I had electrician upgrade my service to 200amp and installed a new murray breaker box. Within this box 2 wires feed the breakers and 1 wire is attached to the neutral bus bar on the lower left side of panel(There is also a neutral bar on rt side that is connected to the left side via flat piece of steel(There is also a heavy screw that makes contact to the panels metal shell). The ground rod wire also is connected to this neutral bar. On top right of panel there is a separate bus bar that is screwed directly to the shell of the panel. What is this for? (For green wires) My question- I am adding a few circuits using MC Lite wire. This wire has 1 black, 1 white, and 1 green. Do I connect the green wire to the same neutral bus bar as all other circuits or do I connect green wire to the separate bar on top right? Thank you.
Since this is an upgrade of the service panel the grounding and grounded conductors really don't matter the side they are on. Both neutral and grounds can be on the same buss. If it is a sub panel the neutral and ground need to be isolated and the grounds on 1 side and the neutrals on the other to be code compliant.
Check your local codes if a separate buss bars are needed for grounds and neutrals some do require it and some allow grounds and neutrals on one buss. Also make sure if a cross wire between the two is needed or not. The bar screw direct to panel would be a ground buss, but codes are so varying so some require neutral to be screwed to panel. I would expect an electrician installing a panel making it to local code but better safe to double check. Your local codes would be on your city, county and state web sites.
Since you are adding circuits to your main panel -- where your house's safety bond between neutral and ground is made -- you can wire them into either bar, electrically speaking, and all is well. Separating them makes life easier if the panel later is turned into a subpanel, but can cause wire routing issues that are alleviated by using the bars as shared neutral/ground bars.