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Thread: Ground wire size

  1. #1
    Retired tool & Die and Mechanic Giles's Avatar
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    Default Ground wire size

    Over the years, I have noticed that on some romex type wire and some extension cords have a ground wire that is smaller then the load wires.
    I always thought it was correct to have the same gauge wires.
    When is a smaller ground wire ok?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The ground wire is there for safety...not for normal current flow. Current flow is designed to go from the hot to the neutral. If, for some reason, there is a fault, the ground provides a path that is sufficient to blow the fuse or trip the CB...therefore, any current in the ground is only there a very short time, so it shouldn't heat up like a larger wire could, and is sufficient. For this reason, you can save some money by using a smaller wire there, and most places do.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Test, Don't Guess! cacher_chick's Avatar
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    An equipment grounding conductor is sized based on the over-current protection installed on the circuit. 15A circuit=14ga, 20A=12 ga, 30-60A=10ga, 70-100A= 8ga, 110-200A= 6ga. The grounding conductor may be larger than this minimum requirement, but should not be smaller.

  4. #4
    DIY Member TWEAK's Avatar
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    NEC and most all local codes allow a smaller equipment ground for most general circuits. IIRC, there are some exceptions in the NEC where you can't do this. But for the most part it's allowed and perfectly safe. Most current Romex I get (southwire) has the same size ground as the conductors, though - but I've torn out a lot that had a smaller equipment ground. When running THHN/THWN I always take advantage of the code and run smaller ground... with the cost of copper it really saves money, and is perfectly safe. There's no reason I can see for ever running an equipment ground that's larger than the current carrying conductors.

  5. #5
    Licensed Electrical Contractor Speedy Petey's Avatar
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    The question was WHEN was the reduced ground NM cable used or allowed. I for one do not know for sure, but it was definitely popular in the 50's and 60's. I think once they realized that a #16 ground was not adequate for a 20A or 30A circuit that is when they went to full size grounds for #14 thru #10 cables.
    Most likely early 70's?

  6. #6
    Electrical Contractor/Instructor jwelectric's Avatar
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    The 1962 cycle in Section 250-95 a 20 amp circuit was the lowest listed with a number 16 as the equipment grounding conductor. By the 1968 cycle a 15 amp overcurrent device required a 14 for equipment grounding

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