Single Line Diagram and Grounding

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steve66 said:
The 10 conduits confuse me, but I've never designed an isolated phase installation. 3 phases and a neutral each with two conduits equals 8 conduits. It looks like the ground conductors are in their own conduits.
Is it correct to put the grounds in 2 separate conduits? It seems like they would be installed in each conduit with phase conductors. And do you really need a ground that is the same size as the phase conductors?
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Every conduit needs a full sized equipment grounding conductor, if you are using an EGC wire. For a 4000A breaker, that is 500 kcmil.

Isolated phase installations are, in my opinion, not recommended at all. The isolated phase will magnetize the surroundings, and these magnetic fields will get amplified if there is any ferrous conduit.

You are permitted to do isolated phase installations, given that all conduits take close paths so that the magnetic fields eventually get cancelled, and if you use non-ferrous conduit. Such as aluminum, stainless, PVC, LFNC, and others. Most people route the conduits along close paths anyhow.
 
Carultch said:
Every conduit needs a full sized equipment grounding conductor, if you are using an EGC wire. For a 4000A breaker, that is 500 kcmil.
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Strange. 50 years of installing/inspecting service conduits and I never saw the need :D
 
augie47 said:
Strange. 50 years of installing/inspecting service conduits and I never saw the need :D
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I think the concept is that if there is an unintended connection of an ungrounded conductor with the grounding conductor in a raceway, all of the fault current available will flow through the lone grounding conductor in that raceway and you want enough of a low impedance to clear the upstream protection.
 
ron said:
I think the concept is that if there is an unintended connection of an ungrounded conductor with the grounding conductor in a raceway, all of the fault current available will flow through the lone grounding conductor in that raceway and you want enough of a low impedance to clear the upstream protection.
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So a parallel EGC does not function the same as parallel ungrounded?
 
Smart $ said:
So a parallel EGC does not function the same as parallel ungrounded?
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Rhetorical question, right?

Many years ago I asked someone more knowledgeable than I the question of why the EGC has to be full sized. I was told that due to the importance of the EGC function, it had to be full size in each raceway in there was damage and you were only left with one EGC intact, that it would be able to function. True or not I don't know. Unless someone wanted to research the ROP/ROC, we may never know the real reasoning behind it.

Anyone know how far back does this rule go?

It does seem reasonable to allow some reduction in parallel especially like in this case where you have so many raceways and the single EGC size is so large.

A proposal would have not only have to justify that the reduction would facilitate OCPD operation, but how would you calculate the reduction and when a reduction is allowed.
 
Smart $ said:
So a parallel EGC does not function the same as parallel ungrounded?
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Good question.

A parallel EGC does function the same as parallel ungrounded wires when things go right, but not when things go wrong.

The parallel sets are only placed in contact when they terminate at equipment. The sets are not in contact in the middle of the conduit run. Therefore, the ground wires among each conduit cannot share the fault current, if a fault occurs in the middle of the run.

If there is a fault from an ungrounded conductor onto its grounded conductor, occurring in the middle of the conduit, then most of the current will bypass the load, and travel on this particular set's faulted path, to take a path through the EGC back eventually to the source. The EGC in each parallel set needs to be able to carry current for enough time to trip the OCPD and clear the fault before it overheats and therefore melts.
 
sandsnow said:
... Unless someone wanted to research the ROP/ROC, we may never know the real reasoning behind it.

Anyone know how far back does this rule go?

...
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According to accessible archived info on nfpa website, first appeared in 1971 Technical Committee Reports. There is no substantiation provided in these 'reports'.
 
Carultch said:
Good question.

A parallel EGC does function the same as parallel ungrounded wires when things go right, but not when things go wrong.

The parallel sets are only placed in contact when they terminate at equipment. The sets are not in contact in the middle of the conduit run. Therefore, the ground wires among each conduit cannot share the fault current, if a fault occurs in the middle of the run.

