Accept 5-201a: Parallel EGCs

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George Stolz

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The panel is considering a fairly impressive leap in the sizing of EGCs installed in parallel.

Proposal 201a said:
Submitter: CMP-5
Recommendation: Revise 250.122(F) to read as follows:
(F) Conductors in Parallel. Conductors installed in parallel shall have equipment grounding conductors installed in accordance with (1) or (2). Each equipment grounding conductor shall be sized in compliance with 250.122.
(1) If conductors are installed in multiple raceways or cables as permitted in 310.10(H), wire-type equipment grounding conductors, if installed, shall be in parallel in each raceway or cable and shall not be required to be larger than the largest ungrounded conductor installed in each raceway or cable.
Exception: Under engineering supervision in industrial locations...
(2) If conductors are installed in the same raceway, cable or cable tray as permitted in 310.10(H), a single equipment grounding conductor shall be permitted. Equipment grounding conductors installed in cable tray shall meet the minimum requirements of 392.10(B)(1)(c).

Substantiation: The revision separates the two conditions of multiple raceways or cable and single raceways or
cables. The revised text provides better clarity and usability to the installers and inspectors. The text was also revised
to clarify that for feeders or branch circuits where the equipment grounding conductor in the raceway could become
larger than the ungrounded conductors, the equipment grounding conductor is not required to be larger than the
enclosed ungrounded conductor. This makes this provision consistent with 250.122(A) where it was not clear which
section took precedence. The new exception allows industrial installations which are designed under engineering
supervision to utilize standard cables applied in parallel. The exception also includes similar provisions for raceways.

1.) Comment on Proposal No.: 5-201a
2.) Recommendation: Continue to accept this proposal.
3.) Substantiation: This proposal makes a fundamental change to 250.122(F), allowing individual parallel EGCs to be smaller than previously allowed, except if 250.122(A) was seen to override 250.122(F). This is a sensible change and should be accepted into the 2014 NEC.
 
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Dennis Alwon

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That explains my rejection and clarifies to me what they wanted.

5-195 Log #554 NEC-P05
_______________________________________________________________________________________________
Dennis Alwon, Alwon Electric Inc.
Revise text to read as follows:
(A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122, but in no case shall they not be required to be larger than the circuit conductors supplying the equipment, except when run in parallel according to 250.122(F). Where a cable tray, a raceway, or a cable armor or sheath is used as the equipment grounding conductor, as provided in 250.118 and 250.134(A), it shall comply with 250.4(A)(5) or (B)(4).

Substantiation
The use of the term "in no case" is not quite accurate as there are times when the equipment grounding conductor (EGC) may be larger than the circuit conductors. If you have a 1000 amp feeder with 7 runs of 1/0 copper then the EGC is required to be 2/0 in each conduit.


Panel Statement
The phrase “in no case shall they be required to be larger than the circuit conductors” applies to the example included in the substantiation. The panel reaffirms the size of the circuit conductors supplying the equipment referred to in 250.122(A) is the equivalent area of the circuit conductors in parallel.
 

Strife

Senior Member
The panel is considering a fairly impressive leap in the sizing of EGCs installed in parallel.



1.) Comment on Proposal No.: 5-201a
2.) Recommendation: Continue to accept this proposal.
3.) Substantiation: This proposal makes a fundamental change to 250.122(F), allowing individual parallel EGCs to be smaller than previously allowed, except if 250.122(A) was seen to override 250.122(F). This is a sensible change and should be accepted into the 2014 NEC.

I still believe the EGC should be sized to be able to clear a fault.
If I run 20 parallels of #3(I know no one does this, but it's still a scenario), I don't think a #3 ground will suffice to clear the fault on a 2000A breaker.
I'm also aware that a #3 grounded conductor will not provide enough fault to have the ground clear the fault, but still, doesn't seem right to me.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I still believe the EGC should be sized to be able to clear a fault.
If I run 20 parallels of #3(I know no one does this, but it's still a scenario), I don't think a #3 ground will suffice to clear the fault on a 2000A breaker.
I'm also aware that a #3 grounded conductor will not provide enough fault to have the ground clear the fault, but still, doesn't seem right to me.

The code still requires an effective ground fault path in another section.

