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Thread: Do I need a bigger ground wire?

  1. #1
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    Do I need a bigger ground wire?

    In order to lower the voltage drop, I am increasing my conductors from size 4/0. To 350 MCM. Is it OK to leave the neutral at 4/0 and leave the ground wire at number two. All the Wires are aluminum.

  2. #2
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    You can leave the neutral wire as is.

    The EGC has to be increased in size IMO.
    Bob

  3. #3
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    You don't need any "ground wires" but you do need an EGC.

    The EGC needs to be increased in size by the same proportion as the ungrounded conductors which went from #4/0 to 350 kcmil. Once you figure out that ratio you would increase the EGC accordingly.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  4. #4
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    Looking at 250.122 the EGC is sized based on the circuit breaker. Example: for 200 amp, #2 Alum is required.
    It is not based on the size of the wire used for the ungrounded conductors.
    So even if I used 500 mcm with a 200 amp circuit breaker, I think I could still use #2 grounding. Do you all agree?
    As for the neutral, do I need to show calculations for reducing the size of the neutral, or does voltage drop not apply to the neutral?

  5. #5
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    Look at the next part of the section.

    The _first_ step is selecting an EGC based on the size of the breaker.

    _Next_ if the ungrounded conductors are increased in size (relative to what they would normally be on that breaker) then you must also increase the size of the EGC.

    We have had huge threads about just what the normal size conductor is, but if we take your opening post at face value you would normally have a 4/0 conductor on the breaker, but you are using a 350 kcmil conductor instead. So
    step 1: find the egc based on the breaker
    step 2: look up its area
    step 3: multiply this by the area ratio from 350 kcmil to 4/0 awg,
    step 4: find a conductor with this (or larger) new area

    -Jon

  6. #6
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    I stated the question incorrectly.

    I have to run wire from house to barn and well. It will be a 200 amp circuit with aluminum wire 4/0 4/0 4/0 and #2 EGC.

    I have some 350 MCM wire which I will use for the two hot wires. Since it is larger wire it will work even better and I assume that I can leave the other wires the same. Will this be OK with the inspector?

  7. #7
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    Quote Originally Posted by mlnk View Post
    I stated the question incorrectly.

    I have to run wire from house to barn and well. It will be a 200 amp circuit with aluminum wire 4/0 4/0 4/0 and #2 EGC.

    I have some 350 MCM wire which I will use for the two hot wires. Since it is larger wire it will work even better and I assume that I can leave the other wires the same. Will this be OK with the inspector?
    The inspector should cite this:

    250.122(B) Increased in Size. Where ungrounded conductors are
    increased in size
    from the minimum size that has sufficient
    ampacity for the intended installation, wire-type equipment
    grounding conductors, where installed, shall be increased in
    size proportionately according to the circular mil area of
    the ungrounded conductors
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  8. #8
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    Quote Originally Posted by mlnk View Post
    In order to lower the voltage drop, I am increasing my conductors from size 4/0. To 350 MCM. Is it OK to leave the neutral at 4/0 and leave the ground wire at number two. All the Wires are aluminum.
    How much neutral current is expected? Can be smaller than 4/0 if neutral is well balanced and never sees much load. Can never be smaller than minimum required EGC though on a feeder or min SSBJ for service conductor.
    I live for today, I'm just a day behind.

  9. #9
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    Quote Originally Posted by mlnk View Post
    I stated the question incorrectly.

    I have to run wire from house to barn and well. It will be a 200 amp circuit with aluminum wire 4/0 4/0 4/0 and #2 EGC.

    I have some 350 MCM wire which I will use for the two hot wires. Since it is larger wire it will work even better and I assume that I can leave the other wires the same. Will this be OK with the inspector?
    Unfortunately, even if you are using larger conductor just because you have a spool of it around you still have to increase the EGC size. The idea is that the EGC is sized to the OCPD rating and the assumed conductor size that the OCPD would normally be paired with. Using a larger conductor reduces the resistance in the circuit which will increase the fault current the EGC has to carry and the EGC can act as a choke point for the ground fault current increasing the time it takes for the OCPD to operate. Using a larger EGC will account for this.

  10. #10
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    Quote Originally Posted by pv_n00b View Post
    Unfortunately, even if you are using larger conductor just because you have a spool of it around you still have to increase the EGC size. The idea is that the EGC is sized to the OCPD rating and the assumed conductor size that the OCPD would normally be paired with. Using a larger conductor reduces the resistance in the circuit which will increase the fault current the EGC has to carry and the EGC can act as a choke point for the ground fault current increasing the time it takes for the OCPD to operate. Using a larger EGC will account for this.
    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.
    I live for today, I'm just a day behind.

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