310.15(B)(7) 120/240-Volt, Single-Phase Dwelling Services and Feeders.

Table 310.15(B)(7) has been deleted and replaced with a simple 83% factor.
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Table 310.15(B)(7) has been deleted and replaced with a simple 83% factor.

Code Change Summary: Table 310.15(B)(7) has been deleted and replaced with an 83% calculation.

Section 310.15(B)(7) contains rules on calculating the conductor size for 120/240-volt, 3-wire, single-phase dwelling services and feeders. 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 the table, 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 has been deleted and replaced with a simple 83% 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 close look at the old deleted table shows the same results.

Below is a preview of the NEC.  See the actual NEC text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2011 Code language:

310.15(B)(7) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one-family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(7), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit. The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

2014 Code Language:

310.15(B)(7) 120/240-Volt, 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 be sized in accordance with 310.15(B)(7)(1) through (4).

(1) For a service rated 100 through 400 A, 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 A, 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.

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310.15(B)(7) 120/240-Volt, Single-Phase Dwelling Services and Feeders.

Below is a Real Question from our Electrical Continuing Education Courses for Electrical License Renewal:

What is the minimum required ampacity of the overhead service entrance conductors supplying a 120/240-Volt, 3-Wire, single-phase, 150 amp electrical service?

A: 83 amps.
B: 150 amps.
C: 124.5 amps.
D: 110 amps.
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