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Thread: Voltage drop for Ground wire????

  1. #1
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    Voltage drop for Ground wire????

    Argh this has had me running in loops all afternoon...

    I have a single 120V 20A circuit for some exterior lighting.

    Ensuring my voltage drop for this branch circuit is below 3%, I've upsized the conductors to #8 from #12.

    Rather than just throw down (3)#8 on my plans for the circuit, I check Table 250.122 and see that equipment grounds are baseed only on the circuit overcurrent device, so that would initially lead me to use (2)#8 with (1)#12 ground.

    I settle on (2)#8 with a #12G, but the problem grows in my mind like a bad itch...

    ... Won't the grounding conducter, if it's being used in a fault situation, be subject to the same voltage drop back to the grounding system? Does that voltage drop in excess of 3% mean the stray current will want to find an alternate path, or will the energy just dissipate more as heat on it's way to the grounding system?

    My usual practice has been to match branch circuit grounding conductors to their VD-upsized partners, but now I'm all topsy-turvy on this issue...

    In the process of trying to find a solution, I noted that #8's are what are normally used for 40A circuits before voltage drops are considered (T310.16), and a 40A circuit requires a #10G (T250.122)... Would this be the correct value if a #12 ground is no good?
    :confused::confused::confused:

    Thanks in advance!

  2. #2
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    Take a look at 250.122(B). Your EGC should be a #8.

    Chris

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    I agree with Chris. Your ratio for #12's is 1 to 1, meaning that the EGC is the same size as the phase conductors. So your new larger conductors must have the same 1 to 1 ratio. That means #8 phase gets #8 EGC too.
    Rob

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    All responses based on the 2011 NEC unless otherwise noted

  4. #4
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    Use #8s on a 40 amp circuit out to a couple 20 amp circuit breakers & you can use the #10 as the equip ground but use #8s on a 20 amp breaker than you must use a #8 EG. Simple.
    Tom
    TBLO

  5. #5
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    Quote Originally Posted by raider1 View Post
    Take a look at 250.122(B). Your EGC should be a #8.

    Chris
    Quote Originally Posted by infinity View Post
    I agree with Chris. Your ratio for #12's is 1 to 1, meaning that the EGC is the same size as the phase conductors. So your new larger conductors must have the same 1 to 1 ratio. That means #8 phase gets #8 EGC too.
    You guys rock. Thanks!

    Quote Originally Posted by ptonsparky View Post
    Use #8s on a 40 amp circuit out to a couple 20 amp circuit breakers & you can use the #10 as the equip ground but use #8s on a 20 amp breaker than you must use a #8 EG. Simple.
    I'm following half of your response here... Are you saying if I turn around and decide I'm instead sizing the circuit to 40A with my conductors, while still using a 20A breaker, instead of raising from #12's (thus invoking 250.122(B)), that a #10 ground is all that is needed (per T250.122), and my voltage drops conveniently happen to fall below 3%? :confused:

    PS: This is not to say you do not also rock d^_^
    Last edited by Nickarus; 01-29-09 at 07:47 PM.

  6. #6
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    I'm following half of your response here... Are you saying if I turn around and decide I'm instead sizing the circuit to 40A with my conductors, while still using a 20A breaker, instead of raising from #12's (thus invoking 250.122(B)), that a #10 ground is all that is needed, and my voltage drops conveniently happen to fall below 3%?
    250.122(B) ends up causing some odd EGC sizing.

    Use 6 AWG out from a 20 amp breaker to a load and 250.122(B) forces you to use 6 AWG for the EGC.

    Use 6 AWG out from a 60 amp breaker to a load and 250.122 allows you to use 10 AWG for the EGC.
    Last edited by iwire; 01-29-09 at 07:55 PM.

  7. #7
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    You guys rock. Thanks!
    Typically I Mosh.

    Chris

  8. #8
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    Nickarus which year NEC are you looking at?

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    Quote Originally Posted by raider1 View Post
    Typically I Mosh.

    Chris
    The other day my daughter noticed your avatar, she thought it was pretty cool.

  10. #10
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    Quote Originally Posted by Nickarus View Post
    You guys rock. Thanks!



    I'm following half of your response here... Are you saying if I turn around and decide I'm instead sizing the circuit to 40A with my conductors, while still using a 20A breaker, instead of raising from #12's (thus invoking 250.122(B)), that a #10 ground is all that is needed (per T250.122), and my voltage drops conveniently happen to fall below 3%? :confused:

    PS: This is not to say you do not also rock d^_^
    Sort of... your load and your footage haven't changed one bit so the # 8's would "essentially" be the same calculation you did when you figured VD on the 20 amp breakers. The only difference is that you noe need to set some type of breaker or fuses at the load to get your 2-20 amp branch circuits that you originally set out to install.
    Jim

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