Upsize EGC?

[charlie b]

Bob and Brother, I understand what you are saying. And in the case of long wires with VD issues, I can see some value in increasing the EGC. But let me point out that you could have used #12 wire and a #12 EGC for that 1000 foot run, and not have violated any code article (and depending on what the load was, it is not impossible that the voltage at the end of the run would be enough for proper operation). From that I infer that the amount of fault current that would be available, noting that the current would flow through the impedance of two very long #12 wires, would still be enough to trip the breaker. I base that claim on the fact that we are allowed to make that installation. So when you increase the size of phase conductor (and, irrelevantly, the neutral as well), you are into my scenario #2. The scenario #1 trip was fast enough, or it would not have been allowed. #2 is faster still. So why are we forced to make it faster than scenario #2? :?

 

[iwire]

But let me point out that you could have used #12 wire and a #12 EGC for that 1000 foot run, and not have violated any code article (and depending on what the load was, it is not impossible that the voltage at the end of the run would be enough for proper operation).

I fully understand that and I agree it is a odd thing.

From that I infer that the amount of fault current that would be available, noting that the current would flow through the impedance of two very long #12 wires, would still be enough to trip the breaker.

And I would have to say you are mistaken, I can tell you from first hand experience that I can connect both ends of a 500 run of 14 AWG on a 120 volt circuit and the current did not trip a 20 amp breaker.

I base that claim on the fact that we are allowed to make that installation.

I don't think it has been prohibited yet due to the fact most installers do want the load to actually work.

So when you increase the size of phase conductor (and, irrelevantly, the neutral as well), you are into my scenario #2. The scenario #1 trip was fast enough, or it would not have been allowed. #2 is faster still. So why are we forced to make it faster than scenario #2? :?

I disagree with your thoughts and am simply astonished by them. hmy:

 

[Ponchik]

iwire, i understand the extreme conditions that you have posted. but why to upsize the EGC for not so extreme condition installations.

as charlie has posted in his example the breaker will trip fast but why do we need it to trip faster and if does trip faster does it make a difference on the safety?

I believe one of these days the 3% VD recommendation will be a code rule.

 

[K8MHZ]

I can tell you from first hand experience that I can connect both ends of a 500 run of 14 AWG on a 120 volt circuit and the current did not trip a 20 amp breaker.

And I can tell you from first hand experience that a grounded 12 AWG hitting the bare bus of a 200 amp SQ D panel will trip the main instantaneously, with the only damage to the wire being a small spot on the end burned off.

The point is, and I think you will agree, that the REAL issue needs to have the length of the conductor or an impedance value taken into consideration as that makes a HUGE difference in the amount of fault current created from a bolted short.

 

[don_resqcapt19]

Bob and Brother, I understand what you are saying. And in the case of long wires with VD issues, I can see some value in increasing the EGC. But let me point out that you could have used #12 wire and a #12 EGC for that 1000 foot run, and not have violated any code article (and depending on what the load was, it is not impossible that the voltage at the end of the run would be enough for proper operation). From that I infer that the amount of fault current that would be available, noting that the current would flow through the impedance of two very long #12 wires, would still be enough to trip the breaker. ...

I am not convinced that you would have enough current to trip a 15 amp breaker, at least not in any reasonable amount of time at the end of that 1000' run. The online voltage drop calculator shows a drop of ~45 volts for 15 amps on a 1000' run of #12.

 

[jrohe]

Just to stir the pot a little bit, but say you're using RMC, IMC or EMT as the equipment grounding conductor and do not have an equipment grounding wire. Per 250.122(B), you would have to proportionately increase the size of RMC or EMT if you increase the size of the ungrounded conductors (250.118 says these are "types of equipment grounding conductors"). A proportional increase in the raceway's trade size does not necessarily equate to a proportional increase in the raceway's current-carrying cross sectional area...

Anyone know the impedance of steel EMT versus aluminum EMT versus RMC versus GRC versus IMC versus coated versions? I can guarantee they vary. But in the NEC's eyes, they don't care about the impedance, as long as the raceways are upsized proportionally. No science behind it...

 

[iwire]

And I can tell you from first hand experience that a grounded 12 AWG hitting the bare bus of a 200 amp SQ D panel will trip the main instantaneously, with the only damage to the wire being a small spot on the end burned off.

Of course.

The point is, and I think you will agree, that the REAL issue needs to have the length of the conductor or an impedance value taken into consideration as that makes a HUGE difference in the amount of fault current created from a bolted short.

Think I will agree?

I thought that was the point of my posts so far? :blink:

 

[charlie b]

Just to stir the pot a little bit, but say you're using RMC, IMC or EMT as the equipment grounding conductor and do not have an equipment grounding wire.

Why do you think about the phrase in 250.122(B), ". . . the equipment grounding conductor, where used. . . "?

 

[charlie b]

I can tell you from first hand experience that I can connect both ends of a 500 run of 14 AWG on a 120 volt circuit and the current did not trip a 20 amp breaker.

Just for curiosity, how long did you wait for the breaker to trip? Per Table 9, the impedance of 1000 feet (i.e., both legs of your 500 foot run) of #14 is 2.7 ohms. At 120 volts, that gives a "fault current" of 44 amps. That would trip a 15 amp breaker, if you give it enough time. It would take a bit longer to trip a 20 amp breaker.

If you swapped the #14 hot leg with a #12, but kept the #14 in the role of the EGC, the fault current is 55 amps. If you swapped both wires with #12, the fault current is 70 amps. In each scenario, the fault current gets higher, and the trip time gets shorter. So certainly, safety is improved with each change. But my problem is that there is no clear notion of how fast is fast enough.

