Wiring new electric cooktop

[tomtheelder2020]

The electric cooktop at my mother-in-law's house shorted out and the new one I ordered arrived today. The stove and oven are on the same circuit, which has two 50 amp breakers clipped together. The breakers were tripped by the short, and would not re-set, so they were replaced by an electrician who said it is a 220 V circuit. Installation instructions for the cooktop say two things that sound contradictory:

1) "You must use a two-wire three conductor 208/240 VAC, 60 Hertrz electrical system."
2) "The cooktop must be installed in a circuit that does not exceed 125 VAC nominal to ground."

What am I missing?

Also, the existing cooktop is wired like this:

Stove wire House circuit wire
red to white
black to black
white to bare copper
green to bare copper

The instructions say a white neutral wire is not needed. The cable from the new cooktop has three wires: red, black and green. It looks to me like the new installation should go like this:

Stove wire House circuit wire
red to white
black to black
green to bare copper

Which seems so obvious that I hesitated to even ask it that is right, but hey, don't know much about electrical (queue Sam Cooke).

Last question: the wires from the house circuit are solid copper but substantially smaller diameter than the stranded wires from the new cooktop. There is not enough sheathing exposed in the junction box to read a wire size. How can I verify the existing wiring is adequate for the new cooktop?

 

[Eddie_T]

It sounds like your wiring assumption is correct but in your original wiring when you say white-to-copper is it the same bare copper as the green-to-copper?

A 50 amp breaker will protect 8 AWG wire which is 0.1285" diameter or just a hair larger than a ⅛" drill bit.

Is the oven wired from the same breaker in the panel or is the drop-in a combination cooktop and oven? If the same wire serves both you may need 6 AWG which is 0.1620" diameter or a little larger than a 5/32" drill bit.

Your voltage is not above 125 VAC. Red-black is 240 (or220) VAC but red-green and black-green are 120 (or 110) VAC.

 

[tomtheelder2020]

It sounds like your wiring assumption is correct but in your original wiring when you say white-to-copper is it the same bare copper as the green-to-copper?

Yes. White and green wires from cooktop cable both connected to the single bare copper wire from the house wiring. Seems strange for an appliance to have a double ground but that is what I see.

 

[tomtheelder2020]

A 50 amp breaker will protect 8 AWG wire which is 0.1285" diameter or just a hair larger than a ⅛" drill bit.

Is the oven wired from the same breaker in the panel or is the drop-in a combination cooktop and oven? If the same wire serves both you may need 6 AWG which is 0.1620" diameter or a little larger than a 5/32" drill bit.

Oven is separate from drop in cooktop. Both are wired to the same circuit (two 50A breakers clipped together). Would it be reasonable for the house circuit black and white wires to both be 120 V,* and therefore OK both both to be 8 AWG? I will check the wire diameters - thanks for giving me the specifics.

*Installation instructions for the new cooktop say a neutral wire is not needed.

 

[tomtheelder2020]

Dang. It looks like chatroom program messed up the formatting I had to show the wiring. Let me try again

Stove wire________________ House Circuit wire
___red--------------to------------white
___black------------to------------black
___white------------to------------bare copper
___green------------to------------bare copper

 

[Eddie_T]

That looks like the way mine is wired except mine is SE cable and the concentric braided ground/neutral is used rather than a bare copper solid conductor.

 

[Guzzle]

2 x 125v = 250v which is 240v + 4.2%.
Some appliances work on 240v +/- 10%.