It is currently Fri Feb 23, 2018 5:39 am



Post new topic Reply to topic

Read entire post first for details! Should NEC 250.122 be revised for parallel circuits?
Yes 25%  25%  [ 9 ]
No 67%  67%  [ 24 ]
No Opinion 8%  8%  [ 3 ]
You may select 1 option

Total votes : 36
Author Message
ekstra   ara
 Post subject: NEC 250.122 Equipment Ground Conductors in Parallel Circuits
PostPosted: Sun Feb 11, 2018 9:20 am 
Offline
Plasma Level
User avatar

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1245
Location: Scottsdale, Arizona
NEC 250.122 Equipment Grounding Conductors and Parallel Circuits

This week’s question is a bit off topic from the traditional questions and is about grounding and National Electrical Code 250.122. It is a bit long and is about a possible change to the 2020 edition.

For as long as anyone can remember, if a circuit is made up of parallel sets of individual conductors run in multiple conduits, a full-size equipment ground based on the circuits overcurrent protective device rating is required to be run in each conduit.

The typical example is an 800 Amp circuit. This circuit is generally made up of 2 parallel sets of conductors run in 2 separate conduits – half of the total ampacity in each circuit.

The parallel sets of phase conductors are connected at each end. There are many NEC provisions such as same size, length etc. to help ensure the impedance of each set of conductors is as close to the same as possible for even distribution of current.

However, the NEC requires that the equipment grounding conductor is FULL SIZED based on the 800 A protective device for the entire circuit.

This means that although the phase conductors in each conduit are sized to carry half of the circuit’s load, the equipment grounding conductor in each conduit is required to be sized based on the circuit’s protective device – 800 Amps.

Therefore, according to NEC 250.122, a 1/0 is required in EACH conduit based on the entire 800 circuit even though the ground is also connected at each end – in parallel just like the phase conductors.

If the ground was sized based on half of the overcurrent device rating like the phase conductors, then the ground could be a #3 based on 400A (half of 800A).

It seems (and is being considered) that if parallel phase conductors can be relied on to evenly carry current, then parallel #3 conductors should be treated the same way.

I must state that I am not the person behind this proposal and don’t want to express an opinion either way. This question is simply to gauge the thoughts of others regarding this subject.

For our international friends that do not use the NEC, it would be interesting to hear your views as well

Comments and discussions would be great for this one!

Thanks for reading this all the way to the end!

This week's actual post question is very abbreviated due to a character limitation with the forum. The full question is:

Should NEC 250.122 be revised to permit equipment grounding conductors to be sized based on the individual circuit in each conduit instead of the entire circuit ampacity as it is now?

Yes
No
No Opinion


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 12, 2018 7:12 am 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1880
Location: North Carolina
Jim are you trolling here just due to your committee memberships? That is the kind of question that should be on an introductory power engineering course, right up there with a question highlighting the difference between ampacity and interrupting capacity.

If the fault occurs outside of the raceway say in a motor being fed by two parallel conduits then the parallel conductor argument makes logical sense. However if the fault occurs within a conduit then the fault current is equal to the full line-ground or line-ground-line fault current and thus it must be full sized because at that point the phase conductor and ground conductor are carrying fault current. The fault current doesn't care about the ampacity of the downstream devices. And the fault is in most cases going to happen at a single point in the system, not uniformly across all raceways. Going a little further to shoot down another silly argument if the ground conductor is damaged and makes contact with the phase conductor, then any and all parallel path arguments are nullified. So you can't count a parallel path where for instance the fault is being cleared by travelling "backwards" out the far end of the raceway and then back along the remaining parallel paths. You've got one and only one fault path.

In addition, the ground conductor is generally ALREADY undersized particularly with solidly grounded systems. It is assumed that it normally doesn't carry any intentional current at all. It only has to survive long enough to clear a fault, not carry current continuously as phase conductors do.

The only case where I could possibly see a parallel conductor argument, and granted this is going out on a limb, is if there are so many parallel paths or current limiters are in use to the point where the ground conductor is smaller than the phase conductors. Then I could make an argument for sizing the ground conductor no more than the size of the phase conductors regardless of the scheme that got us to this point. But the number of conditions under which this occurs is probably going to be pretty rare since the ground conductor will usually already be sized substantially smaller than the phase conductors.

So my vote would be no on this one.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 12, 2018 7:20 am 
Offline

Joined: Tue Dec 31, 2013 12:19 pm
Posts: 19
The Equipment Grounding Conductor (EGC) isn't there to carry load current, it is there to carry short-circuit/ground fault current. If there is a ground fault in one conduit, then the EGC in that conduit needs to be sized to create a low enough circuit impedance to create a large enough current to trip the overcurrent protection. If there is a ground fault in one conduit, that's the lowest impedance path of return. EGC's in the other conduits may not have continuity because of the ground-fault, and if they did, they would be a higher impedance and carry less of the ground fault current.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 12, 2018 7:37 am 
Offline
Plasma Level
User avatar

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1245
Location: Scottsdale, Arizona
PaulEngr wrote:
Jim are you trolling here just due to your committee memberships?

Guilty! Yes, I thought I would feel this one out for one of the committee people thinking about this. As I said, I will keep my opinion out of it (for now) but thanks for the response!


