Q. When can I use Table 310.15(B)(6)?
A. For individual dwelling units of one-family, two-family, and multifamily dwellings, Table 310.15(B)(6) can be used to size 3-wire, single-phase, 120/240V service or feeder conductors that supply all loads that are part of or associated with the dwelling unit.
The 120/208V, single-phase feeders from a 120/208V, 3-phase service are sometimes used to supply single-family dwelling units in a multifamily building, and this table can't be used when that is done. Table 310.16 must be used when that is the case.
Feeder conductors are not required to have an ampacity rating more than the service conductors [215.2(A)(3)].
Warning: Table 310.15(B)(6) doesn't apply to 3-wire, single-phase, 120/208V systems, because the neutral conductor in these systems carries neutral current even when the load on the phases is balanced [310.15(B)(4)(6)]. For more information on this topic, see 220.61(C)(1).