Heat is generated when current flows through a cable. The ampacity of a cable is the amount of current a cable can carry without exceeding its temperature rating. Accurately estimating ampacity is critical to minimizing the total lifetime cost of a cable installation, and minimizing maintenance issues. Nehers and McGrath (1957) published a steady-state method to compute the temperature rise in cables. This method forms the the basis of the cable ampacity tables in The National Electrical Code (NEC) 2017 and IEEE 399-1997. This application implements the Nehers-McGrath equations and cross-checks the results against those tabulated in the NEC; the good agreement means that this worksheet can be the basis of more complex cable ampacity calculations.