EGC--------Quiz

[ibew441dc]

Haha, ok, I say it has to be full sized.

Ding-Ding-Ding!!!! We have another winner.....!:grin:

 

[elohr46]

Soares Book on Grounding explains it this way:

In the event of a line-to-ground fault in the equipment suppolied by the circuit, the fault current should divide equally between the equipment grounding conductors. However, if a line-to ground fault occurs in the raceway or cable, current will be fed to the fault from both directions. The equipment grounding conductor will thus be called upon to carry the entire amount of fault current until the overcurrent protective device ahead of the fault opens.

Yes, and if you go to the next article "cables in Parallel" it explains it even better. Putting two or more cables in parallel would require an EGC larger than what is provided by the cable manufacture. Soares book says that you would have to order special cables that have an EGC to comply with 250.122 in each cable.

 

[ericsherman37]

This is a good thread! *in information absorbing mode*

 

[nunu161]

I'm 100% going with 250.122(A) and say it doesnt have to be larger then the ungrounded conductors. based on the math of 500x16=8000kcmil and 800x16=12800. it wouldnt make any sense to have more current carrying capacity on the egc then the phase conductors its only there for ground-fault conditions.

If a phase conductor were to fault its only going to carry the load of that single phase not all phases combined. And if that fault were to occur in the conduit with a single set of 500's its only going to carry the current of that single ungrounded conductor.

And the short time rating of a 500 kcmil is 11,834 amps much higher then the 6000 amps of the OCPD. If the OCPD doesnt trip within the the short-time rating then the problem wouldnt be with the installation it would be with the OCPD.

 

[ryan_618]

There were a few proposals in the 2011 to clarify this issue. I have the panel actions on my other computer, so I'll see if I can post them later today.

 

[davidaengelhart]

In all cases the EGC must be 800 kcmil copper.

But the EGC does not have to be larger than the circuit conductors: NEC 250.122(A).

 

[ryan_618]

Proposal 5-287 seems to propose allowing a smaller conductor than indicated in the table. The proposal was rejected and the panel confirmed that the value in the table is to be used for each EGC, including each EGC in parallel circuits.

*My first post via BlackBerry

 

[CMEFAY]

Changed in 2005......

Changed in 2005......

Interestly, in the 2002 code 310-4 included the clarification "(electrically joined at both ends to form a single conductor)". In 2005 the clarification omits 'to form a single conductor'. So are they saying since 2005 that parallel conductors are not considered a single conductor anymore?

Lori Fay

 

[ryan_618]

Interestly, in the 2002 code 310-4 included the clarification "(electrically joined at both ends to form a single conductor)". In 2005 the clarification omits 'to form a single conductor'. So are they saying since 2005 that parallel conductors are not considered a single conductor anymore?

Lori Fay

They made that change to clarify that if you had 6 current carrying conductors (three circuits in parallel) in a single raceway, you would still have to apply an ampacity adjustment for over three current carrying conductors in a raceway. I don't think conductors in parallel are a single conductor either. They may create a single conductive path, but they aren't a "single conductor".

 

[SEO]

Interestly, in the 2002 code 310-4 included the clarification "(electrically joined at both ends to form a single conductor)". In 2005 the clarification omits 'to form a single conductor'. So are they saying since 2005 that parallel conductors are not considered a single conductor anymore?

Lori Fay

The 1971 NEC also has (electrically joined at both ends to form a single conductor) I think that it was added in the 71 NEC. The 1971 section is 310-10. You have raised an interesting question as far as the omission of this wording.(To form a single conductor).

 

[jusme123]

1-500mcm, 2- 600mcm, and 3-1/0 250.122(a) "...but in no case shall they be required to be larger than the circuit conductors supplying the equipment."

 

[riggenbw]

6000 OCPD feeding a 3 phase load
Raceway is PVC

1)Using 16 parallel sets of 500 kcmil
What size EGC is required in each raceway?

2)Using 15 parallel sets of 600 kcmil
What size EGC is required in each raceway?

3)Using 40 parallel sets of 1/0
What size is required in each raceway?

500 kcmil = #3
600 kcmil = #2
1/0 = #6

 

[SEO]

500 kcmil = #3
600 kcmil = #2
1/0 = #6

Have you looked at section 250.122 (F) ? And table 250.122?

 

[ZZDoug]

Yes the case can be made that 122a doesnt require the grounding conductor to be larger than the phase conductors. But realistically, the intent of sizing from the overcurrent device is that each individual grounding conductor should be large enough to open the OC device. Its that simple, this is grounding 101. Its true that 122f should be better written to obviously overrule 122a when parallel conductors are involved and I can understand why some would be confused about this. But the intent and reasoning of ground conductor sizing is clear.

 

[Team-MEI]

Partial post #64: ??And if that fault were to occur in the conduit with a single set of 500's its only going to carry the current of that single ungrounded conductor??

Not quite correct; the fault would carry current from both ends of the conductor through to the EGC; thus an oversized EGC is required by the NEC for parallel installations. Theoretically, one could expand upon the rule to see where certain boundary limits may need further clarification as in the example of many parallel runs, but the fundamental rule is sound.

 

[don_resqcapt19]

Partial post #64: ??And if that fault were to occur in the conduit with a single set of 500's its only going to carry the current of that single ungrounded conductor??

Not quite correct; the fault would carry current from both ends of the conductor through to the EGC; thus an oversized EGC is required by the NEC for parallel installations. Theoretically, one could expand upon the rule to see where certain boundary limits may need further clarification as in the example of many parallel runs, but the fundamental rule is sound.

Both the phase conductors and the EGCs are connected together at both ends of the raceway. Both will flow current in both directions in the event of a fault within the raceway.

 

[Team-MEI]

True dat. Safety margin?

 

[lauraj]

I stumbled across this today:

215.2

The size of the feeder grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.


Now I'm completely confused. I'm really not sure what this means. Could someone maybe give an example applying this rule?

 

[SEO]

Section 215.2 is referring to the grounded conductor this thread has been about the equipment grounding conductor.

 

[don_resqcapt19]

215.2 is telling us that the minimum size of the grounded conductor is the larger of what is required by 220.61 or the size found in Table 250.122 based on the size of the feeder OCPD. It is also telling us that when the feeder uses parallel raceways or cable that the grounded conductor is sized by Table 250.122 it does not have to be "full size" in each raceway or cable.