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Old 01-30-2019, 05:27 PM   #1
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4 Wire Cooktop on 3 Wire Power Supply


Hi guys,

I know similar topics have already been posted:
(list of some of the threads I've been reading below)

...but I'm still a little confused and not sure how to continue. My wife picked up a used Kenmore glass cooktop, model 790.42732401.

I don't know much about our old cooktop, part of the paper on the bottom is missing and I can't find any model numbers. It's an old Westinghouse coil cooktop and it had three wires, black, red, and bare. The power supply wire going into the junction box has three wires, black, red, and white. It's a dedicated circuit going to a 40 amp double-breaker in the circuit box. Before I disconnected the old cooktop, the wiring was (cooktop to power supply) black to black, red to red, bare to white.

The Kenmore glass cooktop has 4 wires, Black, Red, White, and Green. I looked up the installation guide:
(link below)

On page 4, there are instructions for hooking the 4-wire cooktop to a 3-wire junction box (image attached.) Basically, hook But some of the other threads on here would lead me to think this is a very bad idea. If I'm reading everything right though, I should be ok since my power supply has a white, insulated cable rather than bare wire.

What are the risks in hooking up the cooktop as directed from the attached diagram? Would I be in any more of a dangerous situation than I was with the old cooktop? Since the top is glass with no real exposed metal, is there any real shock hazard?

I'm also not sure what local codes would be involved (Newport News, Virginia).

Thank you very kindly for your help!
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Old 01-30-2019, 05:32 PM   #2
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Re: 4 Wire Cooktop on 3 Wire Power Supply


Well I was going to post links to other related threads and the installation guide, but apparently as a newbie I can't.
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Old 01-30-2019, 05:55 PM   #3
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Re: 4 Wire Cooktop on 3 Wire Power Supply


For many years dryers and ranges were wired with a 3 wire supply -- Black, Red, White. Inside the appliance a jumper ran between the white wire and the metal frame. This was totally code compliant back then. When the electric code was changed requiring 4 wire supplies, the existing 3 wire supplies were grandfathered. It is still acceptable by code to connect a 4 wire appliance using a 3 wire supply. Most appliances are installed with a plug and cord and installers carry both and use the one compatible with the existing receptacle.

In your case you have a junction box instead of a receptacle and plug. What that diagram is showing is to connect black to black, red to red, and white & green together to the insulated white supply.

Considering it is still code compliant to use existing 3 wire supplies, I see no problem with it. My range and dryer are both using 3 wire cords and I feel safe using them.
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Old 01-30-2019, 07:51 PM   #4
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Re: 4 Wire Cooktop on 3 Wire Power Supply


Related threads I've been reading:
https://www.doityourself.com/forum/e...-3-4-wire.html
https://www.diychatroom.com/f18/hook...service-76518/
https://www.diychatroom.com/f18/repl...ooktop-184343/
https://www.diychatroom.com/f18/wiri...-wires-109217/

Kenmore Cooktop Installation Guide:
https://www.searspartsdirect.com/par...e-parts-manual
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Old 01-30-2019, 11:01 PM   #5
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Re: 4 Wire Cooktop on 3 Wire Power Supply


If your old stove has red,black and white.
These are hot1, hot2 and neutral.
Using a bare wire ( ground ) as a neutral is not code in many places.
You should check which code covers your area,
And check what applies.
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Old 01-31-2019, 11:09 AM   #6
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Re: 4 Wire Cooktop on 3 Wire Power Supply


Quote:
Originally Posted by dmxtothemax View Post
If your old stove has red,black and white.
These are hot1, hot2 and neutral.
Using a bare wire ( ground ) as a neutral is not code in many places.
You should check which code covers your area,
And check what applies.
As I said in my post, the old stove had black, red, and bare. The wires coming from the house are black, red, and white.

Was the old stove using the bare wire as a neutral? Or is it possible, that the red and black cables are not both hot, but hot and neutral instead? In that case, is it possible that I don't have enough power to run the new cooktop anyway? I don't think this is the case, because it's going to a 40 amp double breaker.

Considering the old cooktop was using 3 wires, and the "neutral" was bare, wouldn't continuing to use the old cooktop be every bit as dangerous as using the new one? And the old cooktop is all metal, where the new cooktop is glass. I would think that if there was a wiring fault, in the worst case scenario the metal chassis on the bottom would be hot, but the stove would fail to turn on to begin with. So I'd be able to tell that something was wrong, and would certainly turn off the breaker before doing anything that would put me in contact with that metal chassis.

Somebody PLEASE correct me if I'm wrong, but I believe with the NEC my wiring is "grandfathered in" and I should be able to hook the 4 wire cooktop to the 3 wire supply, as long as I'm not running a new circuit the 3 wires is fine. I have no idea what local regulations come into play, I did state that I live in Newport News, Virginia, but local codes don't change the fundamentals of how electricity works and what's safe and what's not.

