voltage drop and steel conduit

[drivetr.]

How does a metallic conduit or raceway cause a voltage drop?

 

[George Stolz]

Can you elaborate? I don't get the question.

 

[ron]

Metallic conduit or raceway does not cause voltage drop.

 

[Pierre C Belarge]

Hello Ron
I hope you have a great holiday and a healthy New Year!!!

Pierre

 

[W6SJK]

Metallic conduit or raceway does not cause voltage drop.

Yes it does, indirectly, if it's steel conduit... the impedance of the cable is higher in ferrous conduit. Check the NEC tables.

 

[Smart $]

How does a metallic conduit or raceway cause a voltage drop?

It does so by hindering the dissipation of I^2R heat when compared to like conductors handling the same amount of current while suspended in free air.

 

[W6SJK]

It does so by hindering the dissipation of I^2R heat when compared to like conductors handling the same amount of current while suspended in free air.

Hindering the dissipation of heat will indeed raise the temp of the conductors, which will increase the resistance a bit, but his question was specific to metallic conduits, not conduits in general.

 

[Smart $]

Hindering the dissipation of heat will indeed raise the temp of the conductors, which will increase the resistance a bit, but his question was specific to metallic conduits, not conduits in general.

Hindering heat dissipation is the major contributor to voltage drop for any type of conduit. Other differences, such as capacitive coupling, are quite small by comparison.

 

[ron]

If the question was "How does a metallic conduit or raceway cause a voltage drop?", my answer is ... it doesn't.
If I have a 120V circuit in any kind of raceway, the L-L or L-N voltage will be the same on either end of the circuit. Now if you try to draw current through that circuit, it's different.

Pierre, you too!

 

[drivetr.]

response

response

I'm referring to Chapter 9, Table 9. I read an article referring to Section 695.7 of the NEC and the author uses Table 9 in calculating voltage drop. He calculates the voltage drop of 500 feet, #700, @ 30% PF in steel conduit. Installing conductors in metalic conduit changes the reactance of the circuit.

 

[winnie]

When you put conductors into a metallic conduit, there is a certain amount of magnetic coupling between the conductors and the conduit. I can only surmise that the values in table 9 are derived from this fact. I do not _know_ the details, but can _guess_ at a few:

1) All conductors of a circuit are supposed to be run together in the same conduit. Since this means that the _net_ current flowing through the conduit will be zero, we have to be looking at the small magnetic field escaping from the 'loop' of the conductors laying side by side. IMHO this means that the numbers in the table are 'rules of thumb' which will actually change based upon the precise conductor configuration, size of conduit, etc.

2) The _reactance_ of conductors in steel conduit is raised because the steel is ferromagnetic, and results in more magnetic flux for the same magnetic field. This increases the inductance of the system, and thus the inductive reactance. Aluminium is not ferromagnetic. So for _reactance_ you have two tabulated values, one for non-ferromagnetic conduit, and one for ferromagnetic conduit.

3) You see transformer coupling to the conduit, and thus some small amount of eddy current flowing in the conduit. The eddy current losses in the conduit are resistive in nature, and show up as extra resistance. Since both aluminium and steel are conductive, but with different resistance characteristics, you have to have three columns: non-metallic, aluminium, and steel. Looking at the table values, my _guess_ is that because aluminium is _more_ conductive, you get more eddy current flow, and thus _more_ eddy current losses. I bet the AC resistance would be even higher if you used copper pipe as your conduit.

-Jon

 

[Smart $]

I'm referring to Chapter 9, Table 9. I read an article referring to Section 695.7 of the NEC and the author uses Table 9 in calculating voltage drop. He calculates the voltage drop of 500 feet, #700, @ 30% PF in steel conduit. Installing conductors in metalic conduit changes the reactance of the circuit.

It would have helped if you had mentioned ["30%" or poor] PF?power factor?in your first post. Reactance is generally not an issue in the typical voltage drop questions I've seen posted here. Perhaps this thread may help. Post again if you have any further questions.

 

[beanland]

Steel Conduit

Steel Conduit

Heat is not the issue. Steel conduit raises the impedance of the set of conductors. The higher inductance (not resistance) increases voltage drop.

 

[drivetr.]

Thank you

Thank you

Thank you all for the answers. Comparing Table 8 & 9, the resistance of conductors in a particular raceway increases depending on the type of material the raceway is made of (but not by much). I believe (and I might be wrong) because when a conductor is placed in a metallic raceway, the reactance (specifically the inductive reactance) of the conductor increases. This increase in reactance causes a greater voltage drop (again, not by much but that depends on the material, length, etc.) because the resistance of the conductor increases.

 

[drivetr.]

Two more questions about Table 9

Two more questions about Table 9

Why is there two rows per wire size? Table 8 has two rows up to and including #8; due to either stranded or solid wire. Why are two rows present for each conductor in Table 9?
Also, does anyone know of an easy method to use Note 2 of Table 9?

 

[ramsy]

Why is there two rows per wire size? Table 8 has two rows up to and including #8; due to either stranded or solid wire. Why are two rows present for each conductor in Table 9?

That's an excellent question. Tbl.9 in the 99 NEC did not include the top row. There is no explanation for this in the notes.

 

[winnie]

I believe (and I might be wrong) because when a conductor is placed in a metallic raceway, the reactance (specifically the inductive reactance) of the conductor increases. This increase in reactance causes a greater voltage drop (again, not by much but that depends on the material, length, etc.) because the resistance of the conductor increases.

Table 9 has separate columns for _reactance_ and for _resistance_.

Under the _reactance_ column, steel gives you a higher number. This is inductive reactance, caused by the ferromagnetic properties of steel. Inductive reactance implies energy being stored in a magnetic field, returned to the circuit at a later part of the AC cycle. Inductive reactance changes the power factor of the system.

Under the _resistance_ column, _aluminium_ conduit gives a higher number. _Steel_ the middle number, and PVC the lowest number. This is _resistance_, meaning real energy lost, usually heating something up. I _believe_ that this increase in apparent resistance is caused by eddy currents induced in the conduit, heating the conduit up.

Why is there two rows per wire size? Table 8 has two rows up to and including #8; due to either stranded or solid wire. Why are two rows present for each conductor in Table 9?
Also, does anyone know of an easy method to use Note 2 of Table 9?

Look at the top of table 9. The top number is ohms to neutral per kilometer, the bottom number is ohms to neutral per kilofoot.

-Jon

 

[drivetr.]

Very Good

Very Good

Very Good Jon,

Thank you

 

[ramsy]

Thanks Jon!

 

[Smart $]

Heat is not the issue.

I admit heat is not the issue as far as the OP is concerned. But while we're on the subject, I noticed Jon, too, mentioned heating in one of his posts. This leads me to the question I've often wondered about, never ran across any details, and never asked... Why are the resistance/impedance tables given at 75?C. Is this the temperature of the wire? ...ambient temperature inside the conduit? ...outside the conduit? ...the conduit, too? In many cases we are simply not permitted to have operating circuits at this temperature for any length of time. If this is meant to project worst-case scenario, is there an accepted method for adjusting to nominal operating parameters?

Steel conduit raises the impedance of the set of conductors. The higher inductance (not resistance) increases voltage drop.

These statements are completely unsubstantiated. The fact is, in many but not all cases, the impedance is lowered. Please review the table at the following link: Customized Table 9. The .htm file was generated in Excel, so it will import quite well back into Excel. (The process may work better if you right click on the download link on the second page and choose "Save Target As" in the local menu; additionally, does anyone know of a better, non-image, free file host?)