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Thread: Sizing equipment grounding conductor for sub panel

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    Sizing equipment grounding conductor for sub panel

    Hello,

    I built a sub panel that is 200 amps 3 phase. I used 4/0 to feed the load carrying conductors and went with a 6 AWG off of table 250.122 for my equipment grounding conductor and failed my inspection. I was told that 250.122 (B) says that my equipment grounding conductor “shall be increased in size proportionately according to the circular mil area of the ungrounded conductors. I did increase my ungrounded conductors from 3/0 to 4/0. I thought I would be ok because in Table 250.122 it says it goes up to anything below 300 amps over current device. What do you think?

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    You need a larger EGC. #6 is required for #3/0 copper conductors @ 200 amps, since you went up one size you'll need to increase the size of the EGC in this case one size to a #4.

    Welcome to the Forum.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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    I agree with Rob. Look at the wording of 250.122(B). The "increased in size" sentence was recently revised to clarify that it means "increased from a size that would have been sufficient for the required ampacity." In your case, a 3/0 would have had sufficient ampacity for a 200 amp load, and you used a larger wire than that.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    I agree as I stated in my pm before the thread was started but I do understand where the op is coming from. If I only need a #6 equipment grounding conductor when I have a 250 amp breaker why do I need a larger conductor when the breaker size is smaller. It is hard to wrap your head around as this section doesn't always make sense.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
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    I believe it's has to do with making sure the OCPD operates quickly with a short circuit.

    Also, the OCP for the conductors could be increased in the future to raise the capacity.

    That it's a percentage instead of a step in the table makes me think it's the first idea.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Quote Originally Posted by Dennis Alwon View Post
    I agree as I stated in my pm before the thread was started but I do understand where the op is coming from. If I only need a #6 equipment grounding conductor when I have a 250 amp breaker why do I need a larger conductor when the breaker size is smaller. It is hard to wrap your head around as this section doesn't always make sense.
    Yep you and many others have called BS on these situations. There are many sizing situations where you get unreasonable EGC results under the existing wording. I think we are going to see a big change in this in the 2020 edition. The talk is to rewrite all this along the lines of the size of the conductors and not the size of the OCPD. Similar to 250.102(C)(1) but different values.
    If I was the inspector I would probably give them a pass.

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    Our mod Don has been proposing for years that the EGC be sized according to the circuit conductor size not the OCPD that would help to end this nonsense.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

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    Quote Originally Posted by infinity View Post
    Our mod Don has been proposing for years that the EGC be sized according to the circuit conductor size not the OCPD that would help to end this nonsense.
    Yes, it really does lead to some bizarre results in some situations. I think Don will ultimately get his way as I think this makes the most sense.

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    Quote Originally Posted by infinity View Post
    Our mod Don has been proposing for years that the EGC be sized according to the circuit conductor size not the OCPD that would help to end this nonsense.
    There would have to something in place for sizing HVAC equipment grounds, unless the wire size of grounds is made larger in general (if that makes sense). When we get up to bigger 30-40 ton units, sizing based on the conductors for HVAC equipment will give us undersized grounds.

    500 copper can, and has been fused at 450amps if the RTU calls for it. If we size the ground based on 500's max amperage at 75 degrees, it's 380 amps and we would get a #3 ground according to 250.122. I realize im mixing apples and oranges a bit here, but i don't know what the mentioned moderator has proposed as far as the new formula for sizing EGCs. Only that it should be done based off conductor size, not OCPD. As its stands today i would have to pull a #2 equipment ground for that same RTU.

    Not saying im against the change, i like the idea. Im just wondering how the more experienced guys would see a situation like this being handled by the NEC.

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    Hopefully they don't change the value on the smaller size wires as all the nm & ser cables are sized based on 250.122.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



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