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Thread: Do I need a bigger ground wire?

  1. #11
    Join Date
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    Expanding a little on what kewired said:

    For what it is worth, the NEC would be perfectly happy with the ungrounded conductors not being oversized in the first place, which would make the trip time even worse.
    Another argument is that the voltage divider effect before the OCPD trips will make the momentary voltage on grounded surfaces higher than it would be if the EGC is also oversized. But since it would be at least one half of the line voltage anyway, I do not see that as particularly important.
    If you take as given that a size increase for VD will only be done if the VD would otherwise be more than 5%, then the need to upsize the EGC to maintain low trip time does make sense.
    If you upsize to reduce 2% VD to 1% VD, then the motivation to upsize the EGC iscbmnot nearly as strong.

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  2. #12
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    Dec 2007
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    Quote Originally Posted by GoldDigger View Post
    Expanding a little on what kewired said:

    For what it is worth, the NEC would be perfectly happy with the ungrounded conductors not being oversized in the first place, which would make the trip time even worse.
    Another argument is that the voltage divider effect before the OCPD trips will make the momentary voltage on grounded surfaces higher than it would be if the EGC is also oversized. But since it would be at least one half of the line voltage anyway, I do not see that as particularly important.
    If you take as given that a size increase for VD will only be done if the VD would otherwise be more than 5%, then the need to upsize the EGC to maintain low trip time does make sense.
    If you upsize to reduce 2% VD to 1% VD, then the motivation to upsize the EGC iscbmnot nearly as strong.

    Sent from my XT1585 using Tapatalk
    I see long runs on irrigation installation where even if EGC is compliant with the rules, there is still enough resistance that fault current is limited. Seen several instances where fuses never blow where they would have if long circuit lengths weren't involved. But this is often further complicated by the fact that 480/277 can sustain an arc fairly easily and that arc is also current limiting. Bolted faults should still blow the fuse, just may take a little longer than if there wasn't long conductor run.
    I live for today, I'm just a day behind.

  3. #13
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    Quote Originally Posted by kwired View Post
    It is not a perfect rule either, it is the best they could come up with for a one size fits all type of rule. Some installations it makes sense, others it is overkill.

    Some installations where conductor isn't "upsized" but is a long run, you will have much slower response of OCPD if there is a short circuit or ground fault even though you are in compliance with conductor sizing - resistance of conductor is going to be current limiting.
    True and you touch on a point that almost everyone overlooks. I've had some interesting conversations over the note at the bottom of table 250.122. The table provides the minimum EGC size conductor, which may not be the size of the conductor that will do the job. The note says, "Where necessary to comply with 250.4(A)(5) or (B)(4), the equipment grounding conductor shall be sized larger than given in this table." Section 250.4 gives the general requirements the fault current path has to meet, and if the EGC size given in table 250.122 will not meet the requirements, such as due to a long run, the conductor is supposed to be upsized. But the NEC does not provide any rule of thumb, plug and chug, way to calculate the size so most people just take the size from 250.122 and call it a day. I can't really blame anyone for that either since calculating the correct upsize can be complex. I'm not sure what the sizes given in table 250.122 are even based on.

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