Electrical characteristics of raceways

[jumper]

I pulled these posts from another thread to ask my own question. The thread was on parallel conductors.

Certainly the raceway type must be the same, IMHO.

Why? I am not being a smart aleck. Real question.

The physical aspects of the sets must be identical.
Electrically that means at a bare minimum that the number of conductors per raceway must match, and the ferrous/non-ferrous nature of the raceway must match.

Why does the type of raceway affect the ampacity and/or resistance of conductors?

Like when using Table 9, Chapter 9. You have columns for steel, PVC, and AL raceways. The numbers for each are the same until you get up to larger sizes like #1 or such.

310.10(H)(3) says: (3) Separate Cables or Raceways.
Where run in separate cables or raceways, the cables or raceways with conductors
shall have the same number of conductors and shall have
the same electrical characteristics.

What actually happens if they do not? For example, 2 parallel sets - 1 in PVC and 1 in EMT.

I understand length, insulation, size, of conductors but how does the raceway type come into play?

 

[iwire]

I pulled these posts from another thread to ask my own question. The thread was on parallel conductors.







Why does the type of raceway affect the ampacity and/or resistance of conductors?

Like when using Table 9, Chapter 9. You have columns for steel, PVC, and AL raceways. The numbers for each are the same until you get up to larger sizes like #1 or such.

310.10(H)(3) says: (3) Separate Cables or Raceways.
Where run in separate cables or raceways, the cables or raceways with conductors
shall have the same number of conductors and shall have
the same electrical characteristics.

What actually happens if they do not? For example, 2 parallel sets - 1 in PVC and 1 in EMT.

I understand length, insulation, size, of conductors but how does the raceway type come into play?

As I understand it the type and even size of the conduit will change the overall impedance. That will change the current division between the sets

Here is an analogy that I am sure will be picked apart. :lol:

Considered you have a single stream of water passing through your yard that you want to split into two streams flowing exactly the same rate.

It will be easy to get close to the same flow but as you try to get the exact same flow even the smallest changes to the streams direction, width, depth, material type etc will become significant.

That's my story, take it for its worth.

 

[jumper]

As I understand it the type and even size of the conduit will change the overall impedance. That will change the current division between the sets

Here is an analogy that I am sure will be picked apart. :lol:

Considered you have a single stream of water passing through your yard that you want to split into two streams flowing exactly the same rate.

It will be easy to get close to the same flow but as you try to get the exact same flow even the smallest changes to the streams direction, width, depth, material type etc will become significant.

That's my story, take it for its worth.

Appreciate it. This sentence in your analogy is the heart of my question:

As I understand it the type and even size of the conduit will change the overall impedance.

What exactly causes this? Changes in the conductors I get, but type of raceway?:?

Where are the darn eggheads at?

 

[iwire]

Appreciate it. This sentence in your analogy is the heart of my question:



What exactly causes this? Changes in the conductors I get, but type of raceway?:?

Where are the darn eggheads at?

I see.

Well for me that is as far as I go, I suspect the answers will be to nerdly for me to really comprehend.

 

[wwhitney]

What exactly causes this? Changes in the conductors I get, but type of raceway?:?

Where are the darn eggheads at?

Other heads more egg-shaped can give you a better answer, but the basic idea as I understand it is that the electromagnetic field set up by the current carrying conductors interacts with the conduit. This interaction depends on the conduit material and can add a small amount to the overall impedance of the conductor. So the choice of conduit material affects the conductor impedance, and if you are trying to match impedance nigh perfectly for the purpose of paralleling, then you need to match conduit material and even conduit size.

Cheers, Wayne

 

[jumper]

I see.

Well for me that is as far as I go, I suspect the answers will be to nerdly for me to really comprehend.

Yeah it is a nerd question, not a common sense one. Common sense is that we do it because the code says so.

