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] The paper described two-dimensional highly symmetric simplified calculations which have formed the basis for many cable application guidelines and regulations.
The Neher–McGrath paper summarized years of research into analytical treatment of the practical problem of heat transfer from power cables.
[2] From the basic principles that electric current leads to thermal heating and thermal power transfer to the ambient environment requires some temperature difference, it follows that the current leads to a temperature rise in the conductors.
The ampacity, or maximum allowable current, of an electric power cable depends on the allowable temperatures of the cable and any adjacent materials such as insulation or termination equipment.
The Neher–McGrath method is the electrical industry standard for calculating cable ampacity, most often employed via lookup in tables of precomputed results for common configurations.
to compensate for heat generated in the jacket and insulation for higher voltages.
is called the dielectric loss temperature rise and is generally regarded as insignificant for voltages below 2000 V. Term
is the effective thermal resistance between the conductor and the ambient conditions, which can require significant empirical or theoretical effort to estimate.
With respect to the AC-sensitive terms, tabular presentation of the NM equation results in the National Electrical Code was developed assuming the standard North American power frequency of 60 hertz and sinusoidal wave forms for current and voltage.
and of estimating the local increase in ambient temperature obtained by co-locating many cables (in a duct bank) create a market niche in the electric power industry for software dedicated to ampacity estimation.