Parallel Grounding Conductors: 250.122(F)

[EdwardSWard]

I'm aware of article 250.122(F) that "full sized" EGC's are required in each raceway where phase conductors are run in parallel in different raceways.

Question 1: Can anyony explain why?. Insulated EGC's will be the same length as they're terminated at the same points on each end. Thus, won't they each carry the same amount of fault current? Isn't it the phase conductors that will carry different amounts of fault curent depending on the location of the fault?

Question 2: Article 215.2(A)(1) states: "The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel".
Is there a typo in this article's usage of "grounded" instead of "grounding"? If so, just for the first "grounded" but not the second? (i.e. 250.122(F) doesn't apply if the feeder has parallel neutrals?)

 

[walton]

Question #1 - If the fault occurred in or along one of the paralleled circuits the EGC in that conduit would carry more than it's proportional share of the fault current. The shortest and lowest impedance path to ground from the fault to the supply panelboard is through the EGC in that conduit. Paralleled EGC need to be sized to carry a major portion of the fault current. See 2005 NEC Handbook explanation.

Question #2 - I think it has to do with calculated neutral loads?

 

[Bea]

Question 2: Article 215.2(A)(1) states: "The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel".
Is there a typo in this article's usage of "grounded" instead of "grounding"? If so, just for the first "grounded" but not the second? (i.e. 250.122(F) doesn't apply if the feeder has parallel neutrals?)

If you have a large enough over-current device and parallel the Neutral (grounded) conductor there is a good chance the Neutral conductor will be smaller than the required equipment ground conductor and will be code compliant hench the statement.

 

[EdwardSWard]

I did see the Code's explanation and that's what I don't get.
As most EGC's are insulated, wouldn't the fault current enter the EGC's where they are both bonded together at the load's enclosure. Thus they are the same length and impedance and equally share the fault current.

MY point about 215.2(A)(1) is that it refers to 250.122 for the size of the "grounded" conductor, when 250.122 is about "grounding" conductors. So what does the article really mean to say about 250.122(F) not applying?

 

[infinity]

I did see the Code's explanation and that's what I don't get.
As most EGC's are insulated, wouldn't the fault current enter the EGC's where they are both bonded together at the load's enclosure. Thus they are the same length and impedance and equally share the fault current.

According to the explanation in the NECH the reason being that the multiple EGC's could be of different lengths and therefore each could have a different impedance. By requiring a full sized EGC in each raceway it guarantees that any EGC can carry the major portion of the fault current. IMO this is kind of unlikely.

MY point about 215.2(A)(1) is that it refers to 250.122 for the size of the "grounded" conductor, when 250.122 is about "grounding" conductors. So what does the article really mean to say about 250.122(F) not applying?

This is the way I'm reading the requirement:

A grounded conductor is required to be large enough to carry a line to neutral fault therefore it's required to at least as large as the minimum size grounding conductor listed in 250.122. But grounding conductors when run in parallel must be sized according to the OCPD ahead of the feeder, that rule, 250.122(F), does not apply to the grounded conductor.

For example a 2000 amp feeder would require a 250 kcmil grounding conductor in each raceway based on the 2000 amp OCPD. The sum of all the grounded condcutors run in parallel in each raceway is only required to be equal to the minimum size of 250 kcmil.

 

[EdwardSWard]

Thanks Rob, I now see 215.2(A)(1) probably refers to min. neutral sizing for unbalanced loads.
I think the language of 215.2(A)(1) would be a lot more clearer if it said:
"The size of the feeder [/I][/I]circuit grounded conductor shall not be smaller than the grounding conductor required by 250.122, except that 250.122(F) shall not apply ......."