4-Wire 120/240 vs 3-wire 120/240 Please reply

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Old 03-18-13, 09:47 PM
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4-Wire 120/240 vs 3-wire 120/240 Please reply

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This thread concerns me. I always thought in older homes, you could hook up a new oven to exisiting wiring. 50amp range cords are mostly sold with three prongs, I rarely see a four prong plug. Most of the major department stores sell appliances and (at a cost) install them with 3 prong plugs to existing 240 outlet.
Is there something wrong with this? I understand it's ideal to have a 4 wire config, but I didn't think it was mandatory to rewire.
In fact, at the wiring boxes for new ranges, the terminal for the neutral is bonded to the chassis and you have to change this for four wire. So it seems they are most likely expecting you to use a 3 wire power supply.
And the real concern: If three wires are allowed, whats the difference if the 240 is running over an insulated white neutral within say 8/3 or a bare ground running within say 8/2 with ground (which is technically insulated within cable, but not inside panel)????
I ask this because I have seen houses where the 240 was ran to a J box in the attic with 2 hots/ground, then to oven with 8/3 or 8/3 with ground. The average person would not think to look for a J box and assume the wiring at the oven outlet was correct. Is this J box configuration a major screw up??
 
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Old 03-18-13, 10:10 PM
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Just to be clear both 3-wire and four wire 120/240 use 3 conductor cable. With 3-wire it is ungrounded 3-conductor cable. With 4-wire it is 3-conductor with ground. Use of two conductor cable hasn't been code compliant since somewhere around WWII*. In a 3- wire the ground and neutral are combined. This though can in some cases make any othe apliance with a grounded metal case hot to ground. Touch the kitchen faucet and the refrigerator and zap you just provided an alternate path for any 120 load on the stove. If it should loose the neutral you are the only path.

*Use of 2-conductor cable means using the bare ground wire as a neutral. Bare neutrals haven't been code compliant for a very long time. Maybe one of the pros knows how long.
 
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Old 03-18-13, 10:23 PM
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I always thought in older homes, you could hook up a new oven to exisiting wiring. 50amp range cords are mostly sold with three prongs, I rarely see a four prong plug. Most of the major department stores sell appliances and (at a cost) install them with 3 prong plugs to existing 240 outlet.
IMX, if an appliance will work with a 3-wire circuit, the cord for that is often a separate purchase, to be attached to the appliance to replace the 4-wire cord it came with. In addition, the OP asking about the supply for his Bosch oven does not have a cord-and-plug attached appliance. His oven has whip which is to be spliced to the supply wires in a J-box.

The installation instructions for that oven specify either a 4-wire service or a 3-wire service with an insulated ground. He didn't have either of those. Not providing one of those would probably result in the oven's not working properly, would likely void the warranty on the appliance, and would not be in compliance with Section 110.3(B) of the NEC. Attempting to use the bare ground in the existing cabling to supply both neutral and ground would violate many more sections and likely create a hazard. We don't advise doing that.

Is there something wrong with this? I understand it's ideal to have a 4 wire config, but I didn't think it was mandatory to rewire.
There's nothing wrong with connecting a 240V appliance to a 3-wire circuit if that circuit is already in place and the appliance is designed to work with that. Rewiring to provide a 4-wire supply is not mandated. However, if the circuit wiring must be modified for any reason, then only a 4-wire service may be installed under current code in most jurisdictions.

In fact, at the wiring boxes for new ranges, the terminal for the neutral is bonded to the chassis and you have to change this for four wire. So it seems they are most likely expecting you to use a 3 wire power supply.
We replaced our range a couple of years ago. That wasn't true for it. The existing 4-wire run for our range dates from when the house was built, in 1989.

And the real concern: If three wires are allowed, whats the difference if the 240 is running over an insulated white neutral within say 8/3 or a bare ground running within say 8/2 with ground (which is technically insulated within cable, but not inside panel)????
This is a real concern. The bare or insulated EGC is not a neutral conductor and should not be used as one. The EGC is an emergency drain to earth, and should only carry live potential in the event of a fault. The insulated neutral, OTOH, carries the difference between the two hot legs whenever the appliance is in operation and, more importantly, carries the "other half" of the 120V loads, such as the controls.