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 12, 2018 7:49 am 
Offline

Joined: Sat Feb 03, 2018 6:54 pm
Posts: 2
Location: Ottawa, IL
This requirement results in a conductor larger than what is required in 250.102(C)(2) for supply side bonding jumpers. It appears to me that the supply side bonding jumper is a "worst case" as compared to an EGC. The parallel circuit covered by 250.122(F) originates at an OCPD. The supply side bonding jumper, does not originate at an OCPD, and may have to carry more fault current than the EGC of a parallel circuit.
There is also the ongoing discussion as to "what is the conductor size" for the parallel circuit. Do the words "but in no case shall they be required to be larger than the circuit conductors supplying the equipment" in 250.122(A) apply to the total area of all of the conductors that make up the parallel set, or do those words apply to the conductors in a single raceway of the parallel set? Right not what happens in some areas where metal raceways are being used, if the inspector wants the wire type EGC to be sized per this rule, that wire type EGC is just not installed as the code does not require it.
Don G


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 12, 2018 8:15 am 
Offline
Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 164
Location: Colorado
While the fault current will divide between the multiple ground conductors and the majority of the current will travel down the shortest conductor, I still believe a full sized ground is unnecessary. I am not fully convinced that dividing it by the number of conduits is exactly the answer. Maybe the answer could be in the form of ampacity from the chart/ number of conduits*1.25


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 12, 2018 9:32 am 
Offline
Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 442
Consider the ground fault that occurs in one of the conduits close to the source. Even if both ground paths remain intact, Ohm's Law does not divide the fault current evenly due to the uneven path lengths. Now consider the fault caused by damage to one of the conduits that damages a conductor and the ECG within. In this case, only one path back to the source might be available. I see no reason to change.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 12, 2018 9:51 am 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1880
Location: North Carolina
engrick wrote:
While the fault current will divide between the multiple ground conductors and the majority of the current will travel down the shortest conductor, I still believe a full sized ground is unnecessary. I am not fully convinced that dividing it by the number of conduits is exactly the answer. Maybe the answer could be in the form of ampacity from the chart/ number of conduits*1.25


Ampacity gives us a first stab at it. But the ampacity in this case is that necessary to withstand a ground fault until the overcurrent protection clears. That's why the tables for ground conductors give a smaller size. The fault current will not as you say divide between the conductors if the fault occurs within the raceway as a result of damage to the ground conductor leaving only a single conductor to withstand the fault current.

As to holding enough current to trip the overcurrent protection device made by others, I disagree to some extent. This "double dipping" approach where the overcurrent protective device doubles as both ground fault and short circuit protection does not actually work in practice all that well because the ground circuit impedance is usually nowhere close to that necessary to trip the overcurrent protection once you start getting much over residential currents and voltages. There are several purposes for a ground conductor but the major one is to keep voltages on metal enclosures reasonably low during a ground fault essentially to minimize ground potential rise. The fact that it can clear an overcurrent device (at all) is secondary as is lightning protection.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 12, 2018 10:21 am 
Offline

Joined: Mon May 06, 2013 9:28 am
Posts: 8
Location: Oregon
I'm surprised this is under serious consideration. Due to inductance, the ground wires in the non-faulted conduits will present a high impedance fault path for a fault occurring within one conduit. This is particularly true for ferrous conduit. The EGC size is based on sufficient thermal capacity to trip the upstream phase overcurrent device without cable damage in the event of a ground fault. The NEC requirement is logical and reasonable.

The zero sequence impedance for various EGC configurations was tested by R.H. Kaufmann of GE back in the 1960s. The bottom line of those tests was that if the EGC was not in the same conduit with phase conductors, it was not effective as a low impedance conductor during ground faults.

Anyone proposing a change to the existing requirement will need to present solid test data showing that reduced sizes in for the EGC will allow sufficient fault current flow to trip the upstream phase overcurrent device.

Or maybe we just required GF protection on every circuit. 8-)


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Wed Feb 14, 2018 10:14 am 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1880
Location: North Carolina
David Castor wrote:
Or maybe we just required GF protection on every circuit. 8-)


Why is this an issue? NEC is charged with safety, not monetary cost implications.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Wed Feb 14, 2018 10:41 am 
Offline

Joined: Mon May 06, 2013 9:28 am
Posts: 8
Location: Oregon
PaulEngr wrote:
David Castor wrote:
Or maybe we just required GF protection on every circuit. 8-)


Why is this an issue? NEC is charged with safety, not monetary cost implications.


I assume that's not really a serious question. As far as why GF is not required on every circuit, you're free to submit a proposal. It would clearly be safer.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 19, 2018 1:22 pm 
Offline

Joined: Sat Feb 03, 2018 6:54 pm
Posts: 2
Location: Ottawa, IL
David Castor wrote:
...
Or maybe we just required GF protection on every circuit. 8-)

At one time this section had an exception that said you did not need a full sized EGC in each raceway if the circuit was protected by a ground fault device listed for the purpose of protecting the EGC. The following is from the 2005 NEC
Quote:
250.122(F)...(2) Where ground-fault protection of equipment is installed, each parallel equipment grounding conductor in a multiconductor cable shall be permitted to be sized in accordance with Table 250.122 on the basis of the trip rating of the ground-fault protection where the following conditions are met:
(1) Conditions of maintenance and supervision ensure that only qualified persons will service the installation.
(2) The ground-fault protection equipment is set to trip at not more than the ampacity of a single ungrounded conductor of one of the cables in parallel.
(3) The ground-fault protection is listed for the purpose of protecting the equipment grounding conductor

As I recall, this was removed from the code because there was no ground-fault protection device that is listed for the purpose of protecting the EGC.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: NEC 250.122 Equipment Ground Conductors in Parallel Circ
PostPosted: Mon Feb 19, 2018 2:38 pm 
Offline

Joined: Mon May 06, 2013 9:28 am
Posts: 8
Location: Oregon
I'm old enough to remember that exception. ;) I don't think we ever used it. I don't remember when it was removed.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 13 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 4 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
© 2017 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883