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By estimating the temperature of the cables, the safe long-term current-carrying capacity of the cables can be calculated. J. H. Neher and M. H. McGrath were two electrical engineers who wrote a paper in 1957 about how to calculate the capacity of current (ampacity) of cables. [ 1 ]
[1] [2] The other type of flow within a conduit is pipe flow. These two types of flow are similar in many ways but differ in one important respect: open-channel flow has a free surface, whereas pipe flow does not, resulting in flow dominated by gravity but not hydraulic pressure .
For systems such as underground power transmission cables, evaluation of the short-term over-load capacity of the cable system requires a detailed analysis of the cable's thermal environment and an evaluation of the commercial value of the lost service life due to excess temperature rise.
If the accepted fill rating is exceeded, then all the cables in the conduit are derated, lowering their acceptable maximum ambient operating temperature. Derating is necessary because multiple conductors carrying full-load power generate heat that may exceed the normal insulation temperature rating. (NEC 310.16)
The Manning formula or Manning's equation is an empirical formula estimating the average velocity of a liquid in an open channel flow (flowing in a conduit that does not completely enclose the liquid).
The exception to this is when a storm sewer operates at full capacity, and then can become pipe flow. Energy in pipe flow is expressed as head and is defined by the Bernoulli equation . In order to conceptualize head along the course of flow within a pipe, diagrams often contain a hydraulic grade line (HGL).
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Derating curves are included in data sheets for such capacitors. Derating can also provide a safety margin for transient voltages or currents (spikes) that exceed normal operation or prolong life. For example, the life of electrolytic capacitors is dramatically increased by operating them below their maximum temperature rating.