250.2 Definitions.
Effective Ground-Fault Current Path. An intentionally
constructed, permanent, low-impedance electrically conductive
path designed and intended to carry current under groundfault
conditions from the point of a ground fault on a wiring
system to the electrical supply source and that facilitates the
operation of the overcurrent protective device or ground fault
detectors on high-impedance grounded systems.

250.4 (A) (3) Bonding of Electrical Equipment. Non–current-carrying
conductive materials enclosing electrical conductors or equipment,
or forming part of such equipment, shall be connected
together and to the electrical supply source in a manner that
establishes an effective ground-fault current path.
(4) Bonding of Electrically Conductive Materials and
Other Equipment. Electrically conductive materials that
are likely to become energized shall be connected together
and to the electrical supply source in a manner that establishes
an effective ground-fault current path.
(5) Effective Ground-Fault Current Path. Electrical
equipment and wiring and other electrically conductive material
likely to become energized shall be installed in a
manner that creates a permanent, low-impedance circuit
facilitating the operation of the overcurrent device or
ground detector for high-impedance grounded systems. It
shall be capable of safely carrying the maximum groundfault
current likely to be imposed on it from any point on
the wiring system where a ground fault may occur to the
electrical supply source. The earth shall not be considered
as an effective ground-fault current path.
 
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George Stolz

Moderator
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Location
Windsor, CO NEC: 2017
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I still believe the EGC should be sized to be able to clear a fault.
If I run 20 parallels of #3(I know no one does this, but it's still a scenario), I don't think a #3 ground will suffice to clear the fault on a 2000A breaker.
I'm also aware that a #3 grounded conductor will not provide enough fault to have the ground clear the fault, but still, doesn't seem right to me.

You don't think 20 parallel #3's would clear a fault supplied by 20 parallel #3's? :blink:
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
It just hit me - the real world scenario to your "20 #3's" would actually be 14 1/0's for 2000A.

What is the right size ground for each conduit per the 2011?
A. #6
B. 1/0
C. 250 kcmil

Which of these are unsafe sizes for use as an EGC?
 

jumper

Senior Member
It just hit me - the real world scenario to your "20 #3's" would actually be 14 1/0's for 2000A.

What is the right size ground for each conduit per the 2011?
A. #6
B. 1/0
C. 250 kcmil

Which of these are unsafe sizes for use as an EGC?

My first inclination is to rush to T250.122 and say C to your first question, but you got a bad habit of slipping tricky questions in here, so I am a bit wary.

You second question is a bit off for me, I have no idea what may be "safe", only whether it is code compliant or not. For all I know a #18 EGC could be "safe"to clear a 2000A fault, but I pretty sure it ain't legal.
 

Smart $

Esteemed Member
Location
Ohio
It just hit me - the real world scenario to your "20 #3's" would actually be 14 1/0's for 2000A.

What is the right size ground for each conduit per the 2011?
A. #6
B. 1/0
C. 250 kcmil

Which of these are unsafe sizes for use as an EGC?
2011 requirements as written are literally open to interpretation. Common interpretation (and perhaps all AHJ's) is that each conduit must contain a full size EGC, i.e. 250kcmil. However, some look at the not required to be larger than circuit conductors and say 1/0's.

Even though current proposal is an improvement, IMO, it should be revised to different requirements for EGC-qualifying vs. non-EGC qualifying raceways. An EGC-qualifying raceway would be in compliance without a wire-type EGC. Such installation should permit a smaller EGC in each conduit, perhaps based on member-wire ampacity.
 

jumper

Senior Member
2011 requirements as written are literally open to interpretation. Common interpretation (and perhaps all AHJ's) is that each conduit must contain a full size EGC, i.e. 250kcmil. However, some look at the not required to be larger than circuit conductors and say 1/0's.

Even though current proposal is an improvement, IMO, it should be revised to different requirements for EGC-qualifying vs. non-EGC qualifying raceways. An EGC-qualifying raceway would be in compliance without a wire-type EGC. Such installation should permit a smaller EGC in each conduit, perhaps based on member-wire ampacity.

member?:?
 

George Stolz

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Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Smart, I think that if they are changing this, it would be interesting to see what their response would be to a suggestion to split the 250 kcmil down to #6's based on the ampacity of the 250 kcmil.

I have never really understood the science behind requiring a 250 in each conduit; if they indeed intended to max out at the ungrounded conductor size, then they may be open to a smaller size yet. I would not like to see the principle you are adding added to the NEC, because I think they have been steering away from that for some time for simplicity's sake, and I agree.

