Because the neutral conductor carries
less current than the phase conductors, utilities can use smaller
neutral conductors. On three-phase circuits with balanced loading,
the neutral carries almost no current.
On single-phase circuits with a
multigrounded neutral, the neutral normally carries 40 to 60% of the
current (the earth carries the remainder).
On single-phase circuits, some
utilities use fully rated neutrals, where the neutral and the phase
are the same size. Some use reduced neutrals.
The resistance of the neutral should be
no more than twice the resistance of the phase conductor, and we are
safer with a resistance less than 1.5 times the phase conductor,
which is a conductivity or cross-sectional area of 2/3 the phase
conductor.
Common practice is to drop one to three
gage sizes for the neutral: a 4/0 phase has a 2/0 neutral, or a 1/0
phase has a number 2 neutral. Dropping three gage sizes doubles the
resistance, so we do not want to go any smaller than that.
On three-phase circuits, most utilities
use reduced neutrals, dropping the area to about 25 to 70% of the
phase conductor (and multiplying the resistance by 1.4 to 4).
Several other factors besides ampacity
play a role in how small neutral conductors are:
• Grounding — A reduced neutral
increases the overvoltages on the unfaulted phases during single
line-to-ground faults. It also increases stray voltages.
• Faults — A reduced neutral
reduces the fault current for single line to- ground faults, which
makes it more difficult to detect faults at far distances. Also, the
reduced neutral is subjected to the same fault current as the phase,
so impacts on burning down the neutral should be considered for
smaller neutrals.
• Secondary — If the primary and
secondary neutral are shared, the neutral must handle the primary and
secondary unbalanced current (and have the mechanical strength to
hold up the secondary phase conductors in triplex or quadraplex
construction).
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