2.9.3
**************************************************************************
NOTE: The 8-wire system should be utilized for
applications serving mixed loads including
electronic data processing equipment. Since EDP
equipment generates high levels of harmonics (* see
footnote below), a full size neutral should be
provided for each EDP circuit. Alternately, it is
recommended that the phase conductor not be loaded
to more than 12A or that an oversized neutral be
specified. To minimize interference from electronic
noise to sensitive data processing components, the
EDP equipment should be placed on the dedicated
circuits.
In the absence of other criteria, use of an isolated
ground conductor is not recommended for the EDP
circuits (See IEEE Std. 1100). If the amount of EDP
load is extensive and the conventional load is
minimal, a modified 8-wire system should be
provided. The preferred configuration would be 3
phases, 3 neutrals, an EDP ground, and a
conventional ground. Non-EDP load should be
connected to one phase, one neutral, and the
conventional ground. The other two phases and
neutrals and EDP ground should be dedicated for EDP
type loads.
As a second choice the 8 wires could be designated
as follows: 3 dedicated phase, 1 dedicated oversize
neutral (#10 with 14 amp maximum phase loading), and
isolated ground, a conventional phase, neutral, and
ground conductor. The non-EDP load should be placed
on the conventional conductors. (An 8-wire
configuration with 3 phases, 3 neutrals, an isolated
ground, and a conventional ground could also be
used. Non-EDP load should be connected to the
conventional ground and least loaded phase
conductor.)
The 5-wire system may be used if no EDP loads are to
be supplied. The 6-wire system is a less reliable,
hybrid configuration in which EDP and non-EDP loads
use a shared neutral. It could also be used for Air
Force shared ground applications with the isolated
ground connector either disconnected or
interconnected with the equipment ground.
Non-linear loads such as computers, copiers, laser
printers, electronic lighting ballasts, and
uninterruptible power supplies cause harmonic
distortion on power distribution systems. The
majority of workstation loads are non-linear,
harmonic producing loads. Designers must ensure
that the building power distribution equipment can
support these non-linear loads. IEEE Std 519 and
1100 provide details concerning the causes, effects,
and means of compensation for non-linear, harmonic
SECTION 12705
Page 21
Previous Page
