There will be two mains fuses, at the top of your electrical panel. Read the sizes on those, to determine how much amperage is available, per phase.
09-09-12: Rodman99999
Rodman99999,
Ya in some cases, but not always the case.
For many years NEC had a 6 switch rule.
If the total number of branch circuits fed from the panel was 6 or less, a main was not required.
Outlet boxes in older homes ARE NOT USUALLY grounded. Especially if the wiring is knob and tube. Unless the wires were run through metal conduit(EMT), metallic liquid-tite, Bx cabling or have a third ground conductor to the box; it is NOT grounded. Grounding(running a pigtail) to the new outlet to an ungrounded box would be a waste of time/effort.
I wondered how long it would take before somebody would point that out.
.
Jim