Determining Resistor for Heating Element
Using resistors is Very Bad Idea. Look at datasheet of Your heating element, this one is for example: http://heatingelements.isquaredrelement.com/Asset/Moly-D-technical-brochure.pdf On page 4 You can find a graph showing resistivity in function of temperature of heating material. At normal operating temperature (about \$1600^{\circ}C\$) molybdenum elements has resistance 7 times larger than at room temperature! If You have an element with \$0.3\Omega\$ resistance at room temperature, it will rise up to about \$2.1\Omega\$ at \$1600^{\circ}C\$. Connect them in series and You've got more than \$4\Omega\$ in operating condition.

I know, it's still too much to run it with 20A circuit breaker - only reasonable solution for such power is to use SCRs with proper driver. Such regulator gives You another two important features:


Soft-start - limit current during start-up, when molybdenum elements are cold and have low resistance.
Regulation of temperature - use more sophisticated regulator with feedback from some temperature sensor and you'll have an automatic regulation.