Q: The ground wire for my feeder/branch circuit is set to “Size automatically,” but it’s larger than what I have set in my wire sizing table for the conductors. Why is Design Master upsizing the ground?
A: Whenever you manually set the conductors to a given size, if that size exceeds what Design Master would automatically size the conductors to, the software will automatically upsize the ground in accordance with NEC 250.122(B).
When you set the size for the conductors, Design Master will compare the wire area of the auto-sized conductors against the size you have set, using the Wire Only Area defined in the Wire Sizing command for Electrical for AutoCAD and in the Wire Sizes command for Electrical RT for Revit. It will then apply that same ratio to the Wire Only Area of the auto-sized ground and find the next wire size of at least that area.
Example: Using the default wire sizing table, feeders that auto-size to 2/0 and 3/0 will both use a #6 equipment ground. However, if you upsize a 2/0 feeder to 3/0, it constitutes a roughly 26% increase in wire area, from 133,100 cmil to 167,800 cmil. As such, the auto-sized #6 ground with an area of 26,240 cmil must be upsized to at least 33,081 cmil; the next closest default size is #4 at 41,740 cmil.