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My old cooktop which is being replaced connects to a j-box with black, red, white, and bare ground wires, which goes to a 40 amp circuit breaker (US here, noticed this site is popular with Europe too).

We just bought a brand-new Bosch cooktop which only has black, red, and bare ground wires and says, "Connect only to a 3-wire, 120/240-volt power supply; the neutral conductor is not required for the operation of the appliance. The potential at the power supply electrical connections shall be 150-volts-to-ground or less." It also mentions wanting a 30 amp circuit breaker and being compatible with 240 or 208 volts (which I believe means it can handle single or 3-phase current, which is good since I don't know which we have).

I may be mixing up two questions here (circuit breaker amps and neutral wire issue) but can I wire this to the existing setup, or do I need to have the electrician come in and change something at the breaker box?

Note that I am fully aware the right answer is to call the electrician in to handle this and double-check everything -- I feel it is important for me to understand how this all works before calling someone in so when I talk to them I can make sure we are on the same page.

  • It's a bit weird but 208/240 doesn't guarantee 3-phase systems (physics). And further, 3-phase systems require 3 hots A, B, and C (Black-Red-Blue or Brown-Orange-Yellow (277/480)). So based on your post it can operate on a 3-phase system but it's only using two of the legs. There are many configs; you can actually get transformers that will produce 3 voltages with a "stinger leg". Won't see it in a dwelling but they're out there. – ChiefTwoPencils Jan 17 '15 at 8:45
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In this case, you would simply not hook up the neutral wire. Instead you can just put a wirenut on it and tuck it neatly into the electrical box.

Typically 240V appliances require the neutral wire so that they can run the electronics at 120v or provide a plug on the appliance. In this case, these devices are hooked up to one leg of the hot and the neutral, giving 120V.

If no neutral is required, then the device can only operate on 240V/208V, though at 208V you should expect the oven to take longer to heat up.

What it means by "Connect only to a 3-wire, 120/240-volt power supply; the neutral conductor is not required for the operation of the appliance," is that the 240V needs to come from two legs of 120V service; this device won't function on 240V mains like you'd find in Europe.

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    The reason that it says 120/240 volt power supply is that it is not designed for hots that are more than 150 volts from ground. Outside North America, 240 volt hots are typical, but this cooktop is not designed to be connected to a 240 volt hot and a neutral that simply specifying a 240 volt supply could imply. – robartsd Jan 16 '15 at 20:41
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    To the layman, meaning that the black and red hots are each 120s (< 150 from ground)? – mtjhax Jan 16 '15 at 20:48
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    If you turn the power off, there's no reason you shouldn't be able to safely connect this! BUT if you're not comfortable, a pro is always the best call. Good luck! – Steven Jan 16 '15 at 21:09
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    @MikeJohnson - don't forget to replace your 40 amp circuit breaker with a 30 amp one, as required by the appliance. Since you're going down in amperage, you shouldn't need to replace the wiring. – DoxyLover Jan 16 '15 at 21:44
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    @MikeJohnson: If you do replace the breaker, be sure that the new breaker can physically accommodate the size of the wires. Not all breakers have holes big enough! – wallyk Jan 17 '15 at 1:19
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Already answered but assuming this is a US house you don't have 3 phase industrial power. Most likely you don't even have the other 2 phases on the utility pole. You have split phase residential power. A neutral isn't needed since it's balanced no current would flow thru it in the cook top. Basically one phase off the primary side with a center tap. The stove doesn't need the center tap. The controls thermostat etc in the unit has a transformer in there that converts the 240v to the low voltage they require to operate. This is different that EU power, here you can get 240 off the two feeds. The red and the black wires. I don't know code but basically the circuit breaker is sized to protect the wiring not the device so changing it is not mandatory.

The neutral at the breaker box is used as the house won't be balanced with things hooked up on one or the other phase in the breaker box. If you for some reason lost the neutral there a large load on one side would over voltage the other side and damage connected loads.

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I had this exact same question. It took a professional electrical wiring book to explain it to me. It is really complicated, so I will have to start by explain how the electricity gets to your house.

The piece of equipment that is responsible for the electricity getting into your house is the Utility-Pole transformer.... and I am realizing this is waaaaayyyyyy to complicated to explain without a picture, so here is the page from Rex Cauldwell's Wiring a House, 4th edition:

Page 1

Page 2

Let's brake this down: Basically, the two outside connectors are 120V power taps that when connected together, you get 240V. But when either one of them is connected to the Centeral Tap, you only get 120V. Again, this is really complicated, and you should really ask a qualified electrician for any detailed explanations, not a 16 year old child who was barely able to wrap his head around this. And don't you dare start talking about different power phases...

  • This doesn't really answer the querent's question... – ThreePhaseEel Jul 26 '17 at 11:47

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