Table 9 values

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ryan_618

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In table 9 of chapter 9 (2002), the impedance of a circuit is greater when installed in an aluminum raceway and the impedance is less when installed in a steel raceway.

Now, I don't claim to be a big theory guy, but isn't inductive reactance more of a problem in steel and therefore create more impedance? :confused:
 
Re: Table 9 values

Sorry, Ryan, my thinking cap is at the dry cleaners. What I see is even stranger than what you describe, and I can?t think my way out of it this early in the morning.

Take a look at any copper cable from 3/0 and up. The ?AC Resistance? column has a higher value for Aluminum Conduit than for Steel Conduit. Now look to the right. The ?Effective Z? column has a higher value for Steel Conduit than for Aluminum Conduit. :eek:

How can that be? The ?Effective Z? is calculated by taking DC resistance and AC impedance, and adding them using the Pythagorean relationship (C*2 = A*2 + B*2). The DC resistance is the same, so the bigger AC resistance should give the bigger Effective Z. :confused:
 
Re: Table 9 values

Don't have any great answers either, but I did check the history of Table 9 and it has be almost exactly the same since 1987. In 87 the table was completely reworked and they add the information concerning aluminum conduit.
 
Re: Table 9 values

Originally posted by ryan_618:
I wonder if this was addressed as errata?
Nope. I just downloaded all the 2002 errata, and Table 9 was not addressed. Still :confused:
 
Re: Table 9 values

Effective Z and XL make sense. It is higher for steel than alum. The steel acts like a iron core in an inductor. Just like the iron core raises the inductance (and impedence) of a coil, the steel conduit raises the inductance (and impedence) of the conductors.

This also fits in with the GEC going to a grounding electrode. It is not allowed in steel (or is it any metal?) conduit because it raises the impedence of the GEC. (Why is this allowed if we ground both ends of the conduit? Would grounding both ends have an effect on the values in table #9?)

That leaves us with AC resistance. It is highest for alum, medium for steel, and lowest for PVC. Like Charlie B, i'm puzzled. I thought it might have to do with the temp. of the conductors, and the thermal conductivity of alum, but that should also give alum conduit the lower number.

I am thinking the answer might be found in transmission line theory (i.e. a radio communications and antenna theory textbook), but I remember very little of that stuff.

Steve
 
Re: Table 9 values

Let?s complicate the issue one level further. We can put a #14 into a 6 inch conduit, if we wish. We could also put 61 of them (thhn) into a 1.25 inch conduit. Both of those circumstances have an impact on the actual impedance. As Steve pointed out, a steel conduit surrounding a conductor raises its impedance. But there is a difference between a conduit located at a great distance from the conductor and a conduit that is nearly filled by the insertion of the conductor. Also, a large number of wires in close proximity will possess the properties of mutual inductance and mutual capacitance. But Table 9 does not ask what size of conduit or how many wires are in the conduit. Continuing to be :confused: :confused:

[ September 02, 2004, 10:26 AM: Message edited by: charlie b ]
 
Re: Table 9 values

Charlie, though you are correct that Table 9 does not address the size of the conduit, the table does make clear that the values are for "Three single conductors in conduit." I guess you're on your own to consider the effects of mutual impedance and capacitance for more than 3 conductors in a conduit.
 
Re: Table 9 values

I may be out in left field here but look at note 2 to table 9. DC resistance is not used to calculate effective Z. The formula is R*cos? + X*sin? where R is AC resistance, X is inductive reactance and ? is power factor angle which for the .85PF is 32deg. Using this formula the values in the table match up.
 
Re: Table 9 values

Aha, a clue! ;) When I get back to my office, I'll take a look at the Table again. Getting less :confused: all the time.
 
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