Single-family dwellings and individual units in two- and multifamily dwellings can use 310.12(A) through (D) for sizing service and feeder conductors supplied by 120/240V single-phase systems. They can use 310.12(A) through (C) for a single-phase system derived from two ungrounded and the neutral conductor from a 208Y/120V system.

The text and table in 310.12 permits smaller-sized conductors to be used for dwelling services and feeders rated at 100A through 400A. While using smaller conductors for dwelling units may seem to be a simple concept and process, following all the requirements in 310.12 involves adhering to all of the procedures in the requirement and understanding its background.

First, some history

As a fairly young and very inquisitive electrical contractor who had just started my business, I did some research into the permitting of smaller conductors for dwellings that was inserted into the 1975 National Electrical Code. I wanted to understand why this change occurred.

With some research, I determined that the text was accepted in Proposal 42C in the 1968 NEC but was held until 1975 for further information. It was then submitted as a proposal from Code-Making Panel 6. C.H. Seaberg from General Electric Co., who was an active member of Panel 6 in 1968, submitted the proposal that year.

I was able to contact Ben Segall, a member of Panel 6 during the 1968, 1971 and 1975 NEC processes, to ask him about the background for this change. He explained that the information provided to the panel originated from a demand load study by electric utility companies from around the United States concerning dwellings with services from 100A up to 400A. 

The study indicated the demand for these dwelling services warranted slightly smaller conductors. In the 1975 NEC, the demand factor text was inserted as Note 3 for tables 310.16 through 310.18 and labeled “Three-Wire Single Phase Residential Services.” In the 1999 NEC, it was moved to 310.15(B)(6) and later to 310.15(B)(7). The text and table were moved to 310.12 for the 2020 NEC and it has remained there. 

Application information

History lesson over! Let’s apply 310.12 for dwelling services and feeders. The text in 310.12(A) states that “for a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, are permitted to have an ampacity that is not less than eighty-three (83) percent of the service rating. If no adjustment or correction factors are required, Table 310.12(A) can be applied.” 

However, if the service is located in a high-heat location, such as the west side of a dwelling in Phoenix, then Table 310.12 cannot be used. The calculation method uses the 83% value from 310.12(A) for the size of the service. Then make the ambient temperature corrections based on the multiplier in Table 310.15(B)(1)(1) for the conductors.

Down to the details

An example of this calculation is located in Informative Annex D at Example D7 in the 2023 NEC. For a 200A service with an ambient temperature correction factor at 100°F, the calculation would be 200A x 0.83 = 166A. Using XHHW-2 conductors permits the ambient temperature correction in the 90°C (194°F) column of Table 310.15(B)(1)(1) as a 0.91 correction factor. The answer from the 0.83 calculation of 166A divided by the 0.91 would equal 182.41A or a conductor that is sized for 182A from the 75°C (167°F) column of Table 310.16 [based on the 75°C terminals in 110.14(C)(1)(b)], requiring a 3/0 XHHW-2 be installed for the single-family dwelling. 

If there aren’t any corrections or adjustment factors, then Table 310.12(A) can be used with a 2/0 AWG conductor for the 200A service. Based on the heat issue, the 2/0 conductor from the table could not be used and temperature correction would require the 3/0 AWG.

For feeders, 310.12(B) applies and states that “for a feeder rated 100 amperes through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, is permitted to have an ampacity not less than 83 percent of the feeder rating. If no adjustment or correction factors are required, Table 310.12(A) is permitted to be applied.” 

The calculation for the feeders is the same as what was used for the service conductors. Grounded conductors are permitted to be smaller than the ungrounded conductors sized in accordance with 220.61. Remember, heat is an electrical system’s greatest enemy.

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