Code Change Summary: Changes were made to allow the feeder and service conductors for specific dwelling type occupancies fed from a 208Y/120-volt system to be calculated in accordance with 310.15(B)(7).

For quite some time, Section 310.15(B)(7) provided a bit of additional relief when determining the minimum feeder or service conductor size needed to supply one-family dwellings and the individual dwelling units of two-family and multifamily dwellings supplied by 120/240-Volt, single-phase electrical systems.

After determining the minimum size service or feeder required to supply the dwelling (using the load calculations in Article 220), Section 310.15(B)(7) allowed that value to be multiplied by 83%.

Example: A 120/240-Volt, 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 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 100-amp service requires 4 AWG copper, type THWN overhead service entrance conductors.

The idea behind this permitted reduction of the feeder or service conductor size is based on the principle of load diversity. The more loads connected to the service means the less likely it is that all loads will be in operation at the same time.

In the 2017 NEC^®, everything above is still true, but now, 208Y/120-volt electrical systems are also included. This will mainly benefit large apartment complexes and multifamily dwellings that are often supplied from the 208Y/120-volt secondary side of a three-phase utility transformer. For these types of installations, single-phase 208Y/120-volt feeder conductors are run from a large bank of meters at the three-phase service to each individual dwelling unit or apartment.

A careful read of the code change reveals that the allowance in subsection 310.15(B)(7)(4), to reduce the grounded neutral conductor size, cannot be used for a single-phase feeder to a dwelling unit supplied by a 208Y/120-volt system. Similar language is found in Section 220.61(C)(1). This is because the grounded neutral conductor in a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected 208Y/120-volt system is considered a current carrying conductor per 310.15(B)(5)(b).

Below is a preview of Article 310. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.