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#92980 04/22/05 12:07 AM
Joined: Sep 2004
Posts: 167
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I've made this call before. If you want an EGC in the EMT you have to follow the rules. Bob quoted the code and I don't see any gray area here.

I have had people rempove the undersized EGC to pass inspection, rather than install the correct size.

I can't prove it with a formula, but undersizing the EGC in EMT could do more harm than good.


Larry LeVoir
Inspector
City of Irvine, CA
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#92981 04/22/05 06:55 AM
Joined: Mar 2001
Posts: 2,056
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shortcircuit,

I know the NEC does not limit the length of EMT as a EGC.
However, we often refer to other publications, such as ULs White Book, or other building codes, in order to install equipment in a safe manner.
Based on his question, I think that the Soares table is what Ryan had in mind when he asked about the circuit description.



[This message has been edited by Redsy (edited 04-22-2005).]

#92982 04/22/05 10:46 AM
Joined: Jun 2004
Posts: 613
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Member
Redsy and Ryan_J...

I do understand the theory behind the distance of a metalic pipe run with consideration to the increased impedance that will be apparent with a long run.

Thanx for all the responces...

shortcircuit

#92983 04/22/05 01:02 PM
Joined: Aug 2003
Posts: 1,374
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Quote
Redsy...There is nothing in the NEC that restricts the lenght of a EMT run that is used for an equipment grounding conductor.

I would disagree with that. I think 250.4(A)(5) limits the length, it just doesn't say what the limit is.

Here is a thread I started at the other board about this very topic. I found it to be quite good, if you guys are interested.
http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=11;t=003251;p=1


Ryan Jackson,
Salt Lake City
#92984 04/24/05 09:31 AM
Joined: Jun 2003
Posts: 681
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I am glad that Ryan brought up 250.4(A)(5).
People look at me cross eyed when I mention this section.
I believe that this section is usually passed over, because it is a performance reference, where most of the NEC is a perscriptive document. I absolutely agree with Ryan.
Bob's statement and Alan's comment are also fact. Once an equipment ground conductor is installed, section 250.122 in its entirety has to be followed, including that tiny note at the end of the table.


Pierre Belarge
#92985 04/24/05 10:15 AM
Joined: Jan 2003
Posts: 4,391
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Ryan,

Quote
I think 250.4(A)(5) limits the length, it just doesn't say what the limit is.

Then I do not see how you can enforce it.

In order for a police officer to give me a speeding ticket there must be a speed limit.

In order to meet a performance criteria the wanted result must be spelled out.

I know you (Ryan) has a number in mind you like to use.

How is that fair to any EC?

I just do not see how 250.4(A)(5) can be used to limit the length of a run of EMT.

Lets look at this from another direction.

If 250.4(A)(5) can directly limit the length of a conduit run used as a EGC then 250.4(A)(5) could also be used to force the increase of a copper EGC installed in a raceway.

If the above is true why do we need 250.122(B)?

Your right though, that thread at Holt's was a good one. [Linked Image]

Bob [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#92986 04/24/05 01:14 PM
Joined: Jul 2004
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I am with Bob. 250.4(A)(5) looks more like something that should be in a fine print note than a code article. If they would put an ohm value in there it would be enforceable although I am not sure how the inspector would be testing it without some tool we don't usually carry.
I did do some testing with an ECOS (checks for <1 ohm) and the 50 year old EMT was surprisingly good.


Greg Fretwell
#92987 04/24/05 01:29 PM
Joined: Aug 2003
Posts: 1,374
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Hi Bob. This is a touchy subject. As you know, I am very much a code minimum type of guy when it comes to enforcement. I don't use the crystal ball and say "what if this happens", and I am a far cry from a "good ole' boy".

One of the questions you ask is if 250.4(A)(5) is applied, why is 250.122(B) in the code? That is a great point. I think 250.122(B) is quite general, meaning that if you have so much impedance on your circuit that you have to upsize the ungrounded conductors, you will also have much impedance on the EGC, so it also needs to be upsized.

Now lets consider an application where in my opinion 250.4(A)(5) might trump the rest of article 250. Lets say I have a motor that is protected by an inverse time circuit breaker. Lets say such motor draws 24.2 amps (3 phase 208V, 7 1/2 HP, table 430.250), and my inverse time circuit breaker is therefore 70 amps (24.2*250%, next size up=70). I can use 12 AWG ungrounded and 12 AWG EGC for this circuit, because 250.122(A) tells me that my EGC need not be larger than my ungrounded conductor. Now, if the instaneous trip of this breaker is, say, 300 amps, and the fault current produced by this circuit doesn't create that, I have violated 250.4(A)(5). This would require me to increase either the size of the ungrounded conductors, or the size of the EGC, or both. The note to table 250.122 would override 250.122(A)'s requirement of not exceeding the ungrounded conductor size, because 250.122(A) points us to that table, and that table contains a note that IS part of the table.

While I agree this is probably a rare occurance, it is my opinion that 250.4(A)(5) is referenced by the table for just such as event, regardless of how improbable it may be.

When you were asking how I would base my decision, I would have to base it in the instananeous trip level of the OCPD, which will vary by the installation, of course.

I also think that the intended result of the performance based requirement is in fact spelled out. From the 2005:

Quote
(A)(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 ground-fault 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.

If the conductor has such impedance that it will not initiate the OCPD, then the first bolded item is not satisfied. If the conductor has such impedance that its short time withstand rating is arrived at before the OCPD is inititated, then the second bolded item is not satisfied.


Ryan Jackson,
Salt Lake City
#92988 04/24/05 01:34 PM
Joined: Aug 2003
Posts: 1,374
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Quote
I am with Bob. 250.4(A)(5) looks more like something that should be in a fine print note than a code article. If they would put an ohm value in there it would be enforceable although I am not sure how the inspector would be testing it without some tool we don't usually carry.
I did do some testing with an ECOS (checks for <1 ohm) and the 50 year old EMT was surprisingly good.

You couldn't simply put an Ohm value on it, because it depends on the size of the OCPD. That is the whole basis of table 250.122, and its note.

In your example, you had less than 1 ohm. Thats great. Lets say the circuit is 250 amps. It would take 0.12 Ohms to trip the breaker in the instaneous range. If the breaker were 20 amps, it could be done with more than 1.5 ohms.


Ryan Jackson,
Salt Lake City
#92989 04/24/05 06:25 PM
Joined: Jan 2003
Posts: 4,391
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Moderator
Ryan how did you decide the circuit must be cable of operating the instantaneous trip portion of the breaker?

250.4(A)(5)does not state that.

IMO as long as the OCPD operates before damaging the fault path conductors both the highlighted parts of 250.(A)(5) are satisfied.

I agree with your intent, I just do not see how you can enforce what is not spelled out.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
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