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 2.3.1.3 Emergency overload criteria.
2.3.1.4 Voltage drop limitations.
2.3.1.5 Fault current criteria.
2.3.2 Voltage Rating. The selection of the cable insulation (voltage) rating is based on: the
phase-to-phase voltage of the system in which the cable is to be applied, the general system
category (depending on whether the system is grounded or ungrounded), and the time in which a
ground fault on the system is cleared by protective equipment. It is possible to operate cables on
ungrounded systems for long periods of time with one phase grounded due to a fault. This
results in line-to-line voltage stress across the insulation of the two ungrounded conductors.
Such cable, therefore, must have greater insulation thickness than a cable used on a grounded
system (where it is impossible to impose full line-to-line potential on the other two unfaulted
phases for an extended period of time). Consequently, 100 percent voltage rated cables are
applicable to grounded systems provided with protection which will clear ground faults within
one minute. 133 percent rated cables are required on ungrounded systems where the clearing time
of the 100 percent level category cannot be met, and when there is adequate assurance that the
faulted section will be cleared within one hour. 173 percent voltage level insulation is used on
systems where the time required to deenergize a grounded section is indefinite.
2.3.3 Load Current Criteria. The manufacturer's ampacity recommendations should be used
as load current criteria. The following publications contain ampacity tables for power cables.
(a) IEEE S-135-1-1962, Power Cable Ampacities, Copper Conductors.
(b) IEEE S-135-2-1962, Power Cable Ampacities, Aluminum Conductors.
Ampacity tables indicate the minimum size conductor required, however, conservative
engineering practice, future load growth considerations, voltage drop, and short circuit
considerations may require the use of larger conductors.
2.3.3.1 Skin and Proximity Effects. Careful consideration must be given when grouping
cables, as de-ratings resulting from mutual heating may limit capacity. Paralleling two or more
smaller size cables should be considered over installation of conductors (larger than 500 MCM)
because the current carrying capacity, per circular mil of the conductor, decreases for alternating
current circuits (due to skin effect and proximity effect). The reduced ratio of surface to
cross-sectional area of larger size conductors is a factor in the reduced ability of the larger cable
to dissipate heat. Cables larger than 500 MCM are also more difficult to handle during
installation. When cables are used in multiple sets, consideration must be given to the phase
placement of the cable to minimize the effect of reduced ampacity due to unbalanced distribution
of current in the cables. Length of multiple sets should be the same.
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