The eGauge can calculate Total Harmonic Distortion (THD) on a given CT or voltage reference. Each THD measurement will use one physical register. To do so:
Navigate to Settings -> Installation
Under Registers, click the "Add Registers" button
Name the register as appropriate
Using the dropdown menu, change the register type from "P" to "="
A second dropdown menu will appear. Select the unit type to "total harm. dist"
In the formula field, enter the following: THD(HZ, chan("XX")) where HZ is the system frequency in Hz and XX represents either a CT (CT1, CT2, CT3, etc) or a voltage reference (L1, L2, L3).
Click "Save" at the bottom of the page when finished (valid credentials required)
Sample THD Register for CT1 on a 60Hz system