Contributed by David Sedivy, Senior Product Manager, TPC Wire & Cable Corp.
Choosing the right cable with the right capacity isn’t as simple as looking at conductor size. When reviewing cable ampacity, consider the application and the environment. Depending upon the application and/or environment, a correction factor may need to be applied to obtain the correct, safe ampacity rating. A correction factor is a multiplier applied to the ampacity rating to adjust the value based on a specific condition. The multiplier may be less than, equal to, or greater than one. Areas for review where correction factors apply include:
Ampacity tables in the National Electrical Code list ampacity values that are based on an ambient temperature. Should the temperature of the application differ from the referenced ambient temperature, a correction factor is applied to adjust the ampacity. As the application temperature rises above ambient, the ampacity value is reduced, and vice-versa.
Number of Conductors
As more conductors are energized in a cable, more heat is generated. Therefore a correction factor is applied when a cable holds more than three current-carrying conductors. Ampacity values are reduced to compensate for the added heat generated within the conductors.
Cable on a reel with multiple layers of cable does not have the ability to radiate heat away from the lower layers. When multiple layers of cable are installed on an energized cable reel, you must apply correction factors to compensate for the inability of the cable to radiate heat away, or risk de-grading or damaging the cable.
In certain applications, especially resistance welders, a correction factor based on the duty-cycle is applied to gain the correct ampacity value.
In summary, the first step in applying correction factors in regards to the ampacity value of a cable is to define and understand the application, and to consult the standards or authority having jurisdiction (National Electrical Code, UL, CSA, ICEA, etc.). With this understanding, choosing the correct conductor size for your application can reduce the time and costs associated with unnecessary repairs or replacement of materials.
TPC Wire & Cable Corp.