parallel conductors

[ohmhead]

Well as a electrician with a limited electrical education i need help ?


What i think Skin Effect is
skin effect is current pushed by the magnetic flux induced in the center of the conductor by its own self magnetic force as current flows in it .

Now the current is limited to the outer surface area of that conductor which now limits its rated current flow and heat ,reactance and ac resistance is the product of this .


What i think Proximity Effect is
proximity effect is worst case its when two or more conductors run in parallel next to each other and are of the same phase .

Its effects now are the same as the above effects but now the current flowing in both conductors in the same direction is pushed to the outer surface but with more force and current is now limited to just half the surface area of the single conductor of the outer shell .

I know buss ducks and buss bars are run like this but the surface area of a rectangular bar or buss is more surface area then a round conductor .

If more heat and reactance is a problem with parallel same phase conductors why do we do it i always tought by installing all phases in one conduit it was a better installation ?

 

[Doug S.]

In a 3 phase installation the voltage/current run 120 deg out phase with each other. SO when voltage/current is peaking on one conductor it will NOT be on any of the other two supply conductors. Nullifying your concern.

And if that doesn't help someone will be along shortly to greatly expound on my simple explanation...

 

[ohmhead]

In a 3 phase installation the voltage/current run 120 deg out phase with each other. SO when voltage/current is peaking on one conductor it will NOT be on any of the other two supply conductors. Nullifying your concern.

And if that doesn't help someone will be along shortly to greatly expound on my simple explanation...

Well so what your saying is we only have three phase power and at 120 deg. apart between the phases and there is never any other angle in the phase cycle and can not cancel out one another ever ?

Well i disagree with you heres why at any point in time when say A phase is going positive then C is going more negative or B is at some point in time which is not the same as C or A there canceling out one another magnetically meaning the conductors in the raceway ? i maybe wrong so explain this please comments

 

[ohmhead]

Mutual inductive coupling

Mutual inductive coupling

Well a conductor self inductance from magnetic flux circulating inside and outside itself produced by the current flowing in itself .

Mutual inductive coupling !



Three phase waveforms are 120 degrees out of phase with each other yes,
so when A phase is at its peak value positive then C & B are at 50 % negative value which is a canceling effect in a raceway of long length conductors . Meaning opposing fields in reversal of other conductors this limits heat of eddy currents induced from self induction .

What iam asking is this true of multi paralleled runs of feeders in raceways meaning paralleled phase A in one conduit paralleled phase B in one conduit and also phase C the neutral is in one conduit also by itself does this configuration of conductors and raceways make more heat or is it better to run these with all phases ABCN in one raceway not isolated ?

Just asking for thoughts i maybe wrong if so i would like to have it explained in detail why ?

 

[Rockyd]

Recommending a quick peek at 300.3 and it's entirety. Then maybe a trip over to 300.20 and it's entirety. I'm at a remote jobsite, so I don't have a handbook to see if it expands for you all that is involved. Hope this of some help.

 

[cadpoint]

While this is not simple subject matter, I don't think that many will stab at it to both correct your understanding or present a simple lay-mans description either... frankly I don't blame them, these are tough facts to follows.

It's tough enough to grasp what's happening to the wire as the type of voltage is applied. It goes nuts with the pulsing of 3 phase's!

I'm going to throw out two web pages that I read up on, and like, if you pay the math no mind (ok don't stop reading because of the equations) theres alot of what explained as DC or AC and HV-AC is applied to a wire.

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/skineffect/page1.html

http://www.allaboutcircuits.com/vol_2/chpt_3/6.html

I only searched for dc skin effect, BTW

If I missed my Mark, I won't be the last time!

Enjoy

 

[jim dungar]

Well as a electrician with a limited electrical education i need help ?


What i think Skin Effect is
skin effect is current pushed by the magnetic flux induced in the center of the conductor by its own self magnetic force as current flows in it .

Now the current is limited to the outer surface area of that conductor which now limits its rated current flow and heat ,reactance and ac resistance is the product of this .


What i think Proximity Effect is
proximity effect is worst case its when two or more conductors run in parallel next to each other and are of the same phase .

Its effects now are the same as the above effects but now the current flowing in both conductors in the same direction is pushed to the outer surface but with more force and current is now limited to just half the surface area of the single conductor of the outer shell .

I know buss ducks and buss bars are run like this but the surface area of a rectangular bar or buss is more surface area then a round conductor .

If more heat and reactance is a problem with parallel same phase conductors why do we do it i always tought by installing all phases in one conduit it was a better installation ?

While your basic concept of skin effect is correct, it is never a concern for 60Hz current until conductor are larger than 266kcmil. For all intents skin effect is not an issue for paralleled power conductors whether they are grouped as all of one phase or one of each phase. I believe the NEC ampacity tables include the adjustment factors for skin effect for 300kcmil and larger cables.

Again, your idea of proximity effect is good. Proximity effect primarily comes into play when the paralleled conductors are grouped (segregated) by phase, but again at 60Hz frequencies the effect is negligible and can usually be ignored.

 

[ohmhead]

Well Jim and Cadpoint , Thanks for the info Jim i run lots of parallel runs at work most runs are larger than 500 mcm and in runs of 8 or more .

My understanding was that at 60 hertz if one installed isolated phases in conduit it would produce heat in the conduit do to the induced eddy currents in the metal of the conduit and that the paralleling of one phase was only aloud in underground pvc conduit non magnetic conduit .

If this is possible in conduit at 60 hertz then is this not a problem at 60 hertz ?

