Do you know with certainty what gauge wire you have? Is it stranded or solid? What is the insulation? Those are all factors, as the limit for current capacity is related to the heat produced. Thus, wire that is insulated and in an enclosed space will have a lower capacity than uninsulated wire in free-air.
If you have Open Office, you can fiddle with a spreadsheet resistor.ods I wrote to estimate such things; it's in the zip file here. The current capacity for a given wire is estimated; one of the sheets shows how the estimations are gotten. If you're doing wiring in the US for a house, you definitely want to follow the rules in the NEC...