Unfortunately the USA grid system is complicated and runs over state boundaries. Each grid has a different emission factor as the mix of energy sources(coal, nuclear, natural gas, solar) used to power it are different.
If you create a source through the office-based business template then the emission factor will be automatically associated, if you are creating a custom source and want to add a custom emission factor or you want to override an existing one:
1) Select your source
2) Go to related emission factors
3) Click edit
4) Select Emission factor - you will see the 4 digit code at the end of the emission factor listed, use https://www.epa.gov/egrid/power-profiler#/ to identify the grid code. In the screen shot below the code is "NWPP" for the zip code 97410