The alternative to the table method of determining ampacity is the engineering calculation described in Section 310.15(C). This section provides only the general equation. Other documents beyond the NEC are needed in order to use this method.

Annex B provides some of the information needed, but much more information is also required to have a completely accurate and safe result from this type of calculation. This calculation must be done under engineering supervision.

The ampacity found by using the engineering calculation method is sometimes called a "true ampacity." As described earlier, both true ampacity and allowable ampacity may be considered when choosing a conductor.