squre d panel - panel has 10x2=20 spaces for 20 ckt bkrs - bus bar & rail lengths can handle 6x2=12 bkrs - are kits available to extend the lengths? -
there are breakers designed to give you two circuits in one slot. they are either mini twins or piggy backed. mini twins are two side by side breakers in a one slot package with the toggles next to each other, piggy backed are full sized toggles one right one left on the breaker. both are available for your square d box. so as long as you only need a spare circuit or two, and don't exceed your main breakers amperage you can add to your existing box.
This is an excellant answer. I didn't answer this for fear that the person would indeed overload his bus and a chance of fire if he doesn't have a main breaker to this panel. i THING IT IS STILL WISE FOR HIM TO HAVE A QUALIFIED PERSON TO LOOK AT HIS OVERALL SITUATION.