In the scenario I put forward, I think a #6 would do the job just fine, even in PVC.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
suppose you have a 2000A CB that supplies a feeder circuit that consists of a bunch of PVC conduits that have (3) 1/0 per phase in them with 1/0 EGC in each conduit.

would even a phase to phase fault between two of the 1/0 conductors reliably trip the OCPD in time to protect the conductors?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
It just hit me - the real world scenario to your "20 #3's" would actually be 14 1/0's for 2000A.

What is the right size ground for each conduit per the 2011?
A. #6
B. 1/0
C. 250 kcmil

Which of these are unsafe sizes for use as an EGC?

George,
If I understand you're proposal correctly, you are saying that "B" would be the new code compliant answer, but you also believe "A" would do it. You're proposal makes sense to me and as someone elso noted, some AHJ's already see it this way. So yes, it would help clarify things. But I wonder where the data came from in the first place for Table 250.122 and how this change could be shown from an engineering point of view to make your case.
 

Strife

Senior Member
It just hit me - the real world scenario to your "20 #3's" would actually be 14 1/0's for 2000A.

What is the right size ground for each conduit per the 2011?
A. #6
B. 1/0
C. 250 kcmil

Which of these are unsafe sizes for use as an EGC?

Let's keep in mind that a fault can also occur inside a single conduit.
In this case, as I mentioned before, I don't think a #3 EGC will clear a 2000A breaker, but then again I don't think a #3 ungrounded conductor can PROVIDE enough fault to clear it anyway
Per NEC 250KCMIL would be the ground size, but again, given the resistance of the #3 ungrounded conductor, and all other factors, 250KCMIL EGC it's just a total waste.
So, yes, as it is right now, it's a big waste for really nothing.
Twenty paralel of #6 should definitely clear the fault, if the fault occurs on the load side(past the point where all #6 would connect together.
Personally, I believe the EGC can be smaller, as long as each individual EGC can clear the fault, BUT SHOULDN'T be any bigger than the ungrounded conductor size.

For instance: 200A fed with 2 parallel #3, I believe the EGC should be #6(per NEC 200A EGC size, so I don't believe the EGC should be able to be reduced to #8 for 100A on each feed), but I don't see the need of greater than #3 EGC in a 2000A fed with 20 parallels of #3.
So, to sum it up, I believe the code should be: The EGC shall be allowed to be the smallest of the two(the current EGC sizing we have now, or the size of the ungrounded conductor)
 

Smart $

Esteemed Member
Location
Ohio
...So, to sum it up, I believe the code should be: The EGC shall be allowed to be the smallest of the two(the current EGC sizing we have now, or the size of the ungrounded conductor)
It's already that way (re: 2011 edition). But the way it is written can be interpretted as requiring a full size in each conduit. Note at end of 250.122(F) it states, "Each equipment grounding conductor shall be sized in compliance with 250.122." 250.122 general statement, in part, states, "equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122, but in no case shall they be required to be larger than the circuit conductors supplying the equipment." Note it does not say parallel circuit conductors ampacity or size must be summed in some manner.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Let's keep in mind that a fault can also occur inside a single conduit.
In this case, as I mentioned before, I don't think a #3 EGC will clear a 2000A breaker, but then again I don't think a #3 ungrounded conductor can PROVIDE enough fault to clear it anyway
Per NEC 250KCMIL would be the ground size, but again, given the resistance of the #3 ungrounded conductor, and all other factors, 250KCMIL EGC it's just a total waste.
So, yes, as it is right now, it's a big waste for really nothing.
Twenty paralel of #6 should definitely clear the fault, if the fault occurs on the load side(past the point where all #6 would connect together.

Parallel conductors are joined at both ends - all twenty ungrounded conductors will feed every fault, and all 20 EGCs serve to clear it.
 

Smart $

Esteemed Member
Location
Ohio
Parallel conductors are joined at both ends - all twenty ungrounded conductors will feed every fault, and all 20 EGCs serve to clear it.
Not necessarily...

Say you have an underground PVC installation that one conduit gets cut with a backhoe (operator and spotter didn't see the warning ribbon). Imagine the slow motion account of the event is the EGC is cut first, then the bucket bites into an ungrounded conductor next. Because the bucket is pulling as it cuts, only one end of the cut EGC is in contact with the bucket as it cuts into the ungrounded conductor...

