Replacing 240v electric cooktop with gas cooktop - can I get 120v?

Discussion in 'Electrical Forum discussion & Blog' started by TubeGuru, Mar 25, 2021.

  1. TubeGuru

    TubeGuru Unretired defense industry engineer

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    So, the Jennair downdraft electric cooktop that I posted a question about in 2010 is coming out and I’m replacing it with a Jennair downdraft gas cooktop.

    The 50A breaker in the main panel feeds #6 Al Type SE cable, which goes to the oven/microwave combo, and then runs to the electric cooktop. The 120/240v electric cooktop has three wires, as does the SE feeder (red, black and bare/neutral).

    The new cooktop requires 120v on a 15A circuit for the fan and igniters. Is there a way I can use one leg of the existing 240v feeder to accomplish this? I plan to install a junction box and a 15A receptacle under the counter for the gas cooktop to plug into.

    For some reason I'd feel better pulling new wire, since it appeals to my sense of order and correctness, but want to see if I can safely avoid having to do that if there is a common and well-accepted solution.
     
  2. wwhitney

    wwhitney Well-Known Member

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    Does the new gas cooktop permit hardwiring? Reading NEC 250.140 Exception, I think it would be allowed to use the grounded conductor for bonding if it is hardwired, but not to supply a receptacle. This assumes that the "main panel" you refer to contains your service disconnect.

    Cheers, Wayne
     
    Last edited: Mar 26, 2021
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    While ground and neutral are connected at the main panel, the ground wire is not supposed to be a current carrying conductor...so while it would work, to get 120vac using one of the hot lines and ground, the safer way would be to repurpose one of the hot leads as a neutral. Now, whether that would be according to code, not sure. The safer thing to do would be to replace the run. The stove probably doesn’t need more than a 15A circuit unless it’s a dual-fuel device, then, it could use the existing wiring.
     
  5. TubeGuru

    TubeGuru Unretired defense industry engineer

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    The cooktop is supplied with a three-prong plug, so it is not hardwired as delivered. Now if I went "snip", then it would be hardwired.

    Since the 240v oven/microwave combo unit is still on this circuit, I'm not sure how we could repurpose one of the hot leads... hence my gut feeling that I will have to run a new circuit altogether, which is not going to be fun since the service disconnect is in the garage and the wires run to the attic crawlspace, between the first and second floors, down interior walls, and under the crawl space to the island where the cooktop is located.
     
  6. wwhitney

    wwhitney Well-Known Member

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    To clarify, the OP has a 120V/240V branch circuit without EGC, i.e. 2 hots and a neutral, no ground. Which is not allowed for new circuits, but is allowed for existing circuits for cooking equipment and clothes dryers, with some limitations. And when it is allowed, the neutral is allowed for bonding metal electrical boxes and the frames of the appliances (250.140 Exception). But I don't read that exception as permitting a separate receptacle to be installed for cord and plug connected cooking equipment.

    If the gas cooktop instruction manual has provisions for hardwiring (i.e. open up this wiring compartment, remove the cord, bring in your branch circuit, etc), then you could hardwire it. If not, you need a receptacle.

    For a receptacle you have two options that don't involve running a new circuit. One is to put the receptacle on the small appliance branch circuit serving the countertops. This is allowed (210.52(B)(2) Exception 2) for "Receptacles installed to provide power for supplemental equipment and lighting on gas-fired ranges, ovens, or counter-mounted cooking units."

    The other option is to keep the receptacle on the ungrounded cooking circuit, and grab an EGC from another convenient circuit that originates in the same panel as your cooking circuit (250.130(C)(4)). 250.120(C) says you can't run a lone EGC smaller than #6 without using conduit (minimum ENT aka smurf tube) or armored cable. So you could either use a bare solid #6 or an 8/1 armored ground, or ENT with a #12. If you do this, you need to be sure you don't create a neutral - ground connection--i.e. in the box for the new receptacle, don't bring in a grounding wire or metal conduit from the upstream oven box bonded to the neutral.

    Cheers, Wayne
     
  7. WorthFlorida

    WorthFlorida The wife is still training me.

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    The gas top range only needs power for the ignitor circuit, if any, and the fan. It is a very small load. If there is a outlet nearby you probably could branch off of it for the new outlet, otherwise a new cable run. The circuit requirement for the new gas top range is for the circuit, not the range. All it means a 20 amp breaker is not needed. The circuit breaker protects the wiring and outlet, not the range. Trying to rig something for the existing oven is asking for trouble. It'd be a bit tough to branch off or terminate to a #6 wire at an outlet.
     
    Last edited: Mar 26, 2021
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    In the first post, the op mentioned three wires, and said the neutral was a bare wire, which is not allowed as all current carriers should be insulated unless I'm missing something which is entirely possible!
     
  9. wwhitney

    wwhitney Well-Known Member

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    Yes, this is another exceptional feature of this sort of installation.

    First note that a typical US range or dryer is 120V/240V as it has 120V loads in it. So when connected via a 3 wire cord, the wire at a potential of 0V to ground will be carrying current during normal use. That makes it a grounded circuit conductor, aka neutral, rather than an EGC. Within the appliance, you land that conductor at the neutral terminal, and ensure that the case of the appliance is bonded to the neutral with a bonding strap.

    Second, here's an excerpt from NEC 250.140 Exception, one of the conditions for applying the exception allowing the bonding of the case to the neutral (textual parentheses by me to clarify the parsing):

    (3) (The grounded conductor is insulated), or (the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment).

    Lastly, a good way to turn this into a non-exceptional installation (if desired) is to use a dryer or range that is 240V only (many European brands), and to use the uninsulated wire as the EGC. And note that under the conditions of the exception, that typically means making no change at the other end of the cable at the service panel, as they often have a joint neutral/ground bar.

    Cheers, Wayne
     
  10. WorthFlorida

    WorthFlorida The wife is still training me.

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    My first thoughts were 2 hots and a ground, not a neutral. As I researched everywhere it's called it a neutral. I even called my electrician friend, semi retired and he called it a neutral. Then comments by Wayne, I looked up service entrance cable. The wire mess for neutral is named "concentric neutral conductors". My son's house, built in 1977, has this cable to the electric range. Either 6 or 8 gauge, I don't recall.

    https://allwireandcable.com/product...a5m16WpgpjpWjhFM4zCGSAubTpM_gUuxoCMrwQAvD_BwE
     
  11. wwhitney

    wwhitney Well-Known Member

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    For new installations of type SE cable, you can use the concentric uninsulated conductor as a neutral on the utility side of the service disconnecting means, or as an EGC on the premises side of the service disconnecting means. In the past, ungrounded (without EGC) branch circuits or feeders were sometimes allowed, in which case the concentric uninsulated conductor was used as a neutral.

    Cheers, Wayne
     
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