Parellel service conductors (feeders)

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don_resqcapt19

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Illinois
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retired electrician
My understanding is that the NEC "equal length" rule is aimed at ensuring equal impedance on all paralleled conductors. So verifying the length is really an indirect method of ensuring equal impedance. Therefore, if you measure the actual impedance, you're verifying compliance with the INTENT of the NEC, and should therefore satisfy the inspector.
All the equal length does is to try to ensure equal resistance. There are a number of other factors that go into the impedance like the conductor geometry. This is not a big issue when each set of conductors are in their own conduit, but is an issue in cable tray and wireways as shown by the last part of post #15.
 

ohmhead

Senior Member
Location
ORLANDO FLA
Well if you look at the conduits coming into the bottom of a transformer or a switchboard it can not be done they will be at least a 1 foot difference between conductor lengths crossing from terminals or lugs if you make them all the same length than look at that article in the NEC were it tells you its craftmenship or in a neat & workmen like manor ?

It will now be a bad looking job and not neat looking !

Who ever made this rule was meaning to say just keep it close guys and dont be so impedance wise accurate is not going to happen .

Yes in a tray keep them together.
In a wireway yes keep them together.

But when you turn out to terminate in a tray or your running multi conduits it is not going to happen in the real world of electrical work .
 

Smart $

Esteemed Member
Location
Ohio
What could be the solution if you have 20% difference like it is posted in # 15.
I don't know how much creedance I'd give to that 20%. Two things come to mind: 1) He said during (900A + or -) startup, which means the amp draw is under constant change, and 2) you'd need recording equipment and an amprobe for each wire to make take measurements incidental to each other (i.e. you can't use one amprobe to measure current on several individual paralled conductors at the same time... and even if you had one amprobe for each wire, you can't observe the all the readouts simultaneously).
 

iwire

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Massachusetts
Well if you look at the conduits coming into the bottom of a transformer or a switchboard it can not be done they will be at least a 1 foot difference between conductor lengths crossing from terminals or lugs if you make them all the same length than look at that article in the NEC were it tells you its craftmenship or in a neat & workmen like manor ?

It will now be a bad looking job and not neat looking !

The NFPA considers both 'neat' and 'workmanlike' to be vague and possibly unenforceable.

Further in my own opinion we should not be putting how it looks (behind covers) in front of how it performs.

So if your making them less even then they need to be just so it looks better you should be cited.
 

ohmhead

Senior Member
Location
ORLANDO FLA
The NFPA considers both 'neat' and 'workmanlike' to be vague and possibly unenforceable.

Further in my own opinion we should not be putting how it looks (behind covers) in front of how it performs.

So if your making them less even then they need to be just so it looks better you should be cited.

Well we understand what the code says yes its gray in other areas also .

And we see your point but disagree with what you have stated you can not have equal length conductors in a switchboard or transformer with multiple paralled runs .
http://i611.photobucket.com/albums/tt195/stringking/2-17-051cc-19part2-1.jpg


Weve been working distribution for many years lets talk underground parallel runs do you think 11 runs coming up into a switchboard is all the same length?
Meaning from start point to end point its not .
Physically its not and you cant stubb up into the bottom from underground in any order to make it happen with 4 inch conduit .
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Physically its not and you cant stubb up into the bottom from underground in any order to make it happen with 4 inch conduit .
The only other option would be to use PVC and slot the panels (where necessary), and group the phases.

Added: I still wonder why steel locknuts aren't an induction issue.
 
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iwire

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Location
Massachusetts
Well we understand what the code says yes its gray in other areas also.

Who is 'we'? Do you have a mouse in your pocket?;)

I don't see much gray about the code in question 'shall be the same length'

Where is the gray in that?

And we see your point but disagree with what you have stated you can not have equal length conductors in a switchboard or transformer with multiple paralleled runs .

Well yes in most case you could by cutting all the conductors the same length and leaving 'messy' slack at each end. But that was not what I said.

What I said was you should be making every effort to make them the same length without thinking, or concerning yourself with how it looks.