If there is a fault from an ungrounded conductor onto its grounded conductor, occurring in the middle of the conduit, then most of the current will bypass the load, and travel on this particular set's faulted path, to take a path through the EGC back eventually to the source. The EGC in each parallel set needs to be able to carry current for enough time to trip the OCPD and clear the fault before it overheats and therefore melts.
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Supposition borne of reasoned speculation my dear sir. :D
 
jim dungar said:
The concept, of a fault occurring in the middle of a run, is actually put forth in Soares Book on Grounding, (chapter 9 page 133, in my 7th edition).
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I realize Mr. Soares is revered as an authority on the subject of grounding. So are we all to just take his word for it? Where's the proof? (I do not have the book). I am currently looking at it like Obama vs. Obamacare. ;)

Whether substantiated or not is moot. Code requires it, so we do it. :dunce:
 
Smart $ said:
I realize Mr. Soares is revered as an authority on the subject of grounding. So are we all to just take his word for it? Where's the proof? (I do not have the book). I am currently looking at it like Obama vs. Obamacare. ;)

Whether substantiated or not is moot. Code requires it, so we do it. :dunce:
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Its also been a detail in the NEC handbook for years. See 250.22. The ground wires are still in parallel, but some paths are longer than other, so most of the current takes the shortest path.
 
Carultch said:
Every conduit needs a full sized equipment grounding conductor, if you are using an EGC wire. For a 4000A breaker, that is 500 kcmil.

Isolated phase installations are, in my opinion, not recommended at all. The isolated phase will magnetize the surroundings, and these magnetic fields will get amplified if there is any ferrous conduit.

You are permitted to do isolated phase installations, given that all conduits take close paths so that the magnetic fields eventually get cancelled, and if you use non-ferrous conduit. Such as aluminum, stainless, PVC, LFNC, and others. Most people route the conduits along close paths anyhow.
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So both the number of conduits, and the number of conductors in each conduit are shown incorrectly on the posted one line.
 
Smart $ said:
I realize Mr. Soares is revered as an authority on the subject of grounding. So are we all to just take his word for it? Where's the proof? (I do not have the book). I am currently looking at it like Obama vs. Obamacare. ;)

Whether substantiated or not is moot. Code requires it, so we do it. :dunce:
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He was. Been dead for a while.
The book is just dedicated to his memory now. My first one was a little bigger than a 3x5 booklet.

He might cringe at some of it now.
 
FWIW, I did a rough math analysis using one run of 350 vs (2)1/0 and #4 EGC using resistance values in C9T8. It works out that the EGC cannot be reduced in size by the same method as the ungrounded. Works out that #6 would afford the same level of protection... so implementation would be a problem, as Carultch mentioned.
 
steve66 said:
So both the number of conduits, and the number of conductors in each conduit are shown incorrectly on the posted one line.
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If it were my choice, I would standardize on 500 kcmil Cu for this application. That would be 11 parallel sets, assuming that this is sized as an ordinary feeder. Each set would consist of a 3 1/2" conduit, and 5 qty 500 kcmil Cu conductors.
 
Carultch said:
If it were my choice, I would standardize on 500 kcmil Cu for this application. That would be 11 parallel sets, assuming that this is sized as an ordinary feeder. Each set would consist of a 3 1/2" conduit, and 5 qty 500 kcmil Cu conductors.
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Utilities often limit us to ten sets from a transformer.
 
steve66 said:
So both the number of conduits, and the number of conductors in each conduit are shown incorrectly on the posted one line.
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The way that I interpret the callout on the single line is:
10 qty 4" conduits, with each phase consisting of 10 qty #600 kcmil. I'm not sure why they are indicating "2x5" instead of 10.

There doesn't seem to be a callout for the equipment grounding conductor. I suppose they are counting on using the conduit to do this, which the NEC allows you to do. I've seldom depended on exclusively conduit as the EGC.
 
iwire said:
Utilities often limit us to ten sets from a transformer.
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I've never known about this. Thank you for letting me know.

I guess this simply means that it is common that you are forced to buy wire exceeding 600 kcmil. I usually use 500 or 600 as the limit for when to decide to parallel.
 
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