I need to run a 120 volt 40 amp circuit about 50 feet. What size copper wire do I need to use?

#8 AWG copper

I am curious. What could you possibly need a 120v/40A circuit for?

Good question.

This should be good...

Thanks for your interest and help guys. Turns out the specs on the website for a cooking stove were wrong. A dedicated 15A circuit will do. I ran a 20A anyway. The stove is a gas cooking stove with the option for convection oven cooking or gas oven cooking. Not knowing much about convection ovens, I figured the elec needed could be 40A 220V like an electric range. The web site confused me with the 120V 40A specs. I had not see the new stove yet when I posted my question, so I hadn't seen the power cord. Thanks again.

Thanks for getting back to us.