Understanding NEC 250.122

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Muzikp

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I have two current carrying conductors that need to be upsized to accommodate for voltage drop. I have only one equipment grounding conductor. I am upsizing current carrying conductors from #12 (20amp) to #6 (this is a 480V street lighting circuit that travels very far). To upsize my equipment grounding conductor properly do I use the ratio of increased circular mils of one of the conductors or both?

Option A
26251 mils (#6) divided by 6530 mils (#12) = 4.02
6530 mils x 4.02 = 26,251 mils (#6 awg required for ECG)

Option B
26251 mils (#6) times 2 conductors = 52502 divided by 6530 mils (#12) = 8.04
6530 mils x 8.04 = 52,502 mils (#2 awg required for ECG)

It doesn't seem logical that the single ground wire of the same size as the conductors could carry enough current back to the breaker when it took two conductors to get enough amperage over the distance in the first place. #2 awg EGC seems realistic.

Thanks in advance for the help
 
Muzikp said:
I have two current carrying conductors that need to be upsized to accommodate for voltage drop. I have only one equipment grounding conductor. I am upsizing current carrying conductors from #12 (20amp) to #6 (this is a 480V street lighting circuit that travels very far). To upsize my equipment grounding conductor properly do I use the ratio of increased circular mils of one of the conductors or both?

Option A
26251 mils (#6) divided by 6530 mils (#12) = 4.02
6530 mils x 4.02 = 26,251 mils (#6 awg required for ECG)

Option B
26251 mils (#6) times 2 conductors = 52502 divided by 6530 mils (#12) = 8.04
6530 mils x 8.04 = 52,502 mils (#2 awg required for ECG)

It doesn't seem logical that the single ground wire of the same size as the conductors could carry enough current back to the breaker when it took two conductors to get enough amperage over the distance in the first place. #2 awg EGC seems realistic.

Thanks in advance for the help
Click to expand...

Option A is the correct application of 250.122.
 
I agree with David option #1. If you look at T250.122 you'll see that for 20 amps the minimum size EGC is #12 making the ratio to a 20 amp conductor 1 to 1 therefore no calculation is really needed just keep the EGC/ungrounded (1:1) ratio the same when you upsize the ungrounded conductor.
 
Infinity explained it well.

For 30 amp and less the minimum ampacity of the circuit conductors leaves you with a conductor size same as minimum required EGC, so for 30 amp and less you basically need same size EGC as the ungrounded conductors as it is 1:1 ratio.
 
Muzikp said:
It doesn't seem logical that the single ground wire of the same size as the conductors could carry enough current back to the breaker when it took two conductors to get enough amperage over the distance in the first place. #2 awg EGC seems realistic.
Click to expand...

If you are thinking of the line and return conductors, remember that the two of them are in series, not parallel. So only one of them really needs to be temporarily replaced by the EGC until the branch breaker trips. Not both.

If you are thinking of two separate circuits sharing a raceway, the chance that they both fault at once is slim to none. So only one EGC is needed to carry the fault current of either one of them.
 
Carultch said:
If you are thinking of the line and return conductors, remember that the two of them are in series, not parallel. So only one of them really needs to be temporarily replaced by the EGC until the branch breaker trips. Not both.

If you are thinking of two separate circuits sharing a raceway, the chance that they both fault at once is slim to none. So only one EGC is needed to carry the fault current of either one of them.
Click to expand...
And if you have 2 ungrounded conductors (IE 240 volt circuit on a 120/240 system) and fault both to the EGC at the same time, the EGC carries unbalance current for the most part and the fault is primarily line to line.
 
Thanks for the clarification. Very helpful, especially the light bulb that went off when Carultch added the additional info. :thumbsup:
 
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