I ask this because I have seen houses where the 240 was ran to a J box in the attic with 2 hots/ground, then to oven with 8/3 or 8/3 with ground. The average person would not think to look for a J box and assume the wiring at the oven outlet was correct. Is this J box configuration a major screw up??
Yes.
 
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Old 03-18-13, 10:37 PM
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Use of 2-conductor cable means using the bare ground wire as a neutral
This is exactly what I saw and what concerns me. The person hooking up the oven would not know the ground is being used as neutral, because of the J box in the attic. The 240V outlet box for oven looks correctly wired. (correctly being 2 hots and neutral).
Do I need to call this person and tell them to immediately stop using oven, shut off breaker and wait until I can make it there with an electrician? It's over 100 miles away and I cannot go there for a couple of days. This is not really my responsibility, I was working on something else, but I need to relate this info to them. I also need it for myself, because until I understand this, I don't understand electric (and I thought I did). I guess I do understand it a little, you need a separate ground, but then why would they let you hook up an oven to a 3 conducter cable (2 conductor/1 grounded conductor), if it was very dangerous?
 
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Old 03-18-13, 10:43 PM
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Do I need to call this person and tell them to immediately stop using oven, shut off breaker and wait until I can make it there with an electrician?
IDK. How long has it been wired this way?

why would they let you hook up an oven to a 3 conducter cable (2 conductor/1 grounded conductor), if it was very dangerous?
It isn't. The grounded conductor is the neutral. Don't confuse that with the (bare or green insulated) grounding conductor. And don't connect, join or bond those two beyond the service entrance.
 
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Old 03-18-13, 11:05 PM
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IDK. How long has it been wired this way?
It's looks like original wiring, no signs of mods.

It isn't. The grounded conductor is the neutral. Don't confuse that with the (bare or green insulated) grounding conductor. And don't connect, join or bond those two beyond the service entrance.
I'm confusing things. I was trying to address a safety issue and learn at the same time. Let me make it clear:
This guys oven is connected to a standard 50 amp range outlet, three prong.
Only problem is the white neutral at outlet doesn't go to panel, it actually goes to a J box it the attic, which goes to panel as bare ground.

So from the other direction, it's 40 amp breaker at panel, white hot and black hot, ground to J box in attic. From there, it's 8/3 w/g to oven 50amp outlet.

The reason I complicated things is because the bare ground going from j box in attic to panel would serve the same purpose as if it was a white insulated cable, they would go to the same bar in the panel. Only difference is it's not insulated. (I know it's wrong but I'm reaching here)
 
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Old 03-18-13, 11:16 PM
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It needs to be changed. At a minimum, as you realize, the neutral needs to be wired as an insulated neutral all the way.

The proper and safe way to do it is to replace everything with 4-wire, 3-conductor cabling. That way the insulated neutral can function as it should and there should never be any power on the bare or insulated ground unless there's a fault.

Good catch, BTW.
 
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Old 03-18-13, 11:25 PM
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Thanks, I'll call the guy and let him know. His panel is pretty accessable, on the outside of the garage wall. It just gets me though that this is the way the house was originally wired.
 
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Old 03-19-13, 12:01 AM
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His panel is pretty accessable, on the outside of the garage wall.
Does he have conductors pulled in conduit or cables for his branch circuits?
 
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Old 03-19-13, 06:14 AM
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It's cable, What I meant by on the outside of the garage wall is just the location. It's not actually mounted outside the wall. It's within studs, stucco on outside of house and drywall inside garage. If need be, drywall can be cut and patched to access knockouts and pull new cable.
Thanks
BTW House was probably built around 78, has copper wire
 

Last edited by Handyone; 03-19-13 at 06:34 AM. Reason: Year Built
  #11  
Old 03-19-13, 06:56 AM
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Sounds like a bit of a job, but definitely a doable one.
 
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