Weve been working distribution for many years lets talk underground parallel runs do you think 11 runs coming up into a switchboard is all the same length?
Meaning from start point to end point its not .
Physically its not and you cant stubb up into the bottom from underground in any order to make it happen with 4 inch conduit .

You are not the only one that has run large parallel groups of conductors. :)


My only point here is your should be placing equal length head of 'pretty'
 

iwire

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Location
Massachusetts
The only other option would be to use PVC and slot the panels (where necessary), and group the phases.

Added: I still wonder why steel locknuts aren't an induction issue.

Generally when you can use the isolated phase option there is no bottom to slot.

Really the only place you can get away with isolated phase installations is from a pad mount transformer outside and the service gear inside.

Als_Isolated_Phase.JPG
 

ohmhead

Senior Member
Location
ORLANDO FLA
Who is 'we'? Do you have a mouse in your pocket?;)

I don't see much gray about the code in question 'shall be the same length'

Where is the gray in that?



Well yes in most case you could by cutting all the conductors the same length and leaving 'messy' slack at each end. But that was not what I said.

What I said was you should be making every effort to make them the same length without thinking, or concerning yourself with how it look




You are not the only one that has run large parallel groups of conductors. :)


My only point here is your should be placing equal length head of 'pretty'

Well we still disagree with length when you turn up your conduits under there stacked row one on top of row two this is what iam telling you .

Your not seeing what iam explaining your just looking at wire iam saying pipe & conduit can not let the conductors be equal ever paralleled with large arrays of runs not just a few .

look at piping by itself ?
 

iwire

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Location
Massachusetts
Well we still disagree with length when you turn up your conduits under there stacked row one on top of row two this is what iam telling you .

Your not seeing what iam explaining your just looking at wire iam saying pipe & conduit can not let the conductors be equal ever paralleled with large arrays of runs not just a few .

look at piping by itself ?

And you must think I am an idiot. :grin:

I am not saying that you can always make the raceways the same length, it is usually not possible. You can try to pick the best arraignment of them to keep the lengths equal. Sometimes a flat side by side layout is best sometimes making the group in a square or rectangle works best. With the conduits all you can do is your best and thats it.:smile:

What I have been talking about is the conductors, you can give up some neatness in the enclosures to keep the conductors closer to the same length.


In other words Don't cut some shorter just because they will look nicer do the best you can to keep the conductors of each phase the same length even though the raceways are different lengths.

I am not suggesting the imposable. :smile:
 

ohmhead

Senior Member
Location
ORLANDO FLA
And you must think I am an idiot. :grin:

I am not suggesting the imposable. :smile:

Well i know you are a good electrician Iwire and would never suggest that your not .

Most of our work is heavy commercial and we are picky about neat work i dont use the method shown in your photos Iwire not that it can not be done our company just installs ABCN in each conduit its just what we do never had a problem with a inspection ever .
Yes you can cut it closer but we never do it we like it neat and expandable to temp . Heres what it looks like
http://i611.photobucket.com/albums/tt195/stringking/P1010095_01.jpg

http://i611.photobucket.com/albums/tt195/stringking/P1010091.jpg
 
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pdesai_pe

Member
pdesai_pe

pdesai_pe

Is isloation phase as shown on IWIRE picture is an option from the standard switchgear manufactuer such as sq. D. or it is field modifications? Their thechnical group is not aware of this as an option.

what should be done if there are 10 sets of 600MCM for 4000Amp switchboard?
 

ohmhead

Senior Member
Location
ORLANDO FLA
Is isloation phase as shown on IWIRE picture is an option from the standard switchgear manufactuer such as sq. D. or it is field modifications? Their thechnical group is not aware of this as an option.

what should be done if there are 10 sets of 600MCM for 4000Amp switchboard?

Well lets see you will have four conduits with matching phased conductors in each conduit of the same phase to try and keep them the same length meaning 3 conduits with 3 x 600 mcm installed and one conduit with 2 x 600 mcm .
But you could install four per conduit ?

