4000 amp 480v 3ph switch gear

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wwhitney

Senior Member
Location
Berkeley, CA
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Retired
iwire said:
In my picture there are four current carrying conductors in each conduit which requires an 80% derate.

Had they been installed in the conventional way (A, B, C, N) there would be no need for derating.
Click to expand...
Thanks for pointing that out. So if you fix the maximum number of CCCs in a conduit (to either 3 or 6 as desired):

For a three phase feeder with a neutral, isophase requires (a bit over) 4/3 as many conduits, because you don't get to take credit for the neutral as a non-CCC.

For a three phase feeder without a neutral, the hit from isophase isn't as severe: just round up the number of conduits to the next multiple of 3.

Cheers, Wayne
 
I

Ingenieur

Senior Member
Location
Earth
wwhitney said:
Thanks for pointing that out. So if you fix the maximum number of CCCs in a conduit (to either 3 or 6 as desired):

For a three phase feeder with a neutral, isophase requires (a bit over) 4/3 as many conduits, because you don't get to take credit for the neutral as a non-CCC.

For a three phase feeder without a neutral, the hit from isophase isn't as severe: just round up the number of conduits to the next multiple of 3.

Cheers, Wayne
Click to expand...

someone also mentioned that conduit must be derated for isophase to 20% fill
I haven't verified that

as far as using isophasing and limiting to 3 conductors, at that point what is the advantage?
terminations would still be as complex as the abc method?

there must be a reason no one does it (or very rarely done)
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Ingenieur said:
someone also mentioned that conduit must be derated for isophase to 20% fill
I haven't verified that
Click to expand...
I think you are the only one that mentioned that for conduits. I mentioned it for wireway, which is where you may have got the notion mixed.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
wwhitney said:
I don't understand this comment--isophase doesn't restrict you to one conduit per phase (see the pretty picture iwire posted). So if you go above 6 CCC per phase, run more conduits. Same as in a mixed phase installation.

Cheers, Wayne
Click to expand...

True, I was kinda stuck on one conduit per phase. Although once you get into several conduits per phase, you start running into the same thing you were trying to get away from with polyphase. I think it would depend on the configuration and real estate available for conduits as to the degree it would start screwing with the artistry.
 
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dionysius

Senior Member
Location
WA
There is a strong lobby here on this thread that is opposed to isophasing. I consider the derating issue is being over exaggerated by those very same folks to "justify" their position. Let them produce the code abstracts to support their statements.
 
iwire

iwire

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Location
Massachusetts
dionysius said:
There is a strong lobby here on this thread that is opposed to isophasing.
Click to expand...

That is absoulutly false.

I have not seen one person other than Phil state they where against it.

The fact remains regardless if you are for it or against it the NEC PROHIBITS isophase installations except for a very small number of installations.
 
iwire

iwire

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Staff member
Location
Massachusetts
Ingenieur said:
then why is it done so infrequently?
Click to expand...

I would say it is because in most cases it is an NEC violation to do so.

how much difference will a few inches delta make over 100' run
Click to expand...

The difference between passing and failing inspection if the inspector wants to be picky.

The NEC requires the conductors 'be the same length" they do not give us a percentage that we can be off.
 
D

dionysius

Senior Member
Location
WA
iwire said:
That is absoulutly false.

I have not seen one person other than Phil state they where against it.

The fact remains regardless if you are for it or against it the NEC PROHIBITS isophase installations except for a very small number of installations.
Click to expand...

The NEC does not prohibit isophase installations. They embrace them but an absence of good analysis (see my previous post on the topic) discourages overt strong language for obvious reasons. There is nothing to be afraid of in promoting this. Incumbency does carry clout which is why you see the 5% number being used as the lever.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
dionysius said:
The NEC does not prohibit isophase installations. They embrace them
Click to expand...

That is incorrect.:happyno:


Here is the code section from 2014 NEC

300.3(B) Conductors of the Same Circuit. All conductors of
the same circuit and, where used, the grounded conductor
and all equipment grounding conductors and bonding conductors
shall be contained within the same raceway, auxiliary
gutter, cable tray, cablebus assembly, trench, cable, or
cord, unless otherwise permitted in accordance with
300.3(B)(1) through (B)(4).


