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A new fine print note was added to the 1993 NEC in section 220-22 cautioning
against reducing the size of the neutral conductor when high harmonic currents
are present. This change correlates with the Underwriters Laboratories
Electrical Council meeting held in Itasca, Illinois in April, 1992. A paper
presented at this meeting discusses the third harmonic problem. An example is
cited where three 120 volt personal computers are connected to a three phase
208/120 volt system where one computer is connected to phase A, one to phase B,
and one to phase C. Each computer used 1.2 amperes of current, but the neutral
carried 2.1 amperes.
The standard rule "double the size of the neutral for computer loads"
is proven again. The problem is in the switching action of the computer power
supplies. New computer power supplies are being developed to overcome this
problem. Transformers also have a problem handling these third harmonic
currents. A "K-factor" rating is assigned to special transformers as
an index of the unit's ability to support a load current with a harmonic content
while remaining within its designed operating temperature limits.
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