Per the code if you have more then 3 current carrying conductors its correction time so i guess you could install 4 conductors in one conduit two in the next but then your conduit would be increased in size and cost goes up so you would now have to run 700 mcm to install 4 conductors in that one conduit . But 4 inch conduit can handle this just go to a larger conduit .

Bottom line is there still not the same length so its a waste of time and money .

And the worst case is its a inductance issue in my opinion thats why we dont do it on our projects and the cost of it is major when your talking over 11 runs per phase .
 
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don_resqcapt19

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Illinois
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retired electrician
Is isloation phase as shown on IWIRE picture is an option from the standard switchgear manufactuer such as sq. D. or it is field modifications? Their thechnical group is not aware of this as an option.

what should be done if there are 10 sets of 600MCM for 4000Amp switchboard?
The switchgear is just standard gear. You can install the conductors in an "isolated phase" arrangement in any switch gear. It makes for much neater installation than the standard arrangement of one conductor of each phase in each raceway.
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
Well lets see you will have four conduits with matching phased conductors in each conduit of the same phase to try and keep them the same length meaning 3 conduits with 3 x 600 mcm installed and one conduit with 2 x 600 mcm .
But you could install four per conduit ?

Per the code if you have more then 3 current carrying conductors its correction time so i guess you could install 4 conductors in one conduit two in the next but then your conduit would be increased in size and cost goes up so you would now have to run 700 mcm to install 4 conductors in that one conduit . But 4 inch conduit can handle this just go to a larger conduit .

Bottom line is there still not the same length so its a waste of time and money .

And the worst case is its a inductance issue in my opinion thats why we dont do it on our projects and the cost of it is major when your talking over 11 runs per phase .
Ohm,
You can't install a parallel system unless you use the same number of conductors in each raceway of the parallel system.
How is the cost for an "isolated phase" installation higher than a standard installation? The termination labor goes down and the pulling labor stays the same. The isolated phase installation is cheaper.
Yes, there is some additional impedance on the feeder when you install it in an "isolated phase" configuration, but not really enough to worry about. The only thing you have to watch is any ferrous metal that encircles one or more, but not all of the raceways.
As far as the length, it is much more likely that all of the A phase conductors in an "isolated phase" installation will be the same length as each other than in a "standard" installation. There is no code or technical reason that would require the conductors of A phase be the same length as those of B or C phase.
 

iwire

Moderator
Staff member
Location
Massachusetts

Very neat, isolated phase comes out neater and closer to meeting the equal length requirements of the NEC.

Don't just say 'Thats no good because we do not do it' keep an open mind and think about it.

Yes, there is some additional impedance on the feeder when you install it in an "isolated phase" configuration, but not really enough to worry about. The only thing you have to watch is any ferrous metal that encircles one or more, but not all of the raceways.


Don, one other consideration is derating for four CCCs in a raceway.
 
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ohmhead

Senior Member
Location
ORLANDO FLA
Don_resqcapt Well yes i agree each should be the same number so how can this be done with 11 runs of 750 mcm in 4 inch conduit ?
How do you save cost with 11 runs

So 11 runs how many do you put in each run your way ?

Can you do this with 11 conduits or will it take a extra conduit ?

Think odd numbers not even .

You would need more to fit in the phases equal in each conduit it can not be do with odd numbers .


Yes i understand paralled feeders very well.http://i611.photobucket.com/albums/tt195/stringking/IMG_0630.jpg

There is no differance in pulling a feeder with same phase or the old standard way i do it everyday on every job labor its the same set up and labor and the impedance its nothing to worry about were looking at one foot of difference the old way to your new way .

I dont see the difference and underground piping is not the same distance ever old way or new way .

Every job we do has switchgear with many multiple runs this is common and its a nite mare to get these into that window Iwire i know you know what i mean !

Iwire i have a open mind and i disagree maybe i will learn something new today ?
 
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benaround

Senior Member
Location
Arizona
pdesai pe,

You say that these conductors run from the Utility Transfomer to the Main Disconnect, so

you will only terminate the Main Disconnect side and the Utility will terminate the Trans.

side. I hope the Utility does not cut the wires so they are unequal. :)
 
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