Why use AC resistance instead of DC resistance?

At 60 Hz, skin effect and proximity effect raise a conductor's effective resistance above its DC value, especially for large conductors. NEC Table 9 lists the AC resistance so that voltage-drop and fault calculations on alternating-current circuits are accurate, unlike Table 8 which gives DC resistance.

Why does conduit type change the numbers?

The magnetic properties of the raceway affect both reactance and, for steel conduit, the AC resistance. Steel conduit increases inductive reactance and effective resistance compared with non-magnetic PVC or aluminum conduit, so Table 9 gives separate columns for each material.

How do I get effective impedance Ze?

Table 9 footnote gives the effective Z at 0.85 power factor as Ze equals R times power factor plus X times the sine of the load angle. This tool lets you set the power factor and computes Ze, which is the value you multiply by length and current for an accurate AC voltage drop.

What is the difference between Table 9 and Table 8?

Table 8 gives DC resistance and conductor properties like area and strand count, used for simple DC drop and for sizing equipment grounding conductors by area. Table 9 gives the AC resistance and reactance per conduit type at 60 Hz for three single conductors, used for AC voltage drop and short-circuit studies.

Are these values for one conductor or a three-phase set?

NEC Table 9 values are for three single conductors in a raceway, which is the typical three-phase or single-phase circuit arrangement. For a one-way voltage drop you use the per-1000-ft impedance times the one-way length; remember to double the length for single-phase line-to-line two-wire circuits.

NEC Chapter 9 Table 9 AC Resistance & Reactance Lookup

NEC Chapter 9 Table 9 is the reference electricians and engineers use for accurate alternating-current voltage-drop and short-circuit calculations. This tool makes the table searchable: pick a conductor and conduit type to read the AC resistance and reactance per 1000 feet and compute the effective impedance.

How it works

Table 9 lists, for three single conductors in a raceway at 60 Hz, the inductive reactance and the AC resistance for copper and aluminum by conduit material. The effective impedance at a given power factor comes from the table footnote:

Ze = R · pf + X · sin(arccos(pf))

where R is the AC resistance, X is the reactance for the conduit type, and pf is the load power factor. Multiplying Ze by circuit length and current gives a far more accurate AC voltage drop than using DC resistance alone, especially for large conductors where reactance dominates.

Example and notes

A 250 kcmil copper conductor in steel conduit has roughly 0.052 Ω/kft AC resistance and 0.052 Ω/kft reactance. At 0.85 power factor the effective impedance is about 0.052 × 0.85 + 0.052 × 0.527 ≈ 0.071 Ω/kft. Note that steel conduit raises both values versus PVC. These figures are for three single conductors; for a single-phase two-wire circuit account for both the supply and return conductor lengths.