310.12 Single-Phase Dwelling Services and Feeders.
Code Change Summary: A new code section replaces the previous rules on single-phase dwelling service and feeder sizing.
In the 2017 NEC®, Section 310.15(B)(7) contained rules on calculating the conductor size for 120/240-volt, 3-wire, single-phase dwelling services and feeders. Back in the 2011 NEC®, a simple table existed showing the service size on the left and the minimum size conductor required to supply the service on the right.
To use Table 310.15(B)(7), one would first determine the minimum size service or feeder by using the load calculations in Article 220 and then select the minimum size conductor from Table 310.15(B)(7).
In the 2014 NEC®, the table was deleted and replaced with a simple 83% calculation which results in the same final value as if the table had just been allowed to remain.
Example: A 120/240-Volt, 3-Wire, single-phase electrical service for a single-family dwelling with a calculated load (from Article 220) of 98 amps, supplied by THWN copper overhead service entrance conductors.
- A 100 amp electrical service will be used to supply the 98 amp load for the single-family dwelling.
- 100 amps X 83% = 83 amps. This is the minimum required service conductor.
- Take 83 amps and use the 75°C column in Table 310.15(B)(16) to determine that the service requires 4 AWG copper, type THWN overhead service entrance conductors.
The table was brought back in 2017 but placed back in annex D for calculation D7.
In the 2020 NEC®, the previous language in 310.15(B)(7) was restructured and relocated to new Section 310.12 and the old table has been brought back from Annex D. The new note under the table makes it clear that the table can only be used if adjustment or correction factors are not required.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC® edition of NFPA 70.
2017 Code Language:
310.15(B)(7) Single-Phase Dwelling Services and Feeders. For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (4).
For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of 2 ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (3).
(1) For a service rated 100 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, shall be permitted to have an ampacity not less than 83 percent of the service rating.
(2) For a feeder rated 100 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, shall be permitted to have an ampacity not less than 83 percent of the feeder rating.
(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2).
2020 Code Language:
310.12 Single-Phase Dwelling Services and Feeders. For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.12(A) through (D).
For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of two ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permitted to be sized in accordance with 310.12(A) through (C).
N (A) Services. 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, shall be permitted to have an ampacity not less than 83 percent of the service rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.
N (B) Feeders. 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, shall be 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 shall be permitted to be applied.
N (C) Feeder Ampacities. In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B).