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Cooktop and oven are running from 2 separate 30 amp circuits.

I want to replace both by a free standing dual fuel range which requires a 40 amp circuit

Can the 2 30 amp circuits be combined in the electrical outlet?

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No. While it might seem safe since each circuit is load-protected for it's gauge, you can have all kinds of problems.

First: Someone might work on the panel in the future and move a breaker and reverse the phase on one of the two circuits. This would cause a 240v short circuit.

Second: Any fault to ground will likely fault to only one of the two grounds which would be undersized for the fault load.

Third: Any difference in resistance in the two circuit hots will cause a difference in current in the two lines and cause nuisance tripping of one breaker.

Fourth: Any difference in resistance between the two neutrals will cause a difference in current that could overheat one of the two neutrals without tripping the breaker.

Fifth: As Ecnerwal points out, The appliance should trip at 40A, but if the current is balanced on both wires, the breakers may not trip until a combined load of 60A is achieved, causing a possible unsafe condition or damage to the new appliance.

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    "Cooktop and oven" - hard for them to be on opposite phases, since those appliances are pretty much always on BOTH, being 240V appliances. – Ecnerwal Apr 15 '15 at 16:10
  • @Ecnerwal Good point. But still easy for a someone to do a little work in the panel and inadvertently cause a 240v short in the circuit pair by reversing one of the two circuits. Editing my post now. – Paul Apr 15 '15 at 16:20
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    True...but pointless, since there are other reasons why this is NEVER acceptable practice; starting with the 60 amp supply to a device that's supposed to trip at 40. – Ecnerwal Apr 15 '15 at 16:23
  • @Ecnerwal Another good reason. – Paul Apr 15 '15 at 16:38
  • You're right this is a bad idea, but there wouldn't normally be any current flowing in the neutral since this is a 240V circuit (assuming US location). – Philip Ngai Apr 16 '15 at 19:00
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No, nope, non, nyet, nein.

Check the wire gauge and number of wires you have. You'll need to feed this from ONE 40 amp breaker, but you may be able to use the existing wire if it's of adequate size and number of conductors. Otherwise plan on upgrading the wire as well...

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