Custom Search
|
|
|
||
 2.3.3.2 Ambient Temperature. Cables must be de-rated when in proximity to other loaded
cables or heat sources, or when the ambient temperature exceeds the ambient temperature at
which the ampacity (current carrying capacity) tables are based. The normal ambient
temperature of a cable installation is the temperature of the environment in which the cable is
installed with no load being carried on the cable.
2.3.3.3 Surrounding Medium. The thermal characteristics of the medium surrounding the
cables are of primary importance in determining the current carrying capacity of the cables. The
type of soil in which the cable or duct bank is buried has a major effect on the current carrying
capacity of the cables. Porous soils, such as gravel and cinder fill, usually result in higher
temperatures and lower ampacities than sandy or clay soil. The moisture content of the soil has a
major effect on the current carrying capacity of cables. In dry sections of the country, cables may
have to be de-rated, or other precautions taken, to compensate for the increase in thermal
resistance due to the lack of moisture. On the other hand, in ground which is continuously wet or
under tidewater conditions, cables may carry higher than normal currents.
2.3.4 EmergencyOverload Criteria. Normal loading limits of insulated wire and cable are
determined based on many years of practical experience. These limits account for a rate of
insulation deterioration that results in the most economical and useful life of such cable systems.
The anticipated rate of deterioration equates to a useful life of approximately 20 to 30 years. The
life of cable insulation may be halved, and the average thermal failure rate almost doubled for
each 5 to 15C increase in normal daily load temperature. The normal daily load temperature is
the average conductor temperature over a typical 24 hour period. It reflects both the change in
ambient temperature and the change in conductor temperature due to daily load fluctuations.
Additionally, sustained operation over and above maximum rated operating temperatures or
ampacities is not an effective or economical practice, because the temperature rise is directly
proportional to the conductor loss, which increases as the square of the current. The intensified
voltage drop may also increase the risks to equipment and service continuity. Maximum
emergency overload temperatures for various types of insulation have been established and are
available as a practical guide. Operation at these emergency overload temperatures should not
exceed 100 hours per year, and such 100 hour overload periods should not exceed five during the
life of the cable.
2.3.5 Voltage Drop Criteria. The supply conductor, if not of sufficient size, will cause
excessive voltage drop in the circuit, and the drop will be in direct proportion to the circuit
length. Proper starting and running of motors, lighting equipment, and other loads having heavy
inrush currents must be considered. It is recommended that the steady state voltage drop in
distribution feeders be no more than four percent.
2.3.6 Fault Current Criteria. Under short-circuit conditions the temperature of the conductor
rises rapidly then, due to the thermal characteristics of the insulation, sheath, and surrounding
materials, it cools off slowly after the short-circuit condition is cleared. A transient
2-7
|
 
|
|
|
||
