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.

What is the NEC Chapter 9 Table 9 AC Resistance & Reactance Lookup?

Searchable NEC Chapter 9 Table 9 giving effective AC resistance in ohms per 1000 ft for copper and aluminum conductors by AWG/kcmil, plus inductive reactance per conduit type (PVC, aluminum, steel), used in precision voltage-drop and short-circuit calculations. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

NEC Chapter 9 Table 9 AC Resistance & Reactance Lookup

Name: NEC Chapter 9 Table 9 AC Resistance & Reactance Lookup
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Worked example

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:

Ze = 0.052 × 0.85 + 0.052 × sin(arccos(0.85))
   = 0.052 × 0.85 + 0.052 × 0.527
   ≈ 0.044 + 0.027
   ≈ 0.071 Ω/kft

Over a 300-ft one-way run carrying 200 A, the voltage drop is:

VD = 0.071 Ω/kft × (300/1000) kft × 200 A ≈ 4.26 V

For a 480 V three-phase system that is less than 1% — acceptable. For a 120 V circuit the same absolute drop would be over 3.5%, which may violate the NEC Informational Note recommending 3% maximum for branch circuits.

Note that the same 250 kcmil in PVC conduit would show lower resistance and reactance than in steel, since steel’s magnetic properties increase both values. Always pick the conduit column that matches your actual raceway type.

Why Table 9 instead of Table 8 for AC circuits

Table 8 gives DC resistance and physical conductor properties. For direct-current systems and for short, low-current runs where reactance is negligible, it works well. But for AC circuits — particularly runs over 100 ft, circuits with large conductors, or high-current feeders — the combination of skin effect (raising effective resistance) and inductive reactance makes Table 8 values meaningfully inaccurate. Table 9 accounts for both effects at 60 Hz.

The reactance component becomes especially significant for conductors larger than about 2/0 AWG. For 500 kcmil or 750 kcmil conductors, reactance can be as large as the AC resistance, and ignoring it can cause a substantial underestimate of voltage drop.

Three-phase vs single-phase circuits

Table 9 values assume three single conductors in a raceway, which matches three-phase circuits directly. For single-phase two-wire circuits, the current flows out on one conductor and returns on the other — the effective circuit length for voltage drop is twice the one-way distance. Multiply your one-way run length by two before applying the Ze formula for single-phase calculations.

Notes

These figures are approximate reproductions for reference purposes. Steel conduit raises both AC resistance and reactance relative to PVC or aluminum. Verify against the current NEC Chapter 9 Table 9 in the edition adopted by your jurisdiction before submitting design documentation.