 

[jrohe]

Why do you think about the phrase in 250.122(B), ". . . the equipment grounding conductor, where used. . . "?

Per, 250.118, RMC, IMC, and EMT are all considered "types of equipment grounding conductors." If the circuit is run in one of these raceways without an equipment grounding wire, then the raceway is the equipment grounding conductor and has been "installed." Therefore, the raceways would be subject to 250.122(B).

Although I do not think this is the intent, it is what it says.

 

[infinity]

Per, 250.118, RMC, IMC, and EMT are all considered "types of equipment grounding conductors." If the circuit is run in one of these raceways without an equipment grounding wire, then the raceway is the equipment grounding conductor and has been "installed." Therefore, the raceways would be subject to 250.122(B).

Although I do not think this is the intent, it is what it says.

What does this mean?

 

[K8MHZ]

Think I will agree?

I thought that was the point of my posts so far? :blink:

I have learned not to take anything for granted.....

 

[K8MHZ]

Just to stir the pot a little bit, but say you're using RMC, IMC or EMT as the equipment grounding conductor and do not have an equipment grounding wire. Per 250.122(B), you would have to proportionately increase the size of RMC or EMT if you increase the size of the ungrounded conductors (250.118 says these are "types of equipment grounding conductors"). A proportional increase in the raceway's trade size does not necessarily equate to a proportional increase in the raceway's current-carrying cross sectional area...

Anyone know the impedance of steel EMT versus aluminum EMT versus RMC versus GRC versus IMC versus coated versions? I can guarantee they vary. But in the NEC's eyes, they don't care about the impedance, as long as the raceways are upsized proportionally. No science behind it...

I know this is a stretch, but, if you increase the size of the ungrounded conductors enough, you will have to increase the size of the conduit/tubing per Chapter 9.

For instance, if I had two 6 AWG conductors in a 1/2 inch EMT and increase the size to 4 AWG, I would need to go from 1/2 inch EMT to 3/4. If I go up to 3 AWG, I will need a 1 inch EMT.

 

[Twoskinsoneman]

Maybe think of it like this. What if you ran a 15a circuit with #14 ungrounded and grounded conductors and 1 #12 EGC. Weird but legal. Then you decided to up the 2 14's to 12's for VD. Do you need to up the #12 EGC? I don't think so.

So I think your conduit EGC will always have the proper impedance to deal with any # or size conductors that can be legally installed there in

 

[infinity]

Maybe think of it like this. What if you ran a 15a circuit with #14 ungrounded and grounded conductors and 1 #12 EGC. Weird but legal. Then you decided to up the 2 14's to 12's for VD. Do you need to up the #12 EGC? I don't think so.

The EGC is not required to be larger than the ungrounded conductors {250.122(A)} so the answer would be no.

 

[charlie b]

I know this is a stretch, but, if you increase the size of the ungrounded conductors enough, you will have to increase the size of the conduit/tubing per Chapter 9.

Increase the conduit size from what starting point? I mean, what is the minimum size EMT needed to qualify as the EGC for any given circuit? For a 20 amp circuit, do you need a 1/2", and for a 30 amp circuit do you need a 3/4"? If your example (6 AWG increased to 4 AWG in a 1/2" conduit), how can you show that the 1/2" was not already increased beyond its originally required size, and therefore need not be increased further? :happyno:

For instance, if I had two 6 AWG conductors in a 1/2 inch EMT and increase the size to 4 AWG, I would need to go from 1/2 inch EMT to 3/4.

In your example, you take the ratio of the areas of the wires, and get an increase of 59%. Then you try to increase the size of the conduit by at least the same 59%. But are we supposed to increase the diameter of the conduit (as you did), or the area of the conduit, or the cross-sectional area of the metal portion of the conduit?

Based on the inability of any designer or installer to answer any of these issues using the wording of the code, I think it safe to interpret 250.122(B) as only applying to EGCs that are comprised of wires. But this is an interesting twist to the whole "increased in size" debate. :happyyes:

 

[kbsparky]

Here is a better example:

Suppose I have a 500 foot run of #6 circuit conductors, with a #10 ground wire to some pole lights.

If I put those #6 wires on a 60 Amp breaker, then it is Code-compliant.

But if I were to downsize that breaker to a 30 Amp, the Code would require me to upsize the EGC to a #6!

#10 EGC on a 60 Amp circuit is fine. But it's a violation on a 30 Amp circuit???

 

[infinity]

Here is a better example:

Suppose I have a 500 foot run of #6 circuit conductors, with a #10 ground wire to some pole lights.

If I put those #6 wires on a 60 Amp breaker, then it is Code-compliant.

But if I were to downsize that breaker to a 30 Amp, the Code would require me to upsize the EGC to a #6!

#10 EGC on a 60 Amp circuit is fine. But it's a violation on a 30 Amp circuit???

Yup. We've said it here many times that this section isn't perfect and could use some re-working of it's wording.

 

[Finite10]

As an aside, the most frequent violation of this rule is the #12 grounding pigtails used off a box when #10's are pulled for Vd, or due to specs.

Anyway, a kind inspector will ask if you pulled #10 for those reasons. The answer is always "No, it's just all I had on the truck."

Many inspectors see the intent of the rule and will take reality into consideratrion. Just don't want more resistance in the EGC leg than in the other(s) when the OCPD is trying to reference physical earth to trip ---IMHO.

 

[don_resqcapt19]

Yup. We've said it here many times that this section isn't perfect and could use some re-working of it's wording.

The easiest fix would be to make T250.122 work like T250.66. That is the EGC size would be based on the size of the largest circuit conductor. The size of the OCPD would not enter into the sizing of the EGC.