NEC Table 9, Effective Z?

[whitebear]

I thought I knew this stuff till someone at work asked me about NEC Table 9.

Neglecting Xc (as explained in note 1), Z= R cos theta + X sin theta (as explained in note 2).

So why when calculating Z (steel or PVC)for a #10, for example, do you come up with 3.424, not 3.6 ohms?

It seems below 1/0 the effective Z is rounded up and above 1/0 effective Z is rounded down. At approximately 600kcmil the effective Z is again below either Xl + R.

Is there a way to correlate the Z of the conductor with the Z or power factor of the load?

Thank you.

 

[cuky2000]

The NEC table 9 provide approximate values within +/- 5% margin.

Even thought the table provide different reactance values, the published impedance of #10AWG in magnetic and nonmagnetic conduits are shown with the same values.

I guess that you can use more accurate results if could be proved.