I fully understand that its worst case is at higher frequency lets say above 400 hertz more so even higher like 10khz .
Were not using litz wire for conductors and we can not laminate each strand in a 500 mcm .

My thoughts were that at any point in the cycle of the three phases one phase would be able to cancel out the magnetic effects of the opposite phase during the cycles of wave transistions at different points during the cycles of the three phases being generated.

Thats why we can run it in metal conduit above ground and its fine the phases cancel each other out yes/ no ?

And if so would this canceling effect would help with the eddy currents induced in the conduit above ground as we do run parallel above ground but in mixed phase conductors of ABC together with no problem every day .

So if your underground in pvc conduit does this induced current self made eddys create a heat source internally by isolated phased conductors in one conduit produce heat generating at a higher ac resistance internally by same phase grouping of conductors .
Meaning heat conductors to a higher level inside that pvc conduit by self induced eddys internal to each conductor .

I kinda see a problem at 60 hertz yes / no / why ?

 

[jim dungar]

The problem with all of the same phase conductors in one magnetic conduit is heating due to magnetic fields, not skin or proximity effect.

Segregated phases must not be installed in individual magnetic raceways, but they are not restricted to underground PVC.

 

[iwire]

Segregated phases must not be installed in individual magnetic raceways, but they are not restricted to underground PVC.

I believe the NEC restricts them to just that.:-?

 

[jim dungar]

Yes, 300.3 restricts segregated conduits to underground PVC, however 5kV busway is available in segregated phase construction. 392.8(D) prohibits segregated cables in cable tray, but I see no such restriction for 370 for cablebus.

 

[don_resqcapt19]

I believe the NEC restricts them to just that.:-?

The code making panel says that you are correct, however they really didn't act on the part of my proposal that said parallel installation in non-metallic wiring methods is permitted by 300.3(B)(3).

3-11 Log #2229 NEC-P03 Final Action: Reject
(300.3(B)(1) Exception)
____________________________________________________________
Submitter: Donald A. Ganiere, Ottawa, IL
Recommendation: Delete the following:
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).
Substantiation: The exception is not required. Such installations are permitted
by 300.3(B)(3) in all locations, not just underground locations.
Panel Meeting Action: Reject
Panel Statement: This exception is necessary to permit installations where
nonmetallic raceways are installed in close proximity with all of Phase A in one
raceway, all of Phase B in another raceway, all of Phase C in another, and all
the grounded conductors in another raceway. Where this is useful is between
underground manholes so the phase conductors can exit the raceways and be
easily racked within the manhole, taking up less space, and making it easier to
do testing and maintenance on the installation. This would not be permissible
without this exception.
Number Eligible to Vote: 13
Ballot Results: Affirmative: 13

 

[ohmhead]

The problem with all of the same phase conductors in one magnetic conduit is heating due to magnetic fields, not skin or proximity effect.

Segregated phases must not be installed in individual magnetic raceways, but they are not restricted to underground PVC.

Well Jim lets look at what proximity effect does internally the current flow inside the conductors side by side . Each conductor from center of conductor pushes out its normal flow of current outward towards the outer side wall now resistance is a thin exterior wall to one side of conductor .

Meaning if you had two conductors side by side from the center of them outward in two opposite directions left and right they now force or repel the currents flowing in same direction away from there center but in opposite directions in parallel not like a single conductor in all directions .

[ 0 - 0 ]
So you only have half of you surface area this makes heat in the conductor in addition to your heat in the conduit .

The eddy currents are inside the conductors also when proximity effect is in the picture not just the metal conduit .

So if there grouped together you have heat generated .


I really hate to say it but thats what skin effect is its the eddy currents re directing the normal flow of current which forces the current outward to the thin outer layer of conductor. proximity is a major player do you agree yes/ no ?

 

[jim dungar]

... proximity is a major player do you agree yes/ no ?

No.

Proximity effect occurs whenever any two current carrying conductors are next to each other. It exists in both segregated and non-segregated bundles of conductors. If the current flow is in the same direction the current distribution in each conductor is greater in its 'outer half', but if the current flow is in opposite directions the current is greater in the 'inner half'.

At normal power frequency of 60Hz, skin effect and proximity effect are all but negligible for typical circuit conductor sizes (i.e. 750kcmil max).

 

[ohmhead]

No.

Proximity effect occurs whenever any two current carrying conductors are next to each other. It exists in both segregated and non-segregated bundles of conductors. If the current flow is in the same direction the current distribution in each conductor is greater in its 'outer half', but if the current flow is in opposite directions the current is greater in the 'inner half'.

At normal power frequency of 60Hz, skin effect and proximity effect are all but negligible for typical circuit conductor sizes (i.e. 750kcmil max).

Well Jim yes i do agree with that very nice thats what iam talking about . Now question you say at 60 hertz its not a factor so we dont have skin effect at 60 hertz ?

What frequency does it start at ?

So i see its 750 mcm or larger only .

And when will parallel same phase conductors in groups its not a increase in skin effect or proximity effect so i dont need to worry thanks Jim glad thats over .

 

[jim dungar]

Well Jim yes i do agree with that very nice thats what iam talking about . Now question you say at 60 hertz its not a factor so we dont have skin effect at 60 hertz ?

What frequency does it start at ?

So i see its 750 mcm or larger only .

And when will parallel same phase conductors in groups its not a increase in skin effect or proximity effect so i dont need to worry thanks Jim glad thats over .

I did not say skin effect does not exist at 60Hz, I said it was negligible (something like 1.01x at 500kcmil but I don't trust my memory) and accounted for in NEC ampacity tables.