Cable tray wiring systems have conductor advantages over conduit wiring systems where the installations involve phase conductors installed in parallel.

For large ampacity circuits, the most practical wiring system installations are those where reasonable size conductors are paralleled for each phase to adequately handle the circuit's ampere requirements.  A more efficient use of conductor material is made by paralleling phase conductors than by using very large single conductors per phase.

A calculated load of 500 ampere - 480 volt480-volt three phase 600 feet long feeder that is installed where it is sometimes exposed to moisture and where the maximum ambient temperature is 110 degrees Fahrenheit is a practical circuit to be used as an example. 

A good conductor choice for such a circuit would be 75 degree75-degree Celsius rated XHHW OR THHN/THWN insulated copper conductors per 201723 National Electrical Code (NEC®) Table 310.1615(B)(16).  The ambient temperature ampacity correction adjusts the calculated 500 ampere circuit ampacity requirement to 610 amperes. (500 amperes / 0.82 = 610 amperes).

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