(1) Paralleled Installations. Conductors shall be permitted
to be run in parallel in accordance with the provisions of
310.10(H). The requirement to run all circuit conductors
within the same raceway, auxiliary gutter, cable tray, trench,
cable, or cord shall apply separately to each portion of the
paralleled installation, and the equipment grounding conductors
shall comply with the provisions of 250.122. Parallel runs
in cable tray shall comply with the provisions of 392.20(C).


Exception: Conductors installed in nonmetallic raceways
run underground shall be permitted to be arranged as isolated
phase installations. The raceways shall be installed in
close proximity, and the conductors shall comply with the
provisions of 300.20(B).
Click to expand...


The only NEC permitted isophase installations are described in the exception above.

Therefore isophase installations cannot be used for any circuit above ground or in a metal raceway or cable. The eliminates most applications of isophase.
 
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I

Ingenieur

Senior Member
Location
Earth
electricians are professionals, they know what they are doing
the fact that isophase is seldom (rarely) used (where allowed, basically underground in pvc) tells me all I need to know
there is a place for its use, but that is very limited
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
iwire said:
That is incorrect.:happyno:

Here is the code section from 2014 NEC
300.3(B) Conductors of the Same Circuit. All conductors of
the same circuit and, where used, the grounded conductor
and all equipment grounding conductors and bonding conductors
shall be contained within the same raceway, auxiliary
gutter, cable tray, cablebus assembly, trench, cable, or
cord, unless otherwise permitted in accordance with
300.3(B)(1) through (B)(4).


(1) Paralleled Installations. Conductors shall be permitted
to be run in parallel in accordance with the provisions of
310.10(H). The requirement to run all circuit conductors
within the same raceway, auxiliary gutter, cable tray, trench,
cable, or cord shall apply separately to each portion of the
paralleled installation, and the equipment grounding conductors
shall comply with the provisions of 250.122. Parallel runs
in cable tray shall comply with the provisions of 392.20(C).


Exception: Conductors installed in nonmetallic raceways
run underground shall be permitted to be arranged as isolated
phase installations. The raceways shall be installed in
close proximity, and the conductors shall comply with the
provisions of 300.20(B).
Click to expand...

The only NEC permitted isophase installations are described in the exception above.

Therefore isophase installations cannot be used for any circuit above ground or in a metal raceway or cable. The eliminates most applications of isophase.
Click to expand...
While most do not agree with me, including the members of CMP3, it is my opinion that 300.3(B)(3) permits the use of isolated phase installations anywhere you want to use them. There is no need for the exception to 300.3(B)(1).
 
I

Ingenieur

Senior Member
Location
Earth
dionysius said:
There is a strong lobby here on this thread that is opposed to isophasing. I consider the derating issue is being over exaggerated by those very same folks to "justify" their position. Let them produce the code abstracts to support their statements.
Click to expand...


that is your ego speaking
everyone has said it has a limited application but is not, in most cases, the best method
as I said I've seen it used a few times: adjacent pad mounted gear, short runs without much room on the termination ends
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
don_resqcapt19 said:
While most do not agree with me, including the members of CMP3, it is my opinion that 300.3(B)(3) permits the use of isolated phase installations anywhere you want to use them. There is no need for the exception to 300.3(B)(1).
Click to expand...

Even if we all agreed that was the case it would be limited it to nonferrous / nonmagnetic wiring methods methods that also comply with 300.20 etc.

Do you believe this to be a true statement?

dionysius said:
The NEC does not prohibit isophase installations. They embrace them
Click to expand...
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
.....and to expand on 300.20 (b), that makes it a pain to come into a metal cabinet. You can get creative, but imo it's a bit mongrel to slot out a cabinet or put a non ferrous repair plate in - not something I would generally do. In practice, you are looking at a padmount with spade metering, UG to a switchboard as the only common isophase candidate.
 
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