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I am installing a 240 volt 40 amp electric cooktop. Will 12/3 NM cable suffice for this?

Secondly, I know I need a double-pole breaker because 240 volts is delivered using two out-of-phase 120 volt circuits. My question is, is a double-pole breaker labeled "20 amps" really two 20-amp circuits that together can supply 40 amps @ 240 volts? Or do I need a double-pole breaker labeled "40 amps"?

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Not even remotely close. You need 8 gauge copper or 6 gauge aluminum wire.

A 20 amp dual pole breaker supplies 20 amps at 240 volts (line to line), or 20 amps at 120 volts twice (line to neutral and line to neutral). You need a 40 amp at 240 volt breaker.

...and these two questions in combination strongly suggest that you get professional help before you cause yourself an expensive problem. Like the price of a new house expensive.

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    @Tester101, Look up NEC ampacity table. This will help you decide what gauge wire to use for a given application to avoid an electric fire. Also, what kind of outlet are you using for the stove, or is it hard wired? If the outlet is a 240 only type, or if the hard-wired stove does not need 120 for anything, you can use 8/2 romex instead of 8/3. – mkeith Mar 9 '15 at 16:26
  • Picknit: A breaker is a part of the supply chain leading from the (cooktop, in this case) to the electric company's generator. If configured in a useful manner (connected to a panel, which is connected to a meter, with service that is turned on, when the power grid is operational) it supplies current as a part of that chain, and cuts it off if its rating is exceeded for sufficient time and magnitude. – Ecnerwal Mar 9 '15 at 16:39
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    Fully, FULLY agree with the last paragraph in the strongest possible terms. – Speedy Petey Mar 9 '15 at 19:46
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    Appreciate the response, however I disagree with the implication that by asking the question I have proved myself to be incompetent. I've done a lot of DIY electrical work in my life, I just never had a need for 240V before. I do things carefully, and when I don't understand something (such as 240V double-pole breakers), I research it before proceeding (as I am doing now). If it's beyond my abilities, I hire someone. I also get permits when needed, so this will get a pro's eye on it when the inspector comes. So I'm just saying, you can't always judge a newbie based on his question. Peace. – Timbo Mar 9 '15 at 20:09
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    If the idea is if you don't already know something call a professional this site is useless. And if the idea is this site cannot render correct responses to electrical questions someone needs to get on burninating a tag or 5 for everyone's safety. A truly ignorant person doesn't know/care and does it wrong, a smart person understands their limitations and seeks insight. @Timbo, I agree with that. – ChiefTwoPencils Mar 9 '15 at 21:57

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