 

[jumper]

Other heads more egg-shaped can give you a better answer, but the basic idea as I understand it is that the electromagnetic field set up by the current carrying conductors interacts with the conduit. This interaction depends on the conduit material and can add a small amount to the overall impedance of the conductor. So the choice of conduit material affects the conductor impedance, and if you are trying to match impedance nigh perfectly for the purpose of paralleling, then you need to match conduit material and even conduit size.

Cheers, Wayne

Okay cool. Looking at T9 of Chapt.9 it seems that PVC adds the least amount of resistance, then AL followed by steel.
This is carried over into the total Z, impedance. Yes?

 

[wwhitney]

Okay cool. Looking at T9 of Chapt.9 it seems that PVC adds the least amount of resistance, then AL followed by steel.
This is carried over into the total Z, impedance. Yes?

Well, I don't fully understand the ways in which the interaction of the conductor's electromagnetic field with the conduit affects the overall impedance. For copper wire, Table 9 lists Reactance (X_L), AC Resistance, and Effective Impedance at 0.85 PF.

Looking just at the AC Resistance columns, the trend starting at size 3/0 and larger is that Aluminum Conduit > Steel Conduit > PVC; for smaller sizes, they are listed as equal to two significant figures, but that probably just means the difference is less than 1%. In the Reactance Columns, Steel > Aluminum = PVC at all sizes. And in the Effective Impedance at 0.85 PF columns, the trend starting at sizes 3/0 and larger is that Steel Conduit > Aluminum Conduit > PVC.

I don't know why for large sizes, the trend for AC Resistance is Aluminum > Steel, but for Effective Impedance at 0.85 PF it is Steel > Aluminum.

Cheers, Wayne

 

[electrofelon]

I am also interested in this question. Now it makes total sense if all of the conductors are not in the same raceway, or neutral is returning via another path, but if this is not the case, what is happening? I suppose the electromagnetic fields are still not canceling perfectly?

 

[__dan]

Magnetic field density in iron can be ~ 1000 to 2000 times the density in air. Intuitively, if the magnetic field density is a lot less through the air, it is spreading out a lot more or occupying a lot more physical space for cable tray or PVC raceways compared to EMT or RGS.

Whether it causes more or less impedance for one material over the other, It is causing an impedance difference from one material to the other. That will give you an uneven split of the current flow and potentially excess current over the rating, on the wiring method with the lower impedance.

Coax will have a lower capacitance and inductance compared to a parallel string of the same resistance wire. The impedance is lower by the geometry of the configuration.

To split the current flows equally, the geometry and the material methods would have to also match.

 

[Sahib]

I think the insistence by code for same nature of raceways for parallel conductors is for a different reason because if both supply and return conductors are enclosed in same raceway, the residual magnetic field and so voltage drop is negligible. The reason is different type raceways may adversely affect OCD operation during a faults, if used carelessly.

 

[Julius Right]

At first, in my opinion, one has to pick the cable type and size up from the table if this is tabulated. If not, you have always the alternative C: to calculate it.
You may follow the basis of Neher and McGrath or IEEE 835 or a known software.
By the way the resistance depends on temperature, skin effect and proximity effect usually. In a steel or iron conduit one has to add conduit loss [yp as per Neher and McGrath] and to multiply the skin and proximity effect by 1.7 for steel and 0.8 for iron.
IEC 60287-1-1 recommends 1.5 factor for skin and proximity effects.
No one is referring to aluminum conduit. It seems-according to Table 9-this factor has to be more.:weeping:

 

[jim dungar]

Why does the type of raceway affect the ampacity and/or resistance of conductors?
I understand length, insulation, size, of conductors but how does the raceway type come into play?

Really simplistic answer:
The current wants to 'magnetize' the raceway, similar to how a transformer core gets magnetized. Effectively the current does work magnetizing and heating steel. Aluminum and PVC don't magnetize very well so they don't impede the current hardly at all.

 

[Tony S]

Heat dissipation governs both the % fill of any conduit and the ampacity of the conductors.

 

[jumper]

Thanks everyone. I appreciate it.