Question on parallel conductors?
- Thread starter Leagle454
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texie
Senior Member
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- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
You mention multiple phases. If you are asking if you can run all the conductors of a parallel circuit in a single raceway, then yes it is sometimes done and perfectly compliant as long as derating is addressed. If less than 24" no derating applies.
- Location
- New Jersey
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- Journeyman Electrician
The conduit does not need to be longer than 24". If it is longer than 24" and contains more than 3 CCC's then derating would apply. There is no objectionable current because all of the conductors of the circuit are in the same raceway.
Than what am I thinking of when the induction and EMF can cause over current? I’ve heard that it can melt the EMT the conductors are ran in.
Here’s an example of something similar. https://www.electricallicenserenewa...ation-Courses/NEC-Content.php?sectionID=297.0
qcroanoke
Sometimes I don't know if I'm the boxer or the bag
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- Roanoke, VA.
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- Sorta retired........
That is if you ran all A phase in a seperate metallic conduit all B phase in a seperate metallic conduit all C phase in a seperate metallic conduit and all neutrals in a seperate metallic conduit. Causes a lot of heat
- Occupation
- Licensed Electrician
From who? It would have to get up to 2000 degrees to do that.
gar
Senior Member
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- Ann Arbor, Michigan
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- EE
221028-2130 EDT
If you take a single long straight wire and provide its return current path a long ways away from said wire, then the magnetic field intensity varies as the inverse of the distance from the wire. So if the field intensity is X at 1 meter, then it is 0.5 * X at 2 meters.
If the source and return wires are very close to one another, then as you move radially from the wire pair, the residual magnetic field intensity drops off very rapidly. At 1 meter you will find it hard to detect any field.
.
If you take a single long straight wire and provide its return current path a long ways away from said wire, then the magnetic field intensity varies as the inverse of the distance from the wire. So if the field intensity is X at 1 meter, then it is 0.5 * X at 2 meters.
If the source and return wires are very close to one another, then as you move radially from the wire pair, the residual magnetic field intensity drops off very rapidly. At 1 meter you will find it hard to detect any field.
.
What it says in the link I posted is confusing me. “parallel conductors inside an auxiliary gutter must be grouped together to prevent current imbalance in the paralleled conductors due to inductive reactance”. If the phases have to be grouped together to prevent imbalance how is it allowed for all phases to run in one conduit when it would surely create an imbalance?
I mean for example A,B,C phase wires all in one conduit along with theirs neutral and ground wires (EGC)
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
The normal way a circuit is run, _all_ circuit conductors must be in the same raceway. So A,B,C,N in one conduit.
By doing this any current on one conductor is balanced by the other nearby conductors, and there is no induction on the surrounding conduit.
When you use parallel conductors, the general rule is that each parallel set be balanced and closely grouped. For example 3 conduit with the full complement of A,B,C,N in each.
As long as things remain balanced, you can have multiple parallel conductors in a single conduit, eg. 2 conduits with A,A,B,B,C,C,N,N in each. What matters is that things be balanced.
What you couldn't do is have A,A,A,A,B,B,B,B in one conduit and C,C,C,C,N,N,N,N in the other. This would be unbalanced and could induce current on the conduit.
Note that for certain non metallic installations you are permitted to do the isolated phase thing; in this case induction is prevented by getting rid of the conductive conduit.
Jon
By doing this any current on one conductor is balanced by the other nearby conductors, and there is no induction on the surrounding conduit.
When you use parallel conductors, the general rule is that each parallel set be balanced and closely grouped. For example 3 conduit with the full complement of A,B,C,N in each.
As long as things remain balanced, you can have multiple parallel conductors in a single conduit, eg. 2 conduits with A,A,B,B,C,C,N,N in each. What matters is that things be balanced.
What you couldn't do is have A,A,A,A,B,B,B,B in one conduit and C,C,C,C,N,N,N,N in the other. This would be unbalanced and could induce current on the conduit.
Note that for certain non metallic installations you are permitted to do the isolated phase thing; in this case induction is prevented by getting rid of the conductive conduit.
Jon
synchro
Senior Member
- Location
- Chicago, IL
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- EE
The parallel conductors inside of an auxiliary gutter need to grouped in separate sets where each set contains all of the phases and neutral (if used), as Jon explained above. Preferably, the conductors in each set should also be grouped in a consistent manner so that the spacing between the conductors in each set is substantially the same (within practical constraints) as the other sets. If not, then the inductive reactance mentioned in your quote above can have an effect on the current balance between the different sets that are placed in parallel. This is because the inductive reactance for the conductors in each set will increase if their spacing is increased. So if the conductors within one set was closely spaced but another set has them significantly further apart, then the one with closer spaced conductors will have a lower inductive reactance and therefore bear a larger share of the total current.
