I have a new stove but the appliance wires are all 12 gauge and the existing hot wires are 8 gauge. Can I hook up the 12 gauge wires to the 8 gauge wires or do I have to run new lines down to the breaker box

What kind of stove is this, gas or electric? What is the wattage of the stove.

What do the installation instructions tell you to do?

Did you have an electric range and now are going to gas? If you had 8AWG, you most likely had an electric range 120/240V. If you only need 120V and need 12 AWG for the new stove and have 8, pigtail 12 to the 8, Black, white and ground and replace the old stove breaker with a 20AMP. If it was a double 240V breaker before, you will use only 1 breaker and cover the other slot with a snap in cover. The old red, if 240V, can be wire nutted, taped over, and pushed to the rear of the breaker box.

What is the amperage draw of the stove? 8 gauge wire carries more amperage than 12 over the same distance. Where is the 12 gauge wire? Are you replacing an electric stove with a gas stove? If the new stove is gas then you can cap one hot leg and put in a single-gang 120 volt receptacle to plug in the gas stove since it only uses power for lights and controls. Make sure and match the amperage of the receptacle to match that on nameplate of stove. DrTom4444