NEC 250.122 - Size of EGC for 4,000A feeder

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1986

Log # 22086- 13 - (310-4, FPN-(New)): Reject~: Steve Schaffer, Waterbury, CTRECOHHENDATION: Add the following (FPN) to Section310-4 to readFPN): The use of conductors in parallel will makeit necessary to review the conductor short-circuitwithstand rating, especially for the situation wh~reonly one of the cables is involved in the shortcircuit. See Sections 110-10, 240-1, and also ICEAPublication P-32-382 for conductor short-circuitwithstand information.SUBSTANTIATION: When a short circuit occurs on asingle cable of a multiple'run per phase system, thewithstand rating of just that one cable should bereviewed as it is likely that the withstand rating ofjust one cable could be exceeded while the withstandrating of the several cable~ run in parallel may havebeen sufficient.PANEL ACTION: Reject.PANEL COMMENT: When the conductors are in parallel,the short-clrcult current is never only on one slngleconductor.VOTE ON PANEL ACTION: • Unanimously Affirmative.




5(B) used to be 310-4
1989

6- 3 - (310-4): RejectSUBMITTER: Chester Flanagan, San Diego, CARE~MMENDATION: Amend the second line in the firstsentence only as indicated:• . . and larger, comprising each phase or neutral,"and equipment grounding conductor, where wire is used,shall" . . .SUBSTANTIATION: The wording used in the secondsentence does not clearly express the intendedmeaning/idea. This has given rise to a multitude of) wiring-errors; (2) conflicting Code interpretations.Paralleled conductor length requirements areapplicable to equipment grounding conductors (whereparalleled wires are used).It should be noted that the added explanatory "FinePrint Note" may not solve the problem; because: (1)FPNs are relatively new and are not widely used orunderstood; and (2) the "fine" print is hard to read.Based on the above, this proposal should be approvedfor the safety of life and property.PAEE AL_A_C!IO L N : Reject.PANEL COMMENT: Requirements for equipment groundingconductors are already contained in Section 310-4.VOTE ON PANEL ACTION: Unanimously Affirmative.
 
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1968 NEC

310-10 Conductors in multiple.conductors in sizes 1/0 and larger may be run in multiple provided the arrangement is such to assure equal division of total current among all conductors involved, all of the multiple conductors shall be of the same length, same insulation type and terminate in the same manner. where run in separate raceways or cables, the raceways or cables shall have the same physical characteristics.
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i do agree somewhere along the way some people thought it was a problem for grounds, but i have not read a legitimate reason.


think about it, the OP could use all 1/0 ungrounded conductors(more sets) and those would be code compliant. albeit ridiculous
 
free access doesn't have 71' proposals. i think there will likely be a good explanation there either way, likely favoring your interpretation. i'm curious what there substantiation was(if it is meant that way, anyway).
 
i will admit i am finding where panel comments sound like they believe the ground should be full sized, but they never provide a good reason (in my opinion).
 
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2010
sounds like panel members disagree on this. i am now not as confident as i was on what the meaning of these code sections are, i think you all may be right on that but i still have not come across a good reason for the requirement.

