I am trying to determine if the intended load we plan for an office area we are moving into will overload the existing panel.

If I add up all the individual loads and divide by three (Three Phase Panel) will I arrive at the load imposed on each phase wire (assuming the loads are balanced on each Phase)? Or do I add up all the individual loads and divide by 1.73?

Thank you.

Instead of trying to add up the existing loads, try contacting the power company, see if you can get past history of total Demand KW.

Since this is a commercial building, the meter should record DKW along with KWH.

If you want to add loads, I added a panel load spreadsheet, to help organize the loads, and help add them per phase.

You should look over Article 220 of the 2008 National Electirc Code to see if any demand factors can be considered. See Sticky at the beginning of this forum for a link to the NEC.