250.118(4) allows EMT as a means of equipment grounding.

Now, if I also install a equipment grounding conductor in that EMT, as many of us do, and I increase the ungrounded conductor size to compensate for voltage drop or for derating purposes, do I still have to increase the size of that equipment grounding conductor proportionately according to the circular mil area of the ungrounded conductors as described in 250.122(B) ?

shortcircuit

IMHO, if this were an IG circuit I'd say definitely. If it were a routine circuit I guess it would be optional. Since EMT should be an adaquate EGC for any circuit of any size permited in the raceway code, would allow it. I've seen charts that cast doubt on the raceway being a low impedance path after a certain length but that's why some of us pull in an EGC.

Shortcircuit In my opinion if you install it you must follow the rules.

If you want to use the EMT as the EGC thats fine.

If you install a copper EGC in the raceway it must comply with 250.122(B)

Would you install a 14 AWG EGC with 12 AWG circuit conductors on a 20 amp circuit and then say it's OK because it is in EMT?

JMO, Bob

Bob - Your giving good design advice but as an inspector, I can't enforce it unless it's part of the construction documents submitted at plan review.

I don't agree that a raceway, in and of itself, is an adequate equipment ground for every purpose. It still must meet 250.4(A)(5), as indicated in the note to table 250.122. If your circuit length is such that voltage drop is an issue, than the length of the raceway and it's impedance may be an issue as well. With this said, I don't think including the raceway in the equation is valid.

EDIT for clarity: What I mean by referencing the note in table 250.122, is that following 250.118 and 250.122 doesn't automatically grant compliance with 250.4.

Just out of curiousity though, could you post the parameters of the circuit? Length, size of wire, size of pipe, size of breaker?

[This message has been edited by Ryan_J (edited 04-21-2005).]

George


Thanks, however I do believe it is enforceable.


And


Personally I do not see any doubt that if you install a grounding conductor it must comply with 250.122(B).

JMO, Bob

I guess I don't know why an extra ground wire would be required unless it were a health care facility or spec'd out on the plans. If it were a required EGC I would agree with you Bob but, if it's optional I don't see any real concerns.

shorty,

Table 11 of Soares (page 261, 7th edition) addresse the maximum length of EMT, IMC & RGC that may be used as an EGC, based on OCPD and 500% fault levels.

This installation doesn't involve isolated grounding, it is a routine installation with multiple 3 phase 480volt circuits in one raceway about 80 feet in length.

I will increase the ungrounded conductor size for each circuit to compensate for derating due to the number of current carrying conductors in the EMT.

The maximum OCPD will be 60 amps which calls for a #10 EGC when installed. The EMT is a suitable equipment grounding means according to 250.118

I have always installed a EGC in my metalic raceways. An extra step to insure an effective ground fault path in my mind, I guess. I loose locknut, or the helper forgets to tighten a set screw on a coupling and you all know that it happens.

Iwire...I agree that the code does say to increase the size So I will...

Redsy...There is nothing in the NEC that restricts the lenght of a EMT run that is used for an equipment grounding conductor.

George Little...Although the EGC I'm installing is optional, I must follow rule 250.122(B)...You see, with my reason for installing it in the first place, for a back up to a loose locknut or the forgotten tightening of the setscrew on a EMT coupling, I would want it properly sized as if the metalic pipe was not the grounding means at all

shortcircuit

Shortcircuit- You made a good decision. I wish more contractors would pull in EGC in raceways because the quality of materials and tolerances in fittings make a redundant grond very practical,

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.

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).]

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

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

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.

Ryan,


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.

Bob

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.

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:


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.

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 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

You're killing me Bob

Alright, I think you might have won this battle...at least for now.

Do you think the intent of equipment grounding is to facilitate the operation of the OCPD at overload speeds? Such as 30+ seconds? Perhaps I shouldn't have used the term "instananeous", but (as you agree with) I think the intent is painfully clear.