New 220 4 wire stove and old 220 three wire plug

Discussion in 'Electrical Forum discussion & Blog' started by Wardsweb, Mar 17, 2008.

  1. Wardsweb

    Wardsweb New Member

    Messages:
    9
    Location:
    San Antonio TX
    I have started a kitchen remodel where we are replacing all the appliances. The old double ovens were 220 with a three prong plug. The new ovens are four wire. Can the four wire be wired wired into a three prong plug and use a grounding strap on the neutral at the ovens? Do I need to have the house rewired with four wires back to the mains?
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    If you have 3 prong outlets existing, then generally you can plug a new stove into that. The installation manual for the stove would give details.

    If you do any renovation, including just moving those receptacles, you would most likely be required to update to current code.
  3. Chris75

    Chris75 Electrician

    Messages:
    608
    Location:
    Litchfield, CT
    You should have an electrician look at them. There are certain conditions that must be met to keep them.
  4. Bob NH

    Bob NH In the Trades

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    3,317
    Location:
    New Hampshire
    For existing branch circuit installations there is an Exception to NEC 250.140 that permits you to use a 3-wire cord and to connect the frame of the range to the grounded conductor (the neutral) if all of the following conditions are met:

    1. It is a 120/240-volt single phase 3-wire, or a 208Y/120-volt derived from 3-phase Y system.

    2. The grounded conductor (neutral) is not smaller than #10 copper or #8 aluminum.

    3. The grounded conductor is insulated, or if not insulated is part of a Type SE service-entrance cable and the branch circuit originates at the service panel.

    4. Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.
  5. Wardsweb

    Wardsweb New Member

    Messages:
    9
    Location:
    San Antonio TX
    Thanks for the info. The house was built in 1965 and the electrical is original. The three prong receptacle has a red, a black and a green wire dedicated back to a dual breaker at the main entrance panel on the side of the house.
  6. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,540
    Location:
    North Carolina

    Huston we have a problem.

    The grounded conductor must be white
  7. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,718
    Location:
    Central Florida
    I think the question here is not whether he can use the (existing) neutral as a ground, but rather whether he can use the (existing) ground as a neutral. Having said that, I'm not enough of a Code scholar to say yes or no, although it strikes me that the end result is electrically, if not artistically, identical.
  8. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The conditions of the exception to 250.140 that are relevant to the third wire are:

    2. The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.

    3. The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.

    The fact that the third conductor is reported as green indicates that it meets the first option of condition 3, which is sufficient for the exception.

    It is likely that the wire available in 1965 was either the SE cable with red, black, and spiral-wrapped grounded conductor, or the NM that was red, black, and green, or NM that was black, white, and green, or NM that was black, white, and bare.

    The green wire is obviously insulated and appears to comply with the provisions of the exception which says nothing about color.
    Last edited: Mar 18, 2008
  9. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    Bob, in your example, #3 refers to the grounded conductor. NOT the grounding conductor.
    The grounded conductor is a neutral, NOT a ground.

    The ONLY bare grounded conductor allowed in this exception was one that was part of an SE cable.

    The grounding conductor, or "ground", of an NM cable, insulated or not, was NEVER allowed to be used as a grounded conductor (neutral).
  10. kd

    kd New Member

    Messages:
    207
    Wrap white tape on each end of the green wire.
  11. Wardsweb

    Wardsweb New Member

    Messages:
    9
    Location:
    San Antonio TX
    Wow I now realize I know squat about this stuff. I think my best bet would be to have an electrician come in and wire it to code. At least that way, I know it will be right and compliant with the latest safety measures.

    Thanks again guys, ya'll amaze me.

    It's one thing to know and another to know that you don't know.
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,156
    Location:
    New England
    All stoves built today are built to use a 4-wire hookup: two hots, a neutral, and a safety ground. Neutral and ground are connected back at or near the panel. It is safest if there is a real ground at the appliance, rather than the neutral which could get loose, or come off, or be broken accidentally. That way, you really do have a margin of safety.
  13. enosez

    enosez Member

    Messages:
    88
    Location:
    Long Island NY
    Not to start anything, but why would a nuetral have any more of a chance to come loose or break than a ground? The ground on a 6/3 is #10 if not mistaken.

    My understanding was that in the past, before all of the bells and whistles on stoves, neutrals were not needed. It was a 220volt. Now that a 110 circuit is required in some new stoves, a neutral is needed.
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,156
    Location:
    New England
    How many stoves do you know of that don't have either a lamp, or an accessory plug? Those use 110vac, so require the neutral. Only the heating elements are 220vac, not any of the peripherals. Having both a neutral and a ground gives some redundency and safety. If you lost a neutral, you might blow up some of the electronics or the bulb, depending on how it was wired since it might see 220, but...
  15. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

    Messages:
    1,328
    Location:
    Columbus, OH
    Well, back in the day they might have not had a plug or an acessory outlet. If it had a bulb, it could have been rated for 240.... just a thought.
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,156
    Location:
    New England
    Ever tried to buy a 240v light bulb...not in your everyday store! If they had a bulb, oven light, night light, indicator light, etc., they were powered off 110. When I was in the Army, we had one shelter with both 28vdc and 110vac lighting. The bulbs were the same style. If you weren't careful, the 28V bulbs work great as flash bulbs! Working the other way, they worked more like a candle, which isn't catastrophic, but kind of useless.
  17. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    Notwithstanding all "The sky will fall on your head" remarks, the National Electrical Code explicitly permits, for existing branch circuit installations, "the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlets or junction boxes that are part of the circuit for these appliances" to be grounded to the grounded circuit conductor if all of the conditions that I posted earlier are met.
  18. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,540
    Location:
    North Carolina
    A couple of red flags come to surface with this installation and the insulated GREEN conductor raises a flag to its highest.

    If this circuit installation has an insulated green conductor it must be wired with some type of cord. I all my years I can’t remember seeing a SE cable or NM cable that had an insulated green conductor that came installed in the cable. The use of a cord as a permanent wiring method is a big no no.

    If this installation is done in some sort of metal jacketed cable such as MC or AC then the use of a three wire receptacle is NOT allowed.

    In the end the one thing to remember is that if a three wire receptacle is used then any and all exposed metal of the range will be at the same potential as the conductor carrying the unbalanced load.
    In other words the exposed metal will be at a potential of 120 volts anytime that the clock or oven light is energized.
    Last edited: Mar 19, 2008
  19. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    That (the bolded part of the quote) is just plain WRONG. It will be at a potential equal to the voltage drop in the ungrounded conductor that arises from the IR drop of the unbalanced current, which is the same condition that has existed on ranges, ovens, and dryers for as long as they have had lights and clocks.

    With 1 Amp which would be typical of a light, and a 50 ft run to the panel, the potential will be about 0.06 Volt, about 1/25 of the voltage of an alkaline battery. Even if the there were a 100 Amp dead short to the grounded conductor, which would quickly trip the breaker, the voltage would be about 6 Volts which is far below the hazardous level.
  20. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,540
    Location:
    North Carolina
    It has been a long day trying to bring my current classes up to the 08 code cycle.

    The touch potential would put the person in parallel with the clock and or light. In other words the person could become a parallel path with the current these items draw.

    Touching the frame of the range and such items as grounded crock pots made from stainless steel to large bowl mixers could lead to a shocking experience.

    Thanks Bob for keeping me straight.
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