How many volts in a 500 watt work lamp?

This can be easily calculated with the following formula:

Power = Volts * Amps

To re-arrange to answer you question:

amps = power / volts

amps = 500 / 120 = 4.166 amps.

Or here is an online calculator:

http://www.sengpielaudio.com/calculator-ohm.htm