How to calculate current and voltage to heat up a wire?
The wire manufacturer should have available specifications/charts that give the required current to achieve a desired wire temperature for a certain wire size/type. Here is one example: http://wiretron.com/nichrome-resistance-wires/

You also have to take into account the wire mounting. For example if the wire is used as a single strand in free-air, or if it is to be wound on a core material. If the wire is in contact with a thermally conductive material more current is required to sustain a certain temperature.