4-Wire 120/240 vs 3-wire 120/240 Please reply
#1
4-Wire 120/240 vs 3-wire 120/240 Please reply
Separated From http://www.doityourself.com/forum/el...oven-help.html
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??
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??
#2
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.
*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.
#3
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.
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.
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??
#4
Use of 2-conductor cable means using the bare ground wire as a 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?
#5
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?
why would they let you hook up an oven to a 3 conducter cable (2 conductor/1 grounded conductor), if it was very dangerous?
#6
IDK. How long has it been wired this way?
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.
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)
#7
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.
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.

#8
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.
#10
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
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