5-287 Log #3816 NEC-P05 Final Action: Reject(250.122(A))_______________________________________________________________Submitter: Christel K. Hunter, Alcan CableRecommendation: Add new text to read as follows: 250.122 Size of Equipment Grounding Conductors. (A) General. Copper, aluminum, or copper-clad aluminum equipmentgrounding conductors of the wire type shall not be smaller than shown in Table250.122, but in no case shall they be required to be larger than the circuitconductors supplying the equipment, including in parallel circuits. Where acable tray, a raceway, or a cable armor or sheath is used as the equipmentgrounding conductor, as provided in 250.118 and 250.134(A), it shall complywith 250.4(A)(5) or (B)(4).Substantiation: Until the reorganization of the Code in 1999, the EGCs inparallel circuits were not required to be larger than the circuit conductors.When 250.122 was reorganized, the exception that clarified this case wasmoved into (A). Until recently, AHJs have agreed that the language in (A)applies to (F), parallel circuits. However, there has been a change inenforcement in the last few years after the publication of articles interpretingthis code language. Even with the addition of the language “in no case” in the2008 NEC, there are still AHJs requiring that the EGCs be larger than theungrounded conductors in some parallel circuits. The language proposed would clarify that the EGCs need not be larger thanthe ungrounded conductors in parallel circuits. This was clearly the case untilthe 1999 NEC, and no proposal was made nor substantiation provided tochange the application of the code. It appears that the intent of the changesmade at that time were simply to clean up the language and use positivelanguage. However, the editorial changes have resulted in an unintendedtechnical change that was never proposed nor discussed by the CMP.Panel Meeting Action: RejectPanel Statement: The substantiation provided does not support reducing thecurrent sizing requirement. The panel concludes that the rating of theovercurrent device is the determining feature for sizing all equipmentgrounding conductors, including those installed in parallel circuits.Number Eligible to Vote: 16Table 250.122 Minimum Size Equipment GroundingConductors for Grounding Raceway and Equipment3000 400 6004000 500 8007505000 700 120070-297Report on Proposals A2010 — Copyright, NFPA NFPA 70Ballot Results: Affirmative: 13 Negative: 3Explanation of Negative: DOBROWSKY, P.: Accepting this change would match the requirements forsupply side bonding jumpers installed in parallel. MOHLA, D.: This propsal should have been either accepted or as a minimumrequired a clear statement by the panel that that parallel circuits conductors arealso circuit conductors and included. The submitter is correct in stating therequirement that “in no case the equipment grounding conductors are requiredto be larger than the circuit conductors” apply also to parallel circuits.Identicalrequirements exist in 250.24 (C) (1) for grounded conductor brought to ServiceEquipment and in 250.30 (A) (8) (a) for grounded conductor for separatelyderived systems. Grounded conductor on the supply side of overcurrentprotective device performs the same function as EGC on the load side of theovercurrent protective device i.e. provides a path for ground fault currents.Sizing requirements for supply side grounded conductors and load side EGCshould be same as they perform the same function during ground faultconditions. TEMBLADOR, R.: One primary factor that limits the current that flowsduring a ground fault is the impedance of the circuit or circuits. Within aspecific circuit, the weakest link (highest impedance) in the chain of devicesconnected together limits the current. There are multiple circuit paths that are formed during a ground-faultcondition which are primarily comprised of the ungrounded circuit conductor,equipment grounding conductors (EGC) and bonded metal enclosing the circuitconductors. During a phase-to-ground fault in a circuit with paralleledconductors, ground fault current will travel down all available paths to return tothe source. The current that flows divides at the point of the fault and travelsback to the source through both ends at which the ungrounded conductor, andthe bonded metal enclosing the circuit conductors or EGC’s are paralleled, orboth. Current flowing down the EGC’s and bonded metal enclosing circuitconductors does not only flow down one path. It too will travel down allavailable paths to return to the source. Consider a ground fault scenario wherethe EGC’s are larger than the phase conductors. In this instance, the weakestlink in the circuit is the paralleled ungrounded phase conductor and it will limitthe fault current. The equipment grounding conductors should not be required to be larger thanthe ungrounded phase conductors where conductors are paralleled in multipleraceways or cable.
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You see this surprisingly often, where a CMP's justification for the code's saying something is 'that's what the code says.' And it doesn't matter if the code got that way unintentionally, or intentionally but without any technical merit. There's gotta be some CYA going on, because it particularly seems to happen with proposals that would or could make the code less stringent. Nobody wants to be blamed for making things less safe, even if the present requirement is overkill.

That said, at this point a change probably needs a technical substantiation with a fair amount of engineering weight. Among other things, a single conductor needs to trip an OCPD that is potentially many times its ampacity. I'm not gonna pretend I have enough knowledge of the engineering issues to take a position on it.
 
My problem with this concept is that if I use one raceway I only need one EGC which may have to carry all of the fault current. If I use a metal raceway such as EMT I don't need any wire type EGC's but if I want to install them then I need a full size EGC in each metal raceway. Electrically this is all over the place for no apparent reason.
 
jaggedben said:
You see this surprisingly often, where a CMP's justification for the code's saying something is 'that's what the code says.' And it doesn't matter if the code got that way unintentionally, or intentionally but without any technical merit. There's gotta be some CYA going on, because it particularly seems to happen with proposals that would or could make the code less stringent. Nobody wants to be blamed for making things less safe, even if the present requirement is overkill.

