Originally Posted by Arthurrs

You're misapplying Ohm's law here. The wattage of the bulb remains the same, it's the current draw that changes. So a 100 watt bulb draws 0.83 Amps at 120 volts, half of that at 240 volts. Nevertheless we are not exceeding 80% of the wire's voltage rating, shouldn't be a problem. The bulb needs to be changed out to one rated to handle 240 volts, a filament rated for 120 volts will indeed blow with 240 volts across it.