Same Electrical Characteristics

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charlie b said:
I don't know how any VD calculator would alter its results, if you selected a different material for the conduit. I just believe that unless you are dealing with many hundreds of feet, or perhaps even thousands of feet, the influence of the conduit material would be too small to measure.
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It is a noticeable difference. For smaller wire (below the aught ranges) the impedance difference is less than a few percent (steel having higher X and lower R). From there to 500s or so it may be up to a 5-10% difference. Above that it seems to top out with steel having about a 20% higher impedance.

Not much change in the steel to aluminum resistance or impedance ratios but the kicker is the increase in the X/R ratio. That is significant because the steel run has an inductance about 20-25% higher than the aluminum (thus the 20% top-out for the bigger wire).

PS. And yes that included the conductor, CU I believe.
 
The difference in impedance and 'ac resistance' between conductors in conduits of different materials is large enough that it shows up in the tables in Chapter 9. (See table 9, look at the larger conductor sizes, and note that there are different table entries for PVC, steel, and Aluminum conduit).

I take from this that a 500kcmil Cu conductor in PVC conduit has different _electrical_ characteristics than a 500kcmil Cu conductor with the same insulation in Aluminum conduit.

You will note that the 'AC resistance' of the wires is highest with Aluminum conduit. I am guessing that the effect is eddy currents in the Aluminum because of its relatively low resistance. I bet a copper conduit would be even worse.

Due to differences in geometry I would expect two matching conductors in different diameter conduits of the _same_ material would have _slightly_ different electrical characteristics, but my gut feeling is that this difference is similar t0 the expected variation between conductors due to manufacturing variability. (No two conductors will ever have _exactly_ the same characteristics, no matter what the code requires. )

-Jon
 
Ultimately, using two different sizes of raceway would have similar effect to using same size raceways but different lengths of conductors... and we all know Code requires same length conductors. :happyyes:
 
gadfly56 said:
True. It wasn't clear, to me anyway, whether or not the phases were grouped or separated in the OP.
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Hence my suspicion that the calculator program will give different results for the same wire if you specify a one way versus a round trip length.
If it always includes the choke effect from the raceway, it is of little use in NEC compliant installations (except buried duct banks where uniphase is allowed under certain conditions and in sizing GECs for voltage drop. Does anyone do that?
 
infinity said:
A metallic conduit is conductive and a PVC conduit isn't, are those electrical characteristics?
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charlie b said:
I think not. They are physical properties that have no bearing on the electrical circuit that they surround.
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iwire said:
I will have to disagree.

The capacitance coupling of the circuit to the conduit is an electrical characteristic and will be different between PVC, steel or aluminum.
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I agree with your disagreement. :)

Even if the conduit type has no bearing on the circuit involved the two different materials, PVC or metal, are not the same electrical characteristics.
 
infinity said:
I agree with your disagreement. :) Even if the conduit type has no bearing on the circuit involved the two different materials, PVC or metal, are not the same electrical characteristics.
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And I continue to disagree with his (and your) disagreement. Tell me, do your wrist watch and you left boot have different electrical characteristice? Oh, wait, they are not intended to carry current anyway, so I suppose we don't have to ask. :roll:

 
charlie b said:
And I continue to disagree with his (and your) disagreement. Tell me, do your wrist watch and you left boot have different electrical characteristice? Oh, wait, they are not intended to carry current anyway, so I suppose we don't have to ask. :roll:
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Charlie,
If the word in the code section was still "physical" like it was prior to the 2008 code, would the conduits have to be of the same size?
 
Now there you go, Don, throwing "Charlie's Rule" back at me. I have been talking in terms of the intent, and now you bring to bear the actual words of the code. No fair.
 
Charlie,

As I mentioned above, the electrical characteristics of conductors PVC versus Aluminum raceways is different enough that the different values are reported in the code book. For the largest conductors the tabulated 'AC resistance' is different by about 10% (See NEC chapter 9 table 9)

The tabulated values are specifically for 3 phase circuits with 3 single conductors in the conduit...so we are not talking about imbalanced current flow here.

I agree with you that the electrical characteristic difference between two otherwise matching conductors in conduit of similar material but _different_ size is probably small (negligible??) but the effect of different conduit material is clearly a pretty large effect.

Additionally, if you consider what happens during a fault, you have to consider the conduit itself as a conductor, even if not normally current carrying. If, during a ground fault 1 conduit (because of larger size and lower resistance) carries a larger portion of the fault current, then you would expect the conductors in that conduit to also carry a larger portion of the fault current.

-Jon
 
From the point of view of basic physics, the conductors have become a single turn inductor/transformer with the raceway as the core. This will increase the mutual inductance among the wires, but I still do not see any effect on the case of balanced current, since there will be zero magnetic field.
I do not accept the assertion that since the table lists three wires in the raceway it also means that their measurement technique used only balanced currents.
I think it is more likely that they just used an invalid technique or else deliberately measured the single current impedance.
The single current values would be perfectly appropriate for line to neutral or unbalanced line to line loads.
Well, actually it would be a bit off for the latter too because of phase angles....
 
GoldDigger said:
From the point of view of basic physics, the conductors have become a single turn inductor/transformer with the raceway as the core. This will increase the mutual inductance among the wires, but I still do not see any effect on the case of balanced current, since there will be zero magnetic field.
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Where, exactly is the magnetic field zero?

If you have all three phases grouped together, and the net current is zero, then in the _far field_ the net magnetic field is zero.

But if you have a probe sitting between the conductors, then the magnetic field is _not_ zero. Unless all of the conductors somehow occupy exactly the same space (rather than being next to each other) there will be some localized fields.

These localized fields will interact with nearby conductive materials.

I did a bit of digging, and found that I participated in similar conversations on this forum a few years back:
http://forums.mikeholt.com/showthread.php?t=105246
http://forums.mikeholt.com/showthread.php?t=82339&page=2

-Jon
 
I also thought that the conduit type etc was important for calculating the fault current for the system. No? I fail to see however how 2 different size raceways of the same material would really matter. I understand by code it may be an issue but is it really an issue?
 
I did make an incorrect statement. The total current linked to the core (the raceway) will be zero, so the net magnetic field in the raceway will be zero.
I agree with your point that there may be local magnetic fields in parts of the conduit, but I do not see those fields having a significant effect in the induced voltage in the individual wires.
If the table impedance corrections (reactive part) are in fact something well below 10% of the reactive impedance calculated for a single unbalanced current, I could accept them as valid.
More, perhaps, after I have had time to follow the old threads.
 
Dennis Alwon said:
I also thought that the conduit type etc was important for calculating the fault current for the system. No?
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Yes.

Dennis Alwon said:
I fail to see however how 2 different size raceways of the same material would really matter. I understand by code it may be an issue but is it really an issue?
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I sure wouldn't think so, at least at the lower frequency, lengths, and size differences we would normally see. I would have to hear a compelling argument otherwise before I would even be interested in making the impedance calcs.
 
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