I have a new wall oven to install. Both old and new oven are 30Amp units. Old wiring from local oven junction box is 14 AWG (pig tail - 6ft). New oven is 12 AWG, 3ft pig tail (will not reach junction box and is hardwired into new oven controls). I want to put a 30 Amp plug/receptacle in the oven space to connect the two pig tails. Can I connect both 14 gauge and 12 gauge wires through the receptacle (14g) and a plug (12g - new oven)? The 12 AWG oven ground will be connected to the 14 AWG ground old oven wire directly. Ok?
A 30A connection will require a much heavier wiring connection than either 12 AWG or 14 AWG. It is common that a 40A or 50A cord is used to hook up a range unit. These can be purchased in several lengths with molded on plugs at the end.
As always make sure that your outlet, wiring back to the entrance panel and the circuit breakers are sized properly for the load. The circuit components are normally rated a bit higher than the maximum current draw of the range. So a range unit that pulls 30 or 32A should be on a 40A circuit.