SE Cable to a Sub Panel

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LarryFine said:
That installation was non-compliant the day it was installed. I'd try showing the customer the applicable sections of your code book, and if you have to, explain that understanding is not required, but compliance is.
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I guess that's what I'll do if I can't figure out why it is against code. I like the suggestion. It's just frustrating to not be able to understand the problem.
 
electricguy61 said:
In this situation, why not run a separate grounding conductor to the main, and install a ground bar at the sub panel.
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Now you would two uninsulated conductors. Art. 300.3(B) says the egc can be run outside the cable for certain installation but you would still have another uninsulated conductor as a grounded conductor
 
steelersman said:
I've understood the reason for it being unsafe is because you have parallel neutrals (2 paths for the neutral current to travel back to source, thus possibly having current on metal parts that are normally grounded creating a possible shock hazard.

Please explain how it may not be enough current to trip the OCPD if their is a short.
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Have you never seen a short where the wire finely burned into and not trip the breaker?
 
ceb58 said:
Have you never seen a short where the wire finely burned into and not trip the breaker?
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I don't think I have. But what does this have to do with a 3 wire feeder vs. a 4 wire feeder? Are you saying that it is more likely to happen on a 3 wire feeder? If so then please explain cause I'd like to know. That's all.
 
Isn't the point of having a dedicated egc in addition to a neutral that it protects people from death in the event of certain fault situations?

For example, consider the case where there is a shared neutral/egc and the neutral/egc between the main and the sub becomes disconnected. In that case, all the egcs now become "hot" as the neutral/egc get tied to hot through some device. If a person then touches any exposed "ground" from that sub and also a "real" ground, they could be shocked or electrocuted. If there were a separate egc from the main to the sub and the same neutral fault occurred, there wouldn't be the same risk of life from possibly exposed "grounded" equipment surfaces.
 
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SAC said:
Isn't the point of having a dedicated egc in addition to a neutral that it protects people from death in the event of certain fault situations?

For example, consider the case where there is a shared neutral/egc and the neutral/egc between the main and the sub becomes disconnected. In that case, all the egcs now become "hot" as the neutral/egc get tied to hot through some device. If a person then touches any exposed "ground" from that sub and also a "real" ground, they could be shocked or electrocuted. If there were a separate egc from the main to the sub and the same neutral fault occurred, there wouldn't be the same risk of life from possibly exposed "grounded" equipment surfaces.
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I'm not sure I follow this.
 
One of the reasons for separating the equipment ground conductor and the grounded conductor on the secondary side of the service disconnect is to minimize if not eliminate neutral voltage and current from flowing back to the source on the effective ground fault current path (all the metallic portions of the circuit that are bonded to the equipment ground conductor).

Tell your customer there are 3 reasons you will need to repair the improper installation.

1 shock
2 fire
3 death

That should get their attention, if not walk away there are other jobs to be had.
 
steelersman said:
I'm not sure I follow this.
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In the sub being discussed there are 3 wires to main: 2 hot + neutral (I wouldn't call the grounded conductor an egc, as it is intended to carry current and is therefore a "neutral"). If the neutral to the main were to become disconnected, or of high resistance, and someone, for example, switched on a light, there wouldn't be any good return current path. The voltage on the neutral would approach that of the hot that it was connected to through the light and switch. Now, since the egc is bonded to the neutral in the sub, the egc is also pulled up to this "high" voltage. Nice "ground"! So any conductive surfaces that are bonded to this sub's egc (such as the sub's cabinet) are now up at the hot leg's voltage - very dangerous!

However, if you take the same example, except that the egc of the sub is not bonded to neutral at the sub, and instead runs independently back to the main (4 wires to main: 2 hot + neutral + egc), even though the neutral in the sub has the problem of being disconnected from to the main, and being pulled to the hot leg's voltage through the light/switch, the egc is still bonded back to "ground" through the main - preventing the very dangerous situation of having all the subs "grounds" at high voltage.

Now, it is still possible to have a dangerous situation occur even with an independent egc in the sub, but then it requires two faults at the same time - a fault where the neutral is disconnected plus a fault where the egc is disconnected. We hope for only one fault developing at a time, and detecting and fixing the first fault before a one second develops.
 
SAC said:
In the sub being discussed there are 3 wires to main: 2 hot + neutral (I wouldn't call the grounded conductor an egc, as it is intended to carry current and is therefore a "neutral"). If the neutral to the main were to become disconnected, or of high resistance, and someone, for example, switched on a light, there wouldn't be any good return current path. The voltage on the neutral would approach that of the hot that it was connected to through the light and switch. Now, since the egc is bonded to the neutral in the sub, the egc is also pulled up to this "high" voltage. Nice "ground"! So any conductive surfaces that are bonded to this sub's egc (such as the sub's cabinet) are now up at the hot leg's voltage - very dangerous!

However, if you take the same example, except that the egc of the sub is not bonded to neutral at the sub, and instead runs independently back to the main (4 wires to main: 2 hot + neutral + egc), even though the neutral in the sub has the problem of being disconnected from to the main, and being pulled to the hot leg's voltage through the light/switch, the egc is still bonded back to "ground" through the main - preventing the very dangerous situation of having all the subs "grounds" at high voltage.

Now, it is still possible to have a dangerous situation occur even with an independent egc in the sub, but then it requires two faults at the same time - a fault where the neutral is disconnected plus a fault where the egc is disconnected. We hope for only one fault developing at a time, and detecting and fixing the first fault before a one second develops.
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I guess that makes sense. But how would the neutral become disconnected? This doesn't seem likely.
 
steelersman said:
I guess that makes sense. But how would the neutral become disconnected? This doesn't seem likely.
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Yes, hopefully such faults aren't likely - but they don't go ignored by NEC. An example of NEC comprehending an open neutral is noted in "Figure 250-53" in Mike Holt's "Top 101 Rules of Understanding the NEC, Volume 1". "If the neutral opens and a person becomes in series with the raceway, they could be electrocuted".
 
What about the wiring method? A church is a "Place of Assembly" which would be fire rated construction. Would SE cable be permisable wiring method?
 
SAC said:
Yes, hopefully such faults aren't likely - but they don't go ignored by NEC. An example of NEC comprehending an open neutral is noted in "Figure 250-53" in Mike Holt's "Top 101 Rules of Understanding the NEC, Volume 1". "If the neutral opens and a person becomes in series with the raceway, they could be electrocuted".
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Thanks for the responses. Now it makes sense. I don't have the Top 101 rules you mention but I just drew it out on a piece of paper and I understnd it now. I'll use the sketch to show the customer if they want an understanding of why it's a problem.
 
steelersman said:
I guess that makes sense. But how would the neutral become disconnected? This doesn't seem likely.
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IMHO it isn't very likely. I guess you just have to juggle the probability of a fault, the damage of that fault, and the cost of providing protection against that fault. As equipment gets better, as well as _cheaper_ (relative to the cost of labor, not in absolute terms), adding additional protection becomes more and more reasonable.

I recall hearing about wartime European installations where conduit was intentionally (and to local code) used as the grounded circuit conductor on branch circuits.

-Jon
 
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