How to connect 3-wire cook top appliance to 4-wire junction box. 3 wires from new cooktop are black, red & green....existing wires in box are bare ground, 2 black wires and 1 grey wire. So my question is what connects to what? They old cooktop had 4 wires. Any help is appreciated. Thanks. enter image description here

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Wiring diagram from new cooktop

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    I imagine the new stove does not use neutral(grey). Just need to cap/wire nut the grey and use the two blacks to the black and red. The green to the ground. Do not know if I just developed colour blindest, but what you call green looks blue to me. If green it is good, if blue it might mean something else.
    – crip659
    Aug 19, 2023 at 17:38
  • I am 30 % color blind myself...most men are....it is a blueish green...lol
    – Chris
    Aug 19, 2023 at 17:44
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    How many watts is the cooktop rated to pull? Aug 19, 2023 at 19:47
  • Do you have photos of old cooktop wires? Both your photos are showing 3 wires each
    – Zimba
    Aug 20, 2023 at 6:21
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    If available, Brand, Model, and Ratings of both cooktops would also be helpful. Aug 20, 2023 at 7:46

2 Answers 2


The wiring diagram in the photo shows that the neutral isn't used, so cap off the grey, connect the green to the bare wire (keep connected to the box) the black and red to the blacks.

I have concerns, the wires in the j-box look aluminum, no bigger than #10, if so then it shouldn't be on a breaker larger than 20A (what does the cooktop call out for) and you would need wire connectors rated for aluminum connections.

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    "(keep connected to the box)" Indeed. Even better would be to get a proper ground screw in the threaded hole on the back of the box for a neat, solid, code compliant connection. For some reason seeing things like that bother me, and maybe it should or maybe it should not.
    – MacGuffin
    Aug 19, 2023 at 20:06
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    "the wires in the j-box look aluminum" Looks to me like someone tinned the wires, I see what looks to be copper peeking out near the insulation. Also, judging gauge from the photo would be difficult. That is a good point though, I would not think to check for aluminum wire on anything that small.
    – MacGuffin
    Aug 19, 2023 at 20:16
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    @MacGuffin +1 I didn't examine the screw close enough. A green ground screw is nice and 250.126 requires devices to have green screws, but that doesn't include boxes, 250.8 just requires two threads of a machine screw. 250.148 requires the screw to have no other use, that screw likely misses on both sections. Aluminum would be odd I admit, maybe I wouldn't really notice it except one of my barns has a couple of old runs of AL #10 NM cable. Aug 19, 2023 at 21:16
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    The stranded appliance wire is tinned copper. The building wire is surely aluminum. It’s not plausible that somebody tinned the entire exposed length of the ground wire, or that none of the solder was scraped off when the wires were connected to the old stove.
    – nobody
    Aug 20, 2023 at 18:57
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    It appears I was not clear before. I didn't claim that the wire in the wall was definitively copper with the exposed ends tinned later, only that the photo isn't clear enough to say for sure. The bare ground wire is in shadow which can affect how it looks compared to the wire in better light. I see a bit of copper color on the ground wire that could show the tin scraped off but that could be just an optical illusion. I'd suggest finding out which it is definitively or acting as if it could be either.
    – MacGuffin
    Aug 20, 2023 at 19:20

From the schematic pictured this is a 4-burner cooktop with one burner rated at 1400 W, another at 1600 W so 3000 W total for those. Another burner is 1200 W, giving 4200 W total. What about the fourth burner?

Since the schematic shows L1, L2 we know this is powered at 240 V. What is the total wattage of all the burners? How many are there? The maximum current would be at least 4200 W / 240 V = 17.5 A. A fourth burner would put the total maximum current well past 20 A. To power this cooktop it looks like you should have a 30 A breaker with AWG 8 aluminum wire (or 10 AWG copper).

The fine-stranded conductor in the three leads on the cooktop wiring are labeled 10AWG (stranded copper, tinned? Or fine stranded aluminum (if there is such a thing)? indicateing the expected maximum current is over 20 A. But what do the specs state?

It is possible the cooktop has circuitry that automatically limits the total power to less than the sum of the maxima of the four burners. (My 2-burner induction cooktop has two burners rated at 1800 W each at 120 V, but it will not let both run at full power simultaneously. The rated total power is 1800 W at 120 V (so 15 A) and the specs allow it to operate on a 20 A circuit. What are the power specifications for this four burner cooktop?

Can you read the writing on the wire sheath of the house wire and report back the gauge of the wire?

I would expect the breaker for this circuit would be a 2-pole 20 A breaker or a 30 A breaker. Is that what is in the electrical panel? If the wire is #8 aluminum, then you might have a 30 A 2-pole breaker. Do the installation instructions for the cooktop specify the breaker size?

There will be a requirement for aluminum rated connectors. If the house wire is AWG 10, then the relatively cheap AlumiConn connectors would be rated for this connection.

The Ideal Purple twist on connectors are legal to use AFIK, (but not sure about the limit on wire size) but at least one expert on aluminum wiring, Jesse Aronson, thinks the Ideal Purple twist on connectors are not reliable.

Everyone agrees that AlumiConns (King Innovation) are the gold standard for connections up to #10 solid aluminum to copper or to other aluminum. You would need three of these and they are probably available at a big box. Two of the AlumiConns could be the two port (for L1 and L2) and you would want a 3-port for the ground so you could have a third wire of #12 copper to ground the box.

But if the AlumiConns come in a package of say 4, then you could use the 3-port for all three connections and just leave one port unused. If you decide to use the AlumiConns say so and we will give you instructions on how to use them.

AFIK the AlumiConns cannot accept #8 solid aluminum condictor so if you do have #8 wire you will have to use a different and more expensive connector.

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