I purchased a new induction cooktop only to find the old cooktop is has some strange wiring. The house was built in 1980 but the cooktop that came with it was much more modern. The wire powering it has to thick black cables, and then like 10 thin bare silver wires. The existing range had the ground hooked up to a bundle of 3 of the thin bare wires.

I cannot find any reference to this kind of wiring online. For now I've wired up the new cooktop the same way and it works but makes me nervous. Can anyone explain what the existing wiring is without a neutral or single ground? And is it safe, or how urgent is it I get a new wire run?

Existing wiring

  • Is there any identifying text on the cable jacket? If so, can you post a clear photo of it for us? – ThreePhaseEel Feb 14 '17 at 4:08
  • Also is this cooktop 120/240V or 240V-only, and is it hard-wired or cord-and-plug connected? Last but not least, is this cable run from the main panel or a subpanel? – ThreePhaseEel Feb 14 '17 at 4:10
  • It's a 240v line. There is no identifying marks on the cable, it comes right from the main I don't have any subpanels. I've got it all wired up in a junction but this is what the jacket looks like: i.imgur.com/Wb2NoY5.jpg – Chad Feb 14 '17 at 4:52
  • What make and model is the new cooktop? – ThreePhaseEel Feb 14 '17 at 5:07
  • It's a Frigidaire Gallery induction cooktop but that doesn't really matter, this is the wiring in the house, not the cooktop wiring. – Chad Feb 14 '17 at 5:28

Cable Incognito is OK here

Your cooktop circuit was wired using what is known as type SE, style U (or just SEU) cable. This is a jacketed cable with two insulated wires and what is known as a concentric neutral formed by the bare strands wrapped around the inner conductors and then covered by the jacketing. It is suitable for indoor applications as per NEC 338.10(B)(2) and (B)(4). (This is unlike the similar-sounding but not quite related type USE cable, which is a direct-bury rated, jacketed or unjacketed multiplex cable that cannot be used indoors as its insulation is not required to be flame retardant.)

As there is only a main panel involved (no subpanels), and both of the current production Frigidaire Gallery cooktops are 240/208VAC only units, you can strip back some of the SEU cable's jacket, twist all the strands into a bundle after peeling them away from the wires inside, feed it into the junction box with an appropriate cable clamp for strain relief while making sure the jacket is clamped with at least 1/4" of it inside the box and clamp, and then cut and strip the inside wires + cut off the excess bundle length.

From there, use Al9Cu listed split bolts or another type of connector (such as a screw type lug) listed for 90degC operation and copper to aluminum connections with 10AWG copper and 8AWG or larger aluminum to make the connection, along with antioxidant compound. With split bolts, you'll need to insulate the connections yourself as well -- this is done with a self-adhering elastomeric tape first, followed by an overwrap of ordinary PVC electrical tape.

Electrically, what you're doing here is using the concentric conductor to the main panel as an equipment ground as per 338.10(B)(2) with the two inner conductors as the hots as your cooktop has no use for a neutral wire.

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That's Service Entrance Cable (SEU/SER/SE) with a concentric neutral. The two inner wires are the ungrounded (hot) conductors, and the outer wires are the grounded (neutral) conductor. This cable does not have an equipment grounding conductor, so you'll have to follow the manufacturer's installation instructions for installing the cooktop to a 3-wire grounded system.

enter image description here

To terminate the cable, you'll have to strip back the outer sheath to expose the concentric neutral wires. You'll then gather the neutral wires together, and twist them into a single conductor (this might take a bit of practice to get right, and probably isn't a job for a DIYer).

If this is a straight 240V application (not 120/240V), a neutral is not required. In this case, it may be possible to use the concentric wire as a grounding conductor. National Electrical Code allows the concentric conductor to be used as an equipment grounding conductor (338.10(B)(2)), check with local codes to make sure it's allowed in your area. If local codes do not allow this type of installation, you'll have to run an additional equipment grounding conductor to the cooktop. Then you'll follow the instructions for connecting to a 4-wire grounded system.

enter image description here

Since this is aluminum wire, you'll want to make your terminations using connectors rated for aluminum. According to the manufacturer's installation instructions, the cooktop is fitted with a flexible armored cable with copper conductors. This means that you'll have to use connectors listed for connecting copper to aluminum wiring. You'll also want to apply an anti-oxidant compound where required, and make sure all terminals are tightened to the specified torque.

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  • Using the concentric neutral as an EGC is allowed straight-up by the NEC in 338.10(B)(2). – ThreePhaseEel Feb 15 '17 at 4:52
  • @ThreePhaseEel Thank you for looking that up. – Tester101 Feb 15 '17 at 11:43

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