That said, at this point a change probably needs a technical substantiation with a fair amount of engineering weight. Among other things, a single conductor needs to trip an OCPD that is potentially many times its ampacity. I'm not gonna pretend I have enough knowledge of the engineering issues to take a position on it.
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i cannot think of a scenario where only one of the grounds would carry a grossly disproportionate amount than the others(if installed correctly, which should go without saying). and 250.4(A)(4) &(5) already covers what you are referring to and the CMP panel members that oppose allowing it are saying, these already require if there is a ground fault the ocpd must open safely and everything must be able to safely carry the current. the actual times i think that would come in to play is actually with the ungrounded conductors, say one shorts out in a wet pvc pipe, if your paralleling then the impedance of that one ungrounded condcutor alone could be so high that the ocpd wouldn't open. but with grounds i don't see that problem, they will parallel if installed properly.
 
Wire-Smith said:
i cannot think of a scenario where only one of the grounds would carry a grossly disproportionate amount than the others(if installed correctly, which should go without saying). and 250.4(A)(4) &(5) already covers what you are referring to and the CMP panel members that oppose allowing it are saying, these already require if there is a ground fault the ocpd must open safely and everything must be able to safely carry the current. the actual times i think that would come in to play is actually with the ungrounded conductors, say one shorts out in a wet pvc pipe, if your paralleling then the impedance of that one ungrounded condcutor alone could be so high that the ocpd wouldn't open. but with grounds i don't see that problem, they will parallel if installed properly.
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A single ungrounded conductor is precisely what I meant.

But let's also look at your "i cannot think of a scenario where only one of the grounds would carry a grossly disproportionate amount than the others". Very well, forget about multiple conduits and grounds. Let's say we've got one PVC conduit. Let's say you've got a 1200A circuit that requires a 3/0 CU EGC per the table. Someone could do that with, say, (5) 250MCM. Or they could do it with, like, (9) 1/0. Does the EGC size requirement change from 3/0 to 1/0 because you used more, smaller conductors? Even though if one of those ungrounded conductors faults (in the metal box at the far end), it will be slower to trip the breaker by some amount?
 
jaggedben said:
A single ungrounded conductor is precisely what I meant.

But let's also look at your "i cannot think of a scenario where only one of the grounds would carry a grossly disproportionate amount than the others". Very well, forget about multiple conduits and grounds. Let's say we've got one PVC conduit. Let's say you've got a 1200A circuit that requires a 3/0 CU EGC per the table. Someone could do that with, say, (5) 250MCM. Or they could do it with, like, (9) 1/0. Does the EGC size requirement change from 3/0 to 1/0 because you used more, smaller conductors? Even though if one of those ungrounded conductors faults (in the metal box at the far end), it will be slower to trip the breaker by some amount?
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no, unless you mean multiple 1/0 EGC's. the way i'm looking at it is you need the cross sectional area required by 250.122. and don't forget the 1/0 minimum so i would say if you did (5)sets 250's ungrounded and you didn't want to use one 3/0 for some reason, you would have to use (5) 1/0's. (9) sets 1/0's you would need (9) 1/0 EGC's or one 3/0.

table 8
3/0 167,800 cmils

1/0 105,600

105,600 x5= 528,000cmils

just looking at 250.122
(1) 1/0 EGC good for 800A and you have 5 (or 9) of them

i think you would have better protection allowing parallel in that scenario, i'm not sure if anyone would ever do it, but there's always an odd scenario that comes around.

250.122 is based on OCPD and cross sectional area of EGC, EGC sizing is only affected by ungrounded conductors when you increase them for wire length.


250.122(F)(1) says "shall be permitted" to be single wire-type.

i think my disconnect with everyone else's thinking is when i see it says "EGC shall be sized in accordance with 250.122", i look at a parallel set of EGC's as one EGC, i will try to look it up later but i know i have read where it talks about "paralleled conductors are considered one conductor" (paraphrasing) and that's how i am looking at it.
 
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