Can you use 2 runs of 12 gauge wire in for 40 amps circuit?

You might think this is a good idea, but it isn't. The code requires a single cable of the required current carrying capacity. What would happen if one of the cables was damaged and the hot wire severed. Then the other would be carrying 40 amps and you wouldn't know it until your house burned to the ground.

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No, the electrical code states that no conductors of similar conductivity in sizes smaller that #1/0 copper shall be permitted in parallel.