As far as current sharing between parallel conductors, inductive reactance will matter more as the size of the conductors is increased. That's because the resistance of conductors goes down quite a bit faster than the inductive reactance does as the diameter of a conductor is increased. Conversely, as you go to smaller sizes, in typical situations the inductive reactance eventually becomes negligible compared to the resistance. Of course, if conductors are being placed in parallel then the conductors are going to be toward the larger end of the scale, and so inductive reactance may then become a significant factor.
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ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
A service I saw a couple of years ago had three sets of three phase service conductors in two conduits. One conduit contained two sets and the other had a single set. Was that a code violation or a safety problem?
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
That is a code violation, because the conductors of each phase are supposed to have the same characteristics. Consider the phase A conductors; there are 3 of them. 1 is in a conduit with BCN; the other in in a conduit with ABBCCNN.
Is this a safety problem? If all conductors were balanced for length and conductor gauge, then I'd some difference in current flow because of different inductive effects and different temperature. I don't know if the amount of current difference is enough to be a problem.
-Jon
qcroanoke
Sometimes I don't know if I'm the boxer or the bag
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- Roanoke, VA.
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- Sorta retired........
So. Are you saying if I triple parallel I also have to have 3 raceways if I can't get them all in 1 raceway? I can't triple parallel using 2 raceways? Just trying to learn here.
Even if I am retiring in 3 weeks...
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
That's correct. In order for the current to be distributed among the conductor sets, each set must have in common any characteristics which affect impedance, which includes quantity of sets in each conduit.So. Are you saying if I triple parallel I also have to have 3 raceways if I can't get them all in 1 raceway? I can't triple parallel using 2 raceways? Just trying to learn here.
Even if I am retiring in 3 weeks...
As an example, we know that two sets of conductors in a conduit do not have twice the ampacity of one set.
qcroanoke
Sometimes I don't know if I'm the boxer or the bag
- Location
- Roanoke, VA.
- Occupation
- Sorta retired........
We know that the code says they don't or there's an electrical calculation that says they don't?
I'm bored and the race is boring too........winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
The code is base on a calculation.
Ampacity is set by the temperature limits of the conductors, as determined by self heating and external heat inputs. When you have other current carrying conductors in a conduit, they add additional heat, thus reducing the amount of allowed self heating.
Jon
Ampacity is set by the temperature limits of the conductors, as determined by self heating and external heat inputs. When you have other current carrying conductors in a conduit, they add additional heat, thus reducing the amount of allowed self heating.
Jon
synchro
Senior Member
- Location
- Chicago, IL
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- EE
Breaking up the parallel conductors into separate sets where each set contains conductors from all phases and the neutral (if applicable) greatly reduces the effect of metallic conduit on the inductive reactance and losses from eddy currents, hysteresis, etc. That's due to the cancellation that occurs in the magnetic fields of the circuit conductors in each set. And so each of the two sets that are in one conduit will have cancellation of its own conductor's magnetic fields. Because of that, I think putting two sets in one conduit and one set in the other conduit should not have all that much effect on the inductive reactances that are within the closed circuit.
However, Larry has an excellent point about the effect on the ampacity of having more conductors within a conduit. And so there will be differences in heat dissipated and thermal resistance to ambient between the conduit having one set vs. that with two. That will lead to temperature differences between them, and since conductor resistance increases with temperature it will cause some mismatch in the resistances of the one set vs. the pair of sets. And so the arrangement will suffer some amount of current mismatch on the parallel conductors because of thermal effects.
One thing to keep in mind is that current sharing depends on the ratio of the impedances of the parallel conductors including their terminations, etc., instead of the absolute impedance levels. As a result, when the parallel conductors have short lenghts and therefore less impedance, any differences in the impedance of terminations, wiring within enclosures, etc. will have a larger impact on how well the currents are balanced..
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Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
In my opinion there are 2 problems here :
1) the current induced in the gutter -and if all the phases and parallel wire and the neutrals are together in the same gutter this is the case of minimum induction in the gutter since the sum of all currents has to be zero.
2) current unbalance between parallel wire.
It depends on all wire reciprocal positions in the gutter.
See -for instance :
https://www.semanticscholar.org/pap...-Lee/e84e444ab5899f91301f7e738dbd92b01e8d0a23
1) the current induced in the gutter -and if all the phases and parallel wire and the neutrals are together in the same gutter this is the case of minimum induction in the gutter since the sum of all currents has to be zero.
2) current unbalance between parallel wire.
It depends on all wire reciprocal positions in the gutter.
See -for instance :
https://www.semanticscholar.org/pap...-Lee/e84e444ab5899f91301f7e738dbd92b01e8d0a23
How would you separate those same phase wires if they’re all running in the same conduit? You can’t it’s impossible. This is what is confusing me. The phases can be ungrouped when ran in a single conduit but must be grouped once in a gutter?
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