Granted, only one scenario out of many possible... but it is a possibility all the same.
 

Strife

Senior Member
Not necessarily...

Say you have an underground PVC installation that one conduit gets cut with a backhoe (operator and spotter didn't see the warning ribbon). Imagine the slow motion account of the event is the EGC is cut first, then the bucket bites into an ungrounded conductor next. Because the bucket is pulling as it cuts, only one end of the cut EGC is in contact with the bucket as it cuts into the ungrounded conductor...

Granted, only one scenario out of many possible... but it is a possibility all the same.
Let's assume same scenario(20 parallel of #3).
Let's assume a fault down 85 feet at 120V.
The resistance of #6 is 0.491/1000 feet(I believe)
The resistance of #3 is 0.245/1000 feet.
The maximum combined fault current sent back to the breaker between the #3 and the #6 (when you add the resistances), would be 1,918A.
The maximum combined fault between a #3 and a #3 would be 2,881A
The max between a #3 and a #1/0 would be 3,846A

Now let's take same scenario at 150 feet.
The #3 and #6 could deliver maximum 1,086A
The #3 and #3 could deliver maximum 1,632A (still not enough)
The #3 and #1/0 could deliver maximum 2,179A(barely enough, probably the wires will burn before the breaker trips).
The #3 and #250 could actually deliver 2,698A(plenty to trip the breaker in 2-3 seconds).

Now as I mentioned, nobody runs 20 parallels of #3, so let's assume a more feasible scenario of 10 #3/0
Assume 120 feet:
#3/0 and #6 = 1,761A
#3/0 and #1/0 = 5,035A
#3/0 and #3/0 = 6527A
#3/0 and #250 = 7,806A

Same scenario at 300 feet.
#3/0 and #6 = 704A(fail miserably)
#3/0 and #1/0 = 2,014A(probably won't trip the breaker before the wires burn)
#3/0 and #3/0 = 2,610A(probably enough to trip the breaker in 2-3 seconds)
#3/0 and #250 = 3,122A(plenty to trip breaker in 2-3 seconds)

And last the most feasible scenario: 6 parallels of #400
Assume 120feet
#400 and #6 = 1,911A
#400 and #1/0 = 6,489A
#400 and #400 = 15,576A
#400 and #250 = 11,961

And for a 300 feet scenario:
#400 and #6 = 764A
#400 and #1/0 = 2,595A
#400 and #400 = 6,230A
#400 and #250 = 4,784A (still plenty to trip, no need for the #400 ground)

However, at 400 feet we'd have:
#400 and #6 = 573A
#400 and #1/0 = 1,946A
#400 and #400 = 4,672A
#400 and #250 = 3,588A (and even at 600 feet we'd have 2,392A which should still trip the breaker in 10 seconds or so).


As most parallel runs are between 150' and 500' I can see where the :" smallest of either the table or the size of ungrounded conductor" would be the best EGC size.
And once we go over 400 feet the voltage drop will HAVE to be taken in consideration, at which point the ungrounded conductor would be larger, allowing for same size EGC to provide same fault when combining the resistances.
Furthermore I guess it comes down to "can we trip the breaker?"
As the code is spelled now, we only use the size of the EGC, when the size of the grounded conductor plays a factor also. For all it worth, I could probably have a #10 EGC tripping a 2000A breaker if it's only 10' and the grounded conductor is #400.
 

Smart $

Esteemed Member
Location
Ohio
Let's assume same scenario(20 parallel of #3).
Let's assume a fault down 85 feet at 120V.
The resistance of #6 is 0.491/1000 feet(I believe)
The resistance of #3 is 0.245/1000 feet.
The maximum combined fault current sent back to the breaker between the #3 and the #6 (when you add the resistances), would be 1,918A.
....

...
As most parallel runs are between 150' and 500' I can see where the :" smallest of either the table or the size of ungrounded conductor" would be the best EGC size.
....
You are evaluating only one ungrounded and one EGC from the source to the fault as if a non paralleled run. In a parallel conductor and raceways installation, such a fault condition could only exist if those two wires were cut and isolated from their load sides before faulting. Otherwise their load sides would contribute to the carrying of fault current. For example, in your scenario of a fault at 85' you have the ungrounded and EGC going directly back to the source but you also have the same ungrounded and EGC out to the load end connected to 19 ungrounded and 19 EGCs going back to the source.
 
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