and third vertical columns (the second for copper conductors and the third column for aluminum or copper-clad aluminum) provided the conductor sizes in either American wire gauge (AWG) or kcmil, depending upon the size of the conductors. The table permitted 4 AWG copper conductor or 2 AWG aluminum or copper-clad aluminum conductor to be used for a 100A service or feeder without any ambient-temperature correction factor requirements, whereas Table 310.15(B)(16) would require a 3 AWG copper conductor or a 1 AWG aluminum or copper-clad aluminum conductor plus an ambient temperature correction factor to be applied wherever the ambient temperature exceeded the table value of 30°C or 86°F. 


As another example, a 200A service would only require a 2/0 AWG copper conductor without any ambient temperature correction, rather than a 3/0 AWG copper conductor, plus any ambient-
temperature correction, which might result in a larger conductor due to higher ambient temperatures. 


The service and feeder ratings, and the conductor sizes provided in Table 310.15(B)(7) for dwelling units, were based on diversity values submitted for the 1956 NEC by utility companies and by calculation values submitted by the American Institute of Electrical Engineers, the precursor of the Institute for Electrical and Electronics Engineers.


The 2014 NEC requirement uses text and calculations instead of a table. The new text states that in one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a 120/240V, single-phase system are permitted to be sized in accordance with four separate subsections in 310.15(B)(7)(1) through (4). The first subsection, 310.15(B)(7)(1), deals with services rated at 100A through 400A where the service conductors supply the entire load associated with the one-family dwelling or where the service conductors supply the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling. In this application, the service conductors are permitted to have an ampacity of not less than 83 percent of the service rating. The second subsection, 310.15(B)(7)(2), contains the same basic text as (1) but applies to feeder panelboards or switchboards, rated at 100A to 400A, not to the service equipment and service conductors as covered above. Again, as in (1), the feeder conductors are permitted to have an ampacity of 83 percent of the feeder rating.


Neither 310.15(B)(7)(1) nor (2) specifically require the use of the ambient temperature correction factor; however, Informational Note No. 1 in the 2014 NEC states, “the conductor ampacity may require other correction factors or adjustment factors applicable to the conductor installation.” Ambient temperature correction factor is in Table 310.15(B)(2)(a), and the number of current-
carrying conductors in a raceway or cable must be in accordance with Table 310.15(B)(3)(a).


Informational Note No. 2 in 310.15(B)(7) refers to Annex D.7 for an example of a sample calculation. The example states, “Service conductors and feeders for certain dwellings are permitted to be sized in accordance with 310.15(B)(7). If a 175 ampere service rating is selected, the service conductor is then sized at 175-amperes × 0.83 = 145.25-amperes per 310.15(B)(7). If no other adjustments or corrections are required for the installation, then, in accordance with Table 310.15(B)(16), a 1/0 AWG Copper conductor or a 3/0 AWG Aluminum conductor meets this rating, using the 75°C (167°F) column in Table 310.15(B)(16).”


The 2014 NEC requirements in 310.15(B)(7) provide proper sizing of dwelling unit service and feeder conductors where higher ambient temperatures are encountered. Where necessary, using the ambient-temperature correction factor tables for service and feeder conductors for dwelling units should help ensure that these service and feeder conductors subjected to higher temperatures do not have insulation-deterioration issues.