I don't have the expertise to run a new 4 wire circuit, so right now I have two options. Either hook the old cooktop back up, or hook up the new cooktop. And if I'm understanding everything correctly (which I very well might not be, that's why I'm asking for help!) I don't think hooking up the old cooktop would be any safer than hooking up the new cooktop, and in fact might be more dangerous.
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Old 01-31-2019, 11:28 AM   #7
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Re: 4 Wire Cooktop on 3 Wire Power Supply


It is possible that the old stove did not use a neutral. It may have only had two hots and a ground.
Your old three wire setup is fine as long as you don't alter it.
Your new stove should have directions for connecting to a three wire supply. It usually involves some sort of jumper between the ground and the neutral inside the stove and swapping the four wire cord for a three wire cord.
You should verify the connection at the panel for the white wire.
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Old 01-31-2019, 12:36 PM   #8
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Re: 4 Wire Cooktop on 3 Wire Power Supply


There was a period of time where using SEU cable to feed the electric range was acceptable and permitted by code. I run into this in homes typically built in the 1940s - 1960s. Back when a 60 amp service was standard, the electrician would use the same cable for the service entrance and range. Based on your description this appears to be what you have. SEU cable has a bare neutral conductor with its strands wrap around the 2 insulated conductors and is still used today for service entrances.

To answer you question, you are correct. Your setup is grandfathered by code and is acceptable to continue using. Your new cooktop has a diagram on how to hook it up to a 3-wire supply.

If the bare neutral in the junction box is a concern then swap the junction box for a plastic one.

Just a side note...on a range, oven, or cooktop the neutral is only used by the controls and clock which operate at 120 volts. The heating elements only use the 2 hots as they are 240 volts.
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Old 01-31-2019, 12:39 PM   #9
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Re: 4 Wire Cooktop on 3 Wire Power Supply


Quote:
Originally Posted by joed View Post
It is possible that the old stove did not use a neutral. It may have only had two hots and a ground.
Your old three wire setup is fine as long as you don't alter it.
Your new stove should have directions for connecting to a three wire supply. It usually involves some sort of jumper between the ground and the neutral inside the stove and swapping the four wire cord for a three wire cord.
You should verify the connection at the panel for the white wire.
Right, the picture in my post came from those directions. I will have to verify the connection at the panel, to be honest I'm not sure exactly what I'll be looking at though.

I was mainly inquiring about the safety of such a setup. Obviously they changed the code for a reason, but I don't see why it would be any more dangerous than my old cooktop.
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Old 01-31-2019, 12:42 PM   #10
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Re: 4 Wire Cooktop on 3 Wire Power Supply


Quote:
Originally Posted by BRodgers View Post
There was a period of time where using SEU cable to feed the electric range was acceptable and permitted by code. I run into this in homes typically built in the 1940s - 1960s. Back when a 60 amp service was standard, the electrician would use the same cable for the service entrance and range. Based on your description this appears to be what you have. SEU cable has a bare neutral conductor with its strands wrap around the 2 insulated conductors and is still used today for service entrances.
...
To answer you question, you are correct. Your setup is grandfathered by code and is acceptable to continue using. Your new cooktop has a diagram on how to hook it up to a 3-wire supply.

If the bare neutral in the junction box is a concern then swap the junction box for a plastic one.
No, there is no bare neutral in the junction box. The only bare neutral was on the old cooktop. The junction box has three insulated wires, Black, Red, and White.
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Old 01-31-2019, 03:31 PM   #11
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Re: 4 Wire Cooktop on 3 Wire Power Supply


Three insulated wires is good. There were setups that used a bare ground. Those were not correct as I understand. Three wires always required three insulated wires. The insulated neutral could be used as ground, but a bare ground was never allowed to be used a neutral.
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Old 01-31-2019, 10:34 PM   #12
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Re: 4 Wire Cooktop on 3 Wire Power Supply


Alright, I took a look at the breaker panel to verify the white cable like @joed said. Pictures are attached. The cooktop circuit goes to the top left pair of breakers. The white cable appears to be plugged into the neutral bus bar, behind the black and red wires that are plugged into the breakers, so I should be good to go, right?

Thanks for all the help!
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4 Wire Cooktop on 3 Wire Power Supply-20190131_225633.jpg   4 Wire Cooktop on 3 Wire Power Supply-20190131_225644.jpg   4 Wire Cooktop on 3 Wire Power Supply-20190131_225717.jpg  
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Old 01-31-2019, 11:11 PM   #13
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Re: 4 Wire Cooktop on 3 Wire Power Supply


As long as your new cook top requires a 40 amp breaker you are good to go. Sometimes cook tops require a smaller breaker than the standard 40 amp for a full range.
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Old 02-01-2019, 06:46 AM   #14
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Re: 4 Wire Cooktop on 3 Wire Power Supply


Quote:
Originally Posted by joed View Post
Three insulated wires is good. There were setups that used a bare ground. Those were not correct as I understand. Three wires always required three insulated wires. The insulated neutral could be used as ground, but a bare ground was never allowed to be used a neutral.
Yes, it was, many many years ago as Brogers stated. They allowed connecting the neutral to the metal frame of the appliance and of course the neutral was connected to the ground at the panel

There's still a lot of misunderstanding out there about how neutrals and earths (bare wires) operate.

The reason code was changed is because if you have one of these appliances (range, clothes dryer, etc.) and you have a load like a clock that is fed by half of the circuit (ie: neutral-L1 at 120v) if the neutral becomes disconnected and the hots stay connected, and someone who is grounded touches the metal frame of the appliance, there will be a path from L1, through the clock, to the frame of the appliance, to the person, then to the ground the person is standing on.

Of course in reality chances that a neutral would open without the hots also opening is ridiculously small but it isn't impossible. I think they figure if there's 2 wires in the box that are earthed at the panel then chances are less that both will open.

Modern devices are constructed so that the neutral is not connected at any point to the metal frame of the appliance inside of the appliance. The only place the neutral and ground are connected together is at the panel.
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