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Thread: appliance grounds and two-pronged outlets

  1. #1

    Default appliance grounds and two-pronged outlets

    Most of the outlets in my 1920s era house are still wired with the original system of two wires in a flexible metal conduit terminating in a metal box. The outlets have two slots to accept a two-pronged plug. T

    All circuits run back to a modern breaker in the basement that is about six years old.

    My tester indicates that the center screw (and the metal box itself) in these outlets is a ground. I assume this means the conduit itself acts as a ground back to the panel.

    If I want to run an appliance with a three-pronged plug, such as an air conditioner, is it safe to use one of these outlets with a two-prong to three-prong adapter with a ground that attaches to the center screw?

    If this is legit, could I take it one step further and install a three pronged outlet in place of the original two-prong and run a wire from the the grounded (green) screw on the three-pronged outlet to the metal box to create a legitimate ground?

    I'd appreciate any thoughts, advice or warnings about this.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default outlet

    IT would be safer to install a three prong outlet as long as you ensure that the BX cable is providing a ground. But if it doesn't then even a 2 to 3 prong adapter will be ineffective.

  3. #3
    Electrician Chris75's Avatar
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    There are a few appliances that would be in violation to use a 2 wire receptacle,

    250.114 (3)
    a. refrigerators, freezers, and air conditioners
    b. clothes-washing, clothes-drying, dish-washing machines, kitchen waste disposals, sump pumps, aquarium equipment.
    these are your basic appliances that must have a EGC.

    As far as the BX, if it is pre 1950's then it cannot be used as a ground. and you cannot just replace the 2 wire receptacles with GFCI's either... they must be re-wired to meet code.

  4. #4

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    Quote Originally Posted by Chris75 View Post
    As far as the BX, if it is pre 1950's then it cannot be used as a ground. and you cannot just replace the 2 wire receptacles with GFCI's either... they must be re-wired to meet code.
    What changes pre 1950's just wondering for my own house which is 1947. some one just installed 3 pronged outlets in place of the two. I am slowly changing out most of the old wiring for a number of reasons like the whole houes only had 4 circuts originally and a gfci installed on the old wiring did nothing but trip.

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    Electrician Chris75's Avatar
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    Quote Originally Posted by BigLou View Post
    What changes pre 1950's just wondering for my own house which is 1947. some one just installed 3 pronged outlets in place of the two. I am slowly changing out most of the old wiring for a number of reasons like the whole houes only had 4 circuts originally and a gfci installed on the old wiring did nothing but trip.


    No bonding wire installed inside the armor cable, this can be a fire hazard if a short did occur and the OCD did not trip.

    Thats not a ground wire in the lower picture, its a bonding wire run throughout the cable to ensure continuity of the armor.
    Last edited by Chris75; 07-09-2008 at 05:17 PM.

  6. #6

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    Interesting, but I'm a little bit confused.

    Both of these pictures show three wires inside the armor cable. Are these two variations on post-1950s armor cable -- one with a bonding wire and one without? Or is the top picture pre-1950s?

    The reason I ask is that I'm pretty sure that my armor cable has two wires inside. Both are fabric wrapped like the upper picture.

    Please clarify.

  7. #7

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    This sounds pretty half-assed, but years ago, I remember seeing someone use a two-prong to three-prong adapter with the little metal ground tab wired to a clamp on a nearby radiator.

    I think this was an attempt to provide protection for computer equipment in an apartment where the wiring was all two-prong.

    How terrible an idea is this?

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    In the Trades brownizs's Avatar
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    How hard is it for you to pull new Romex for the outlet? I still have lighting that I have not replaced the GreenField, along with the Bathroom, which feeds off of one Bedroom (next project). It is a 2 person job, but can be done.

    Also, doing a websearch on Greenfield, I came across the Army Tech manual on Electrical http://www.globalsecurity.org/milita...424/index.html

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    Electrician Chris75's Avatar
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    Quote Originally Posted by Bob in Maine View Post
    Interesting, but I'm a little bit confused.

    Both of these pictures show three wires inside the armor cable. Are these two variations on post-1950s armor cable -- one with a bonding wire and one without? Or is the top picture pre-1950s?

    The reason I ask is that I'm pretty sure that my armor cable has two wires inside. Both are fabric wrapped like the upper picture.

    Please clarify.
    Its just a picture, the top cable is a 3 conductor cable... red, black, white.

  10. #10

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    Quote Originally Posted by Bob in Maine View Post
    This sounds pretty half-assed, but years ago, I remember seeing someone use a two-prong to three-prong adapter with the little metal ground tab wired to a clamp on a nearby radiator.

    I think this was an attempt to provide protection for computer equipment in an apartment where the wiring was all two-prong.

    How terrible an idea is this?
    Actually the key word here is "Apartment". A typical landlord is not going to run new wiring, so ya got to do what ya got to do! Probably the best that can be done in that situation.

    Other than that, large computer centers where there is a lot of computer equipment will be quite picky about their grounds. They even have separate "orange" colored outlets which have an "isolated ground". These may also be connected to a backup power source and the electricity "filtered/conditioned".

    For a home the best thing is a separate circuit run back to the main panel and a surge protector/power strip.

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