EGC size

[glene77is]

George Stolz,

Good Post from January 2007.

Your post is an example of why these Electrical Forums are so useful.

 

[M. D.]

I think this code making panel member hit the nail on the head in regard to the proposal that dropped the voltage drop language .

TOOMER: The submitter did not provide sufficient
substantiation.

 

[M. D.]

This is a good article written by Ryan Jackson ,. simple and easy to understand ,.. Good work Mr. Jackson:smile:

http://www.iaeiofutah.com/Articles/TheEffectsOfVoltageDrop.pdf

 

[elohr46]

Very interesting discussion guys. I always learn something new here. I have a question: If I have a #6 cable w/#10 grd. and I install it on a 50 amp cb I'm good to go but if I do same thing with a 30 amp cb it is a violation?

 

[M. D.]

Well lets see ,........

# 10 upsized to # 6 ,...The circular mil area of 6 AWG is 2.53 greater than #10 AWG (26,240 cmil ? 10,380cmil = 2.52)

Tlb. 250.122, a 30 Amp OCPD would require a 10 AWG EGC,.. however it must be proportionaly increased in size by 2.52 (10,380 cmil ? 2.52) = 26,157cmil), which means a #6 is now required .

So yup ,.. unless I misunderstand ,....you would have a violation,.. as this is the way it is being taught,... it is most likely the way it will be enforced ....

 

[elohr46]

Well lets see ,........

# 10 upsized to # 6 ,...The circular mil area of 6 AWG is 2.53 greater than #10 AWG (26,240 cmil ? 10,380cmil = 2.52)

Tlb. 250.122, a 30 Amp OCPD would require a 10 AWG EGC,.. however it must be proportionaly increased in size by 2.52 (10,380 cmil ? 2.52) = 26,157cmil), which means a #6 is now required .

So yup ,.. unless I misunderstand ,....you would have a violation,.. as this is the way it is being taught,... it is most likely the way it will be enforced ....

Yeah, now I see why some want to send in a proposal to change the wording in that area.

 

[kbsparky]

So, if the OP were to feed that circuit with a 60 Amp breaker, instead of a 20, then his EGC would only have to be a #10, instead of a #6?

How is that any safer, considering the OPD is now 3 times larger than before, but the EGC can now be much smaller? :-?

 

[M. D.]

So, if the OP were to feed that circuit with a 60 Amp breaker, instead of a 20, then his EGC would only have to be a #10, instead of a #6?

How is that any safer, considering the OPD is now 3 times larger than before, but the EGC can now be much smaller? :-?

Not sure I understand ,...The thought behind increasing the EGC is that when you increase the ungrounded conductor you increase the current available should a fault occur. If the EGC is not large enough handle the fault then damage could result ...

 

[tryinghard]

If you can pull the EGC out and use the conduit, 250.118

 

[M. D.]

If you can pull the EGC out and use the conduit, 250.118

Hey, now there's an Idea for an exception,..

Exception 1. No proportional up-sizing of EGC will be required if circuit conductors are installed in one of the wiring methods found in 250.118

 

[gar]

090108-2150 EST

My following comments have little to do with code, but rather values. These calculations are base on charlie b's post #30.

I am going to assume an ideal 120 V source, no internal impedance.

A 50 ft loop of #12 is 1.588/20 ohms. The maximum possible current is 120/0.0794 = 1511 A. This will easily instantanenously trip a 20 A breaker. The voltage at the short is 60 V RMS relative to ground and neutral at the main panel. This is also a peak voltage of 85 V.

Next a 500 ft loop of #12 and the current is 151 A. Loop resistance is 0.794 ohms. This is still more than 7 times the breaker rating. Voltages remain the same.

Next is 250 ft of #6 and in series 250 ft of #12. The loop resistance is (0.3951/4) + (1.588/4) = 0.099 + 0.397 = 0.496. The short circuit current is 120/0.496 = 242 A. Here the short circuit point voltage is 120*0.397/0.496 = 96 V or a peak of 136 V.

And last is a 500 ft loop 0f #6. The loop resistance is 0.3951/2 = 0.198 . The maximum possible short circuit is 120/0.198 = 606 A. The voltage is back to 1/2 of 120.

I believe all of these are sufficient to trip a QO20 in the instantaneous area without trying to find the curve.

The third combination is the worst because it imparts a higher voltage on the machine frame where the short occurs..

.

 

[Dennis Alwon]

Hey, now there's an Idea for an exception,..

Exception 1. No proportional up-sizing of EGC will be required if circuit conductors are installed in one of the wiring methods found in 250.118

Why would we need that exception? I thought that was common knowledge and legal as it is now.

 

[iwire]

He had me wondering on that one as well Dennis. But I think he means if the raceway is an EGC per 250.118 we could choose to run a copper EGC in addition to the raceway without up sizing copper EGC.

Personal I can live with it how it is.

Few things worry me as much as finding 6, 4 or 2 AWG line conductors run to site poles with a 10 or 12 AWG protecting them. Even if the math shows the small conductor is capable of tripping the OCPD the small conductors are much more likely to damaged then the large conductors or for the smaller connections to corrode to the point where they can no longer take a fault current load.

 

[Dennis Alwon]

He had me wondering on that one as well Dennis. But I think he means if the raceway is an EGC per 250.118 we could choose to run a copper EGC in addition to the raceway without up sizing copper EGC.

That makes sense. :smile: I thought I must be missing something.

 

[M. D.]

Why would we need that exception? I thought that was common knowledge and legal as it is now.

Yeah as long as there is no wire EGC,.. seems silly to have to up-size the wire type ,..if the wiring method can stand alone as the EGC,.. I also read somewhere that even when the wire provides a parallel path the emt will carryy 95% more current .. I'll try to find some verification on that ...

 

[gar]

090210-1034 EST

M. D.:

You can answer that question yourself. Determine the resistivity ratio between steel or iron, and copper. Then calculate the cross section area of the copper wire and of the conduit. The resistance of the copper wire as a ratio to the iron conduit is ( (Area of copper)/(Area of conduit) ) * ( (Resistivity of copper)/(Resistivity of iron) ).

Now you can calculate the ratio of the cuurents in the paralle circuit.

.

 

[M. D.]

thought I would post this link ,.. it is to a study the steel pipe folks did ,.. I did not read nor grasp all of it's content ,.. but I have saved it ,..for further review.. pretty sure I get the gist though,...There is so much to learn in the world



http://www.steelconduit.org/pdf/groundingpart1.PDF

 

[Dennis Alwon]

Here is a thought... There I go thinking again. :smile:
How many of you out there have taken an old electric range and converted the wire to 120 v for a gas range and installing a 15 or 20 amp breaker. I know I have. Now thinking EGC and 250.122(B)-- you see the problem.

 

[stickboy1375]

Here is a thought... There I go thinking again. :smile:
How many of you out there have taken an old electric range and converted the wire to 120 v for a gas range and installing a 15 or 20 amp breaker. I know I have. Now thinking EGC and 250.122(B)-- you see the problem.

I have done the same, but I sometimes forget about 250.122(B) :smile:

 

[infinity]

That is a very valid argument. One would tend to think that the #6 was increased due to wire size but according to capacity it is not. Wow-- Another prime example of how difficult it is to write this section.

There is no question that this section needs help however I still cannot see your original post permitting a #12 egc. :smile:

But Dennis don't you find a contradiction in the two points that you've made in this post? First you're saying that the graphic does not show an increase in conductor size due to it's capacity but then you're saying that the OP does show an increase in conductor size so the #12 is no good. Actually the two examples are the same.