UFC 3 -520-01
June 10, 2002
single phase load becomes dominated by nonlinear loads. Notice that the neutral
current can be as high as 1.73 times the phase current.
Figure 12-2. Effect of Single Phase Nonlinear Loads on Neutral Currents
2.0
1.6
1.2
0.8
0.4
0.0
0%
20%
40%
60%
80%
100%
Percent Nonlinear Loads
12-7.2
Installation Design Criteria .
12-7.2.1 Minimize neutral circuit overheating by specifying separate neutral conductors
for line-to-neutral connected nonlinear loads with high harmonic content. When a
shared neutral conductor must be used for three-phase, four -wire systems, size the
neutral conductor to have an ampacity equal to at least 1.73 times the ampacity of the
phase conductors. Some cable manufacturers provide cable with oversized or extra
neutral conductors included to meet this design requirement.
12-7.2.2 Two paralleled, full size neutral conductors can be used to obtain the required
neutral ampacity for cond uctors sized #1/0 AWG and larger. Size the neutral conductor
between the transformer and the panelboard to be a minimum of 1.73 times the
ampacity of the phase conductors. Select panelboards that have been rated for
nonlinear loads. Circuits for nonline ar loads approaching 100 percent should be
provided a neutral ampacity of a minimum of 1.73 times the phase ampacity.
12-7.2.3 If necessary, install third harmonic filters at specific analyzed locations in the
electrical distribution system. Ensure that the selected locations provide the best
benefit in terms of neutral current reduction.
12-7.2.4 Although other power quality designs could be implemented to reduce the
electrical system neutral currents, the above approach has been selected to ensure that
neutral currents do not exceed wiring system limits. This method also ensures greater
system flexibility in the future.
12-10
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