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Ground Wire Sizing

  1. May 8, 2015 #1
    I am looking for the rule of thumb when selecting a wire gage for ground wires on electrical circuits. Currently we use 18 GA for all of circuitry regardless of the amperage of the circuit-from less than 1A to over 30A we use 18 GA. We have never had an issue but I am wondering why 18 GA was selected and not something smaller assuming that is there is a short the wire will only carry the load for a very short time before tripping a circuit breaker.

    Thanks
     
  2. jcsd
  3. May 8, 2015 #2

    berkeman

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    Is this for AC Mains power distribution? If so, your local building codes and the National Electric Code (NEC) will give you the required values.
     
  4. May 8, 2015 #3
    No house or business wiring...These circuits are used in industrial heaters. Composed of NiCr resistance wires, Mica, and then the standard TGGT or MGT wire sized to handle the load of the heater-which is based on the wattage of the heater.
     
  5. May 8, 2015 #4

    Averagesupernova

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    Personally if I were designing a product that utilized a ground wire for carrying fault current that would ultimately lead to a breaker tripping or fuse blowing I would size it based on the NEC. Here in the USA we have appliances that have #16 wire in the cord that can be plugged into a 20 AMP outlet, so there is a case where all the conductors are undersized in a fault condition. It seems a bit silly to have a ground wire larger than the size of the source wires of the appliance but I would not rule it out. One way to look at it is any time a wire size is reduced it will technically take slightly longer for the breaker to trip in a fault condition.
     
  6. May 8, 2015 #5

    jim hardy

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    The requirement is that it be sized to trip a protective device(breaker) and to allow no dangerous voltage(maximum of 30 volts) from ground faults.
    .

    To that end your country's electrical code will specify minimum sizes.
    Generally it must be large enough to carry the current allowed by the protective device's trip setting. An exemption exists for household cords in portable equipment where 18 gage is okay for 15 amp circuits.

    If it's like our country's code it seems cryptic at first, but the logic behind it is is rigorous. Sometimes painfully so.
    People make a living explaining it for other people.
    https://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/Grounding-Part-1-of-12~20041005.php
    http://www.mikeholt.com/mojonewsarc...ding-versus-Bonding-Part-8-of-12~20050513.php

    I'm no code expert . Try a search on NEC 250-122
     
  7. May 9, 2015 #6

    FOIWATER

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    Yeah it depends on if we are talking about grounds for say, branch circuits, or maybe main panel grounds, for example.

    If its a main panel ground, it has to be able to carry the rated current for the panel. since a fault could occur as far back as that (if a fault occurred between a bus bar and the panel, say)

    If its a fault in a branch, then it has to be sized based on the rating of that circuit (so, the breaker).

    For the states here is an excerpt from the NEC which gives corresponding ground conductor sizes based on circuit capacities.

    https://www.industry.usa.siemens.co...ducts/Documents/Grounding_Conductors_0507.PDF

    They are usually sized based on the fact that ground impedance is typically determined to present some small value of impedance by code and the ampacity is determined from that, since the ground is the only current limiting property if we don't consider conductor resistance and assume all connections are made tight. This is because the fault current travels through the earth outside the occupancy to reach the distribution transformer center tap. That is why the main ground for the panel has to be connected to the water main in case of residency. I used to work as an electrician before I became an engineer, and I often saw main panel grounds attached to the water main right above or as close to the panel as possible. That is a pretty serious issue, since if a section of the water main is removed for maintenance on some plumbing, you no longer have a grounded electrical system. So it is very important that the main ground is attached as soon as the water main enters the house below the main cut off and a few inches from the entrance.
     
  8. May 9, 2015 #7

    jim hardy

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    That is the important and oft misunderstood point.
    Current must get back to the transformer winding from whence it came.
    Its intended path is via the "neutral" wire, or in 230 volt equipment via the other "hot" wire.
    Should a fault occur, now its intended alternate path is via the green wire. That wire is really a "Bonding" conductor but is usually called "Ground". That's a misnomer. If the current has to resort to earth to get back we have a wiring error and code violation.

    There's a subtle quirk of terminology at work here, and it stems from the unfortunate dual use of the word "Ground" .
    "Ground" means earth, as in where our feet are planted.
    "Ground" is mis-used to mean "Power Supply Return", and that mis-use is almost ubiquitous.
    In NEC there's a distinction between Bonding and Grounding.
    The purpose of the green wire is to provide a low impedance path back to the transformer neutral that does not include earth.
    That is called Bonding.
    Bonding ties equipment together so that a path back to the transformer winding from which current came, consisting of all metal conductor, will exist.

    The green wire provides that bond. Metal conduit or cable tray can also be used with 'approved' connectors.
    The point where all the green wires join up with the transformer center tap is in the breaker panel.
    That point is earthed, and connected to the big incoming neutral wire from utility "pole-pig" transformer's center tap. That neutral is the bare wire from your pole around which the two insulated ones are draped.
    Electrical_Service590-DFss.jpg
    tnis photo courtesy http://inspectapedia.com/electric/Electrical_SEC_Sizes_Amps.php

    Observe in this picture that fault current from "Load" can make it back to Utility Power centertap X0 without using earth.


    That point is tied to earth at the meter, which is usually right at the entrance to that panel.
    We used to use the incoming water pipe but with the popularity of PVC pipe we now drive a grounding electrode, usually a copper coated steel rod, at the service entrance.
    The reason for that is to keep high voltages, arising from either lightning or a primary to secondary short in utility transformer, out of the house



    Without the bonding conductor , 'Ground" would become a voltage divider and might not even trip the circuit breaker

    touch2.gif

    It's important to have awareness of the distinction between "Grounding" and "Bonding".

    Bonding ties equipment together.
    Grounding ties equipment to earth.

    Unfortunately we use those two different terms as if they were interchangeable.
    That''s a language problem not an electrical one.

    ps those drawings courtesy of Mike Holt Enterprises, a handy link is here: http://forums.mikeholt.com/showthread.php?t=95495


    see also http://ecmweb.com/code-basics/grounding-and-bonding-part-1-3
     
    Last edited by a moderator: Apr 17, 2017
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