Question 6. We are writing to request clarification of Section 310-15(b)(6) [1999 NEC]. We are electrical contractors doing service work in 34 different jurisdictions on the central coast of California. The inspectors in one local jurisdiction have taken this section (per the clarification on page 153 of the IAEI Analysis of the 1999 National Electrical Code) to apply only to the main power feeders (between the main disconnect and the branch-circuit panel board) and not to the service-entrance conductors. We take the section to mean that for single-phase, 120/240-volt, 3-wire residential service, the service-entrance conductors, as well as the service-lateral conductors, as well as the feeder conductors can all be sized according to Table 310-15(b)(6).
By our interpretation, when we install or replace a typical 100-A, single-phase residential service, we will use 4 AWG copper service-entrance conductors. These local inspectors would have us use 3 AWG (which is not generally available, so we have to use 2 AWG). Which interpretation is correct? — V. K.
Answer 6. Section 310-15(b)(6) in NEC- 1999 was previously the text in note 3 to the ampacity tables of 0 to 2000 volts. If a single set of 3-wire, single-phase, service-entrance conductors in a raceway or cable supplies a one-family, two-family, or multifamily dwelling, the reduced conductor size permitted by Section 310-15(b)(6) is applicable to the service-entrance conductors, service-lateral conductors, or any feeder conductors that supply the main power feeder to a dwelling unit. This section permits the main power feeder to a dwelling unit to be sized based on the conductor sizes in table 310-15(b)(6) even if other loads, such as air conditioning units and swimming pools, are fed from the same. The feeder conductors to a dwelling are not required to be larger than its service-entrance conductors. An example of this would be if you installed the service panel outside and from that location fed an air-conditioning unit, then ran a feeder into the dwelling unit to a subpanel, that feeder would be sized from table 310-15(b)(6), the 4 AWG copper would be 100 amps, and 2 AWG aluminum would also be 100 amps.— O. P. Post, CMP-6
