Who says you cannot parallel conductors used in that table?
The CMP has a different view. From the 2011 ROP:
6-86 Log #1732 NEC-P06
Final Action: Reject
(310.15(B)(6))
_______________________________________________________________
Submitter:
Charles E. Beck, Affiliated Engineers NW, Inc.
Recommendation:
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.
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.
Panel Meeting Action: Reject
Panel Statement:
The submitter is incorrect in his assumption that the
conductors in 310.15 (B)(6) are not permitted to be paralleled in accordance
with 310.4. The conductor ampacities listed in 310.15(B)(6) are based on the
diversity of the total load of an individual dwelling. This means that the
conductors of a 120/240-volt, single-phase dwelling service or feeder with a
calculated load of 200 amps will never carry 200 amps. Due to this fact, the
language and table in 310.15(B)(6) will permit the use of a 2/0 conductor,
which has an ampacity of 175 amps in the 75 degree C column. The same
theory applies to the conductors of a service or feeder with a calculated load of
400 amps. Section 310.15(B)(6) will permit the use of a 400 kcmil conductor
that would only have an ampacity of 335 amps in the 75 degree C column to be
used for a 400 amp service or feeder. When two 2/0 conductors are installed in
parallel for the same 400 amp installation the combined ampacity is 350 amps,
which is 15 amps more than that of a 400 kcmil conductor.
Number Eligible to Vote: 11
Ballot Results:
Affirmative: 11
