This is just FYI, but is certainly open to discussion. I have already submitted this to the NFPA.
Recommended change: (Insert after the second sentence, which ends with, ?. . . associated with the dwelling unit.?) Use of paralleled conductors as a substitute for the conductor sizes shown in this table, or to supply larger service or feeder loads than those shown in this table, shall not be permitted. The feeder conductors to a dwelling unit shall not be required . . .
Substantiation: This change would clearly declare that Table 310.15(B)(6) cannot be adapted to suit the user?s design preferences.
Article 310.4 tells us that paralleling conductors is allowable, under certain conditions. But it does not tell us what changes take place, when we do so. It is commonly presumed that two parallel conductors (in separate raceways) will have twice the ampacity of either conductor alone. But Table 310.15(B)(6) is not about ampacity. The values for amperes shown in the left-hand column are not ampacity values, but rather are load values.
Table 310.15(B)(6) allows for the use of conductors in situations that exceed their ampacity values as given in Table 310.16. For example, a 2/0 copper conductor with 75C insulation has an ampacity of 175 amps, yet it can be used for a 200 amp service. That represents a 25 amp difference between the conductor?s ampacity and the load it will carry. That difference becomes 50 amps, when you use two in parallel. Absent any proof that the additional burden will not harm the conductor, and particularly noting that the user has no knowledge of the design basis that lies behind this Table, the user should not be allowed to presume that a pair of 2/0 conductors will be sufficient for a service of 400 amps.