Foundations of Voltage drop calculation

[Fer Lefer]

George:

Take a look at 90.5. A table is just a table, there is no language to make us do anything about voltage drop in most circumstances.

A FPN (fine print note) is not an enforceable rule, it's just additional information.

This and ALL the others are tables. Right. And they contain values that we use in our day-to-day job, as mandatory. Isn't it? OK, VD is not mandatory, by NEC, except in the two cases you've quoted in that url.

I just wanted to know how you guys handle the notes in Table 9 (By the way, they are NOT FPN). So, if anyone cares about adjusting the values, I'll have to look for information in that issue anywhere else.

Do you know about any book, or paper that can help me with this: not the formulae, not the how's, but the why's?

We do this solely because we want to, not because the rules say we must.

That's one point of view; others tell you to do it.:-?

Resistance changes with temperature changes. If you want a more accurate calculation, knowing the temperature that the resistance is taken is a good thing to know.

If resistance change, why not the reactance? Again, I don't think it is high level engineering, just plain aritmetic plus some trigonometry.:roll:

 

[jim dungar]

If resistance change, why not the reactance? Again, I don't think it is high level engineering, just plain aritmetic plus some trigonometry.:roll:

Very simply, reactance is "caused" by the magnetic field created by the current flow through a conductor. The current flow is not affected by temperature therefore the reactance is not.

 

[Fer Lefer]

Very simply, reactance is "caused" by the magnetic field created by the current flow through a conductor. The current flow is not affected by temperature therefore the reactance is not.

Thanks, Jim, but the point was some replies before this last: The notes of Table 9 state that the values of R and X are only valid for some conditions written there; the question was: how do you handle the changes in R and X under different installation conditions, in order to figure out the voltage drop? :smile:

 

[jim dungar]

Thanks, Jim, but the point was some replies before this last: The notes of Table 9 state that the values of R and X are only valid for some conditions written there; the question was: how do you handle the changes in R and X under different installation conditions, in order to figure out the voltage drop? :smile:

As you stated earlier, you may adjust the resistive values based on your new ambient temperature. The reactive values (X) are not affected by temperature and therefore no adjusment is needed.

Take the values shown in NEC table 9 (which is simply a reference source, just like the IEEE Red book) and work the effective Z formula backwards to determine the XL of your circuit. Then make adjusments for the PF and temperature. Finally caluclate your new effective Z.

 

[chris kennedy]

Of all the voltage drop discussions I've read on this Forum, I have never seen anyone mention a place where V.D. adjustments are mandatory.

"The supply systems covered by Article 647 are subject to mandatory voltage-drop requirements."

These are odd systems that I've inquire about on another thread.

 

[Fer Lefer]

As you stated earlier, you may adjust the resistive values based on your new ambient temperature. The reactive values (X) are not affected by temperature and therefore no adjusment is needed.

Take the values shown in NEC table 9 (which is simply a reference source, just like the IEEE Red book) and work the effective Z formula backwards to determine the XL of your circuit. Then make adjusments for the PF and temperature. Finally caluclate your new effective Z.

I think that I'm not clear: I know that reactance doesn't change with temperature... but they're affected with the position of the conductors: so, the reactance of three sets of 4-4/0 within conduit is not the same as if they were in a cable tray.

Question was: Do you calculate the reactance in this configuration, or you take the values in Table 9, forgetting what the notes say?

Are worth the calc's for VD, Shortcircuit parameters, load flow or whatever analysis yo do, when you need the reactance?

Thanks anyway for the feedback.

 

[Fer Lefer]

Of all the voltage drop discussions I've read on this Forum, I have never seen anyone mention a place where V.D. adjustments are mandatory.

That was my question at the very beginning: Doesn't anybody take into account the notes of Table 9? The resistance and reactance values stated are, let's say + 95% accurate, even if we have THW-LS cables in an aluminium cable tray on 86?F ambient temperature? :-?

Let's forget a little about "mandatory": Are not worth the calc's adjustments for engineering purpouses?

Thanks anyway for the feedback.

 

[jim dungar]

I think that I'm not clear: I know that reactance doesn't change with temperature... but they're affected with the position of the conductors: so, the reactance of three sets of 4-4/0 within conduit is not the same as if they were in a cable tray.

If your cables are not in the configuration of Table 9, you may want to look at other resources (i.e. IEEE color books) for your reactance values, that is your choice. Many people I know take the worst case reactance values (usually conductors in magnetic conduit) and always use them. The estimate of current draw probably provides more variability in the VD than does the difference in reactance for most circuits (especially for those circuits measured in 10's and maybe even 100's of feet).