Welding is a higher voltage application which may occur in an environment which is much harsher than a typical dimmer room, so the insulation required would likely be more expensive—I wouldn't expect welding cable to be cost effective for lighting. I'm having difficulty thinking of reasons why welding cable would be unsafe for lighting loads—possibly due to current or duration of load.Now to further expand the discussion:
1) What is wrong with welding cable?
1. Welding cable is not a listed extra-hard usage cable covered by NEC Article 400. It does not even have a voltage rating, but the default voltage for welding equipment is 60V. Therefore it is not allowed for transmission of line-voltage power.
In the photo above, printed on the jacket of the welding cable is "600 VOLTS (-40°F)". Does that mean it's good for 600V only at forty below? Who would want to weld in that weather?
That 600V is not an NRTL 600V, so it doesn't count.