Ampacity, defined as "the maximum current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating," is extremely important. Four “primary determinants of operating temperature” are relevant in many different decisions about conductor size: Heat produced by current flow in the conductor Ambient temperature Ambient medium Adjacent current carrying conductors Two primary methods are used to determine ampacity: the table method, which results in "allowable ampacity," and an equation that must be used under engineering supervision, which gives "true ampacity." Different methods sometimes yield different values for ampacity of a conductor. When this happens, the lowest value must be used. Other factors that must be considered in determining conductor size include the effect on current on overcurrent devices, requirements for branch circuits, whether the load is continuous or non-continuous and specific equipment requirements.

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