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I have a new cooktop with 4 wires (see pic) - black, red, white and bare. The 240V supply cable has black, white and bare. I measured ~250V across the supply black & white (no load).

The instructions for the cooktop aren’t clear to me ( - shown below). They seem to indicate connecting white and bare (neural & ground), which doesn’t seem right.

My assumption is that in orange supply that black is live/hot, white is neutral and bare is ground. Update: now wondering if perhaps white is the other hot leg and there is no neutral (would explain why I measure 240V between black & white supply cable)

What should I connect to what?

Thanks!

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  • Is running a ground wire back to the main panel an option? – ThreePhaseEel Jan 12 at 21:45
  • I’m not sure - not easily, it runs under the tile floor in a channel. Isn’t the bare copper wire coming out of the orange supply cable the ground? – DavidJ Jan 12 at 21:54
  • I wouldn't be so quick to make presumptions here....note that this ground wire I'm asking you to run can take whatever path back to the main panel is convenient – ThreePhaseEel Jan 12 at 21:58
  • Unfortunately it is an island, so no other ways. Though there is also a seperate 110V line powering some llights and a power outlet. – DavidJ Jan 12 at 22:03
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    @DavidJ no, stealing neutral from a nearby circuit is absolutely forbidden. The neutral would then be overloaded with the normal return current from that circuit plus the return current from this one, and that would be a serious problem because neutrals don't have breakers! Also, that would violate the "wire in tree topology, no cross connections" axiom of wiring, and would cause eddy current and GFCI problems as neutral current would not match the hot current in the other circuit. – Harper Jan 13 at 8:45
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(Building on a ton of comments)

Your cooktop needs two hots + neutral + ground.

Your supply cable has two hots + ground. That is a problem.

The cooktop instructions allow for two hots + neutral, using the neutral for neutral + ground together. This is supposed to be only for old installations (i.e., grandfathered), which is (a) not recommended for safety reasons and (b) likely not allowed in your case anyway. Not an option.

There is, often, an allowed fix (again, supposed for old installations only, but it is safe). That is to retrofit a ground from elsewhere. But that only works if you have two hots + neutral and are missing ground. But you have two hots + ground, and that means the third wire is not insulated (and if it were insulated, it would be green, which is a ground-only color). Not an option.

Unlike ground, you can't pull a neutral from another circuit. That would run the risk of overloading that neutral - and neutrals are not protected by breakers like hots are. Not an option.

I only see 2 options:

  • Switch for a 240 V only cooktop.

  • Run a new cable that includes two hots, neutral and ground. Maybe you can run a cable the ugly way - up to the ceiling (i.e., through cabinets) and along the ceiling around to wherever the breakers are? Or use the old cable (depending on how it was installed) to pull a new cable in to replace it.

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    I am just boggling my mind at how the inspector even passed that. Failing to provision a neutral wire to a 30A stove circuit is just inviting a ground bootleg later on, a scheme outlawed for new work as of 1989. The inspector should have insisted that the builder show that he had a very specific no-neutral range in mind. – Harper Jan 13 at 8:54
  • My guess is originally cooktop elsewhere in kitchen. Kitchen expanded. Island added for new cooktop and workspace. Electrician didn't think ahead or was a general handyman /builder and didn't know what to get, and no inspections. When I redid /expanded my kitchen 18 years ago I got a really professional and knowledgeable and conscientious electrician and looking back (I didn't understand much of this at the time) he really did things right (except he should have sold me on a bigger subpanel). But not everyone is so lucky. – manassehkatz Jan 13 at 14:40
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The cooktop wants a 240 circuit and a 120. The two hots are the 240 (120 each and out of phase to add up to 240). The return wire (not safety ground) is used with just one of these phases to make a regular 120 circuit. You'll need X-3 wg wire, where X is whatever gauge is needed for your installation.

  • The safest "X" is 6... I.e. 6/3. That will support almost any range/oven on the market. – Harper Jan 13 at 8:55

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