Voltage drop for Ground wire????

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Nickarus

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Olathe, Kansas
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!
 
infinity

infinity

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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.
 
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Nickarus

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Location
Olathe, Kansas
raider1 said:
Take a look at 250.122(B). Your EGC should be a #8.

Chris
Click to expand...
infinity said:
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.
Click to expand...

You guys rock. Thanks!

ptonsparky said:
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.
Click to expand...

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^_^
 
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iwire

iwire

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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%?
Click to expand...

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.
 
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Jljohnson

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Location
Colorado
Nickarus said:
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^_^
Click to expand...

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.
 
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Nickarus

Member
Location
Olathe, Kansas
iwire said:
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.
Click to expand...

Yeah I was just chewing on that... Is there something stated that says you can use the smaller result of either approach?

iwire said:
Nickarus which year NEC are you looking at?
Click to expand...

2005

Jljohnson said:
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.
Click to expand...

I should maybe clarify - I'm talking about a single 20A circuit in this case.

Sounds like you're describing this: in the case of supplying (2) 20A circuits worth of load with a 40A breaker/circuit, and fusing each fixture to 20A, that I would only need the #10 per T250.122. Please correct me if I misunderstood!
 
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