Replacement cooktop wiring question

Question

I’m installing a new induction cooktop to replace an old electric coil unit. The new unit has a three-wire connection (black & red hots and a green ground) with instructions for retrofit to existing three or four wire box connections (with or without a neutral wire). House was built in the 1970’s.

The old cooktop was connected with black & red hots, a white neutral and a ground to the junction box (which does not have a ground wire back to the panel but may have a ground path via the conduit - but I don’t know for sure), so it’s kinda in-between the cases in the instructions (both assume there is a grounding wire).

My initial thought was to hook up red-red, Black-Black, green (cooktop ground)-white (house neutral) roughly in-line with the four-wire retrofit instructions. Would this be my best (safest) bet or would it be advisable to attach the cooktop ground to the junction box (although I’m not sure how I ensure the box is a good ground) and cap off neutral? I suppose I could also attach the house neutral and cooktop ground to the junction box grounding screw as a “belt-and-suspenders” approach but I don’t know if that introduces other risks?

Answer 1

We haven't heard back from OP, but I suspect what's going on here is that the range uses metallic conduit all the way back to the panel, and the conduit is the ground, accessible via the green #10-32 screw in the metal junction box. That's a legal setup; OP is good to go.

This is a modern range which does not require a neutral wire. Historically neutral was used for nothing but the oven light and the clock, so it's not hard to get rid of. If there's a neutral in the supply cable, it should be capped off.

In the off chance this is an obsolete 3-wire circuit with a cable, if the neutral wire is a bare wire, it can be permanently converted to a ground. This is irreversible. Inside the panel it would need to be moved from the neutral bar to the ground bar (on a main panel they are probably the same bar). A white neutral wire can never be converted to a ground.

Answer 2

Pay attention to @Harpers comment. Your assumption to connect black-black, red-red, white-white, and green to ground is correct. The problem you're having is that the original cooktop ground looks to be installed incorrectly.

Unfortunately there is no reverse grandfather clause that says if the wiring is incorrect you can revert to a three wire connection. You must correct the wiring. So verify if the ground is correct if it is not you need to run a ground back to the panel and make sure it is installed correctly.

Good luck.