NEC Chapter 9 · Tables 8 & 9

How to use NEC Chapter 9 conductor resistance and reactance

DC resistance (Table 8), AC resistance and reactance (Table 9), effective Z, and the K-factor physics behind them — which one to use for voltage drop, with a worked example and a free link to the official tables.

Need the actual tables? Look up Chapter 9 Tables 8 and 9 in the free NFPA viewer →

Table 8 vs Table 9 — what each one is for

	Chapter 9, Table 8	Chapter 9, Table 9
What it lists	Conductor properties: size in circular mils, stranding, diameter, area, and DC resistance	AC resistance, inductive reactance (XL), and effective Z at 0.85 PF
Materials	Uncoated copper, coated (tinned) copper, aluminum	Uncoated copper and aluminum
Depends on raceway?	No — it is the bare conductor	Yes — separate columns for PVC, aluminum, and steel conduit
Basis	DC, at 75 °C, per 1000 ft (and per km)	600 V cables, 3-phase, 60 Hz, 75 °C, three single conductors in conduit, ohms to neutral per 1000 ft (and per km)
Best for	DC circuits, small AC conductors, quick VD checks, the circular-mil size of a conductor	AC feeders in larger sizes where skin effect and reactance matter, and when power factor is known
Watch out	Solid and stranded rows differ slightly — pick the right one	"Effective Z" is only valid at 0.85 PF — at another PF, compute it from R and XL yourself

Our own summary, not the code text (2023 NEC). Look up Chapter 9 Tables 8 and 9 in the free NFPA viewer (nfpa.org/freeaccess) and verify against the edition adopted by your AHJ. Not engineering advice.

Which resistance value to use for voltage drop

Situation	Use	Why
DC circuit (solar string, battery, DC controls)	Table 8 DC resistance	No skin effect or reactance in DC
AC branch circuit, small conductors	Table 8 or the K-factor method — Table 9 R gives almost the same answer	Reactance is tiny relative to resistance in small sizes
AC feeder, larger conductors, PF near 0.85	Table 9 effective Z	Accounts for AC resistance and reactance at that PF
AC feeder, PF known and not 0.85	Table 9 R and XL: Z = R·cosθ + XL·sinθ	The effective-Z column is fixed at 0.85 PF
Motor starting, inrush, low-PF loads	Table 9 R and XL at the actual PF	Reactance dominates at low PF — resistance-only will under-predict the drop
Conductor temp well below 75 °C	Correct R using the Table 8 notes formula	Resistance rises with temperature; tables are at 75 °C

Worked example — 12 AWG stranded copper, 120 V, 16 A load, 100 ft one-way, PVC conduit

Method	Value used	Calculation	Voltage drop
Table 8 (DC resistance)	1.98 Ω per 1000 ft	2 × 100 ft × 16 A × 1.98 ÷ 1000	6.34 V (5.3%)
Physics / K-factor	K = 12.9 Ω·cmil/ft, 6530 cmil	2 × 12.9 × 16 A × 100 ft ÷ 6530	6.32 V (5.3%)
Table 9 at 0.85 PF	R = 2.0, XL = 0.054 Ω per 1000 ft (PVC)	Z = 2.0 × 0.85 + 0.054 × 0.527 ≈ 1.73; then 2 × 100 × 16 × 1.73 ÷ 1000	5.53 V (4.6%)

Worked-example values for one conductor only — not the tables. Single-phase uses 2 × one-way length (out and back); for three-phase line-to-line, use √3 × one-way length. Verify values against your edition.

Computing resistance from physics (no table needed)

Step	What you do	Example (12 AWG Cu)
1	Take the resistivity K of the metal at operating temperature, in ohm-circular-mils per foot	Copper ≈ 12.9 at 75 °C (aluminum ≈ 21.2)
2	Get the conductor area in circular mils (Table 8, or the AWG formula)	6530 cmil
3	R per foot = K ÷ cmil; × 1000 for per-kft	12.9 ÷ 6530 × 1000 ≈ 1.98 Ω/kft
4	VD (1-phase) = 2 × K × I × L ÷ cmil; VD (3-phase) = 1.732 × K × I × L ÷ cmil	See worked example above

K values are the physical resistivity of the metal (they rise with temperature) — this is the same physics the table is built on, which is why the answers agree.

Common field mistakes

Mistake	What to do instead
Using one-way length on a single-phase circuit	Current goes out and back — use 2 × one-way length (or √3 × for three-phase line-to-line)
Using effective Z when the PF is nowhere near 0.85	Build Z from the R and XL columns at the real power factor
Reading the steel-conduit column for a PVC run	Table 9 values change with raceway type — magnetic steel raises reactance and, in bigger sizes, AC resistance
Treating Table 9 as a single-phase table	Table 9 is ohms to neutral for a three-phase system; scale by the circuit type
Thinking voltage drop is a hard NEC limit	The 3% / 5% figures are informational notes (e.g. 210.19 and 215.2), not requirements — though project specs, energy codes, and a few articles (e.g. 647 and 695) do set enforceable limits

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Resistance, reactance, and a voltage drop you can defend

Look up Tables 8 and 9 in the free NFPA viewer. NFPA offers free read-only access to current NEC editions. This page explains how to use the tables (2023 NEC); it does not reproduce them.

Table 8 is the conductor itself. For every size it gives the circular-mil area, stranding, diameters, and DC resistance per 1000 ft at 75 °C for uncoated copper, coated copper, and aluminum. It is also where you go for the circular-mil area of an AWG size and the dimensions of a bare conductor. Its notes include a formula to correct resistance to another temperature.

Table 9 is the conductor in a raceway on AC. On 60 Hz, skin and proximity effects push AC resistance above DC resistance, and the conductors pick up inductive reactance. Both depend on what the raceway is made of, so Table 9 has separate PVC, aluminum, and steel columns. It is built for three single conductors in a conduit on a three-phase system and gives ohms to neutral — scale it for your circuit.

Effective Z is a shortcut at one power factor. Table 9 combines R and XL into an effective impedance at 0.85 PF, which multiplied by current and length approximates the line-to-neutral drop. If your load runs well above or below 0.85 PF, compute Z = R·cosθ + XL·sinθ from the individual columns instead.

The physics shortcut. Resistance is just resistivity × length ÷ area. In trade units, copper at 75 °C is about 12.9 ohm-circular-mils per foot and aluminum about 21.2. Divide by the conductor’s circular mils and you have ohms per foot — the same answer the DC table gives, because it is the same physics.

Worked example. A 16 A load at 120 V, 100 ft away on 12 AWG stranded copper in PVC. Table 8’s 1.98 Ω/kft gives 2 × 100 × 16 × 1.98 ÷ 1000 = 6.34 V (5.3%). The K-factor method gives 6.32 V — the same. Table 9 at 0.85 PF (R 2.0, XL 0.054) gives an effective Z of about 1.73 and a drop of 5.53 V, because only the in-phase part of the drop counts at that PF. Either way, it is over the 3% branch-circuit recommendation, so bump to 10 AWG or shorten the run.

Run your own numbers in the free voltage drop calculator or the Ohm’s law calculator. Size conductors for ampacity first with the NEC 310.16 ampacity chart, then check drop. Field PM keeps feeder schedules, submittals, and as-builts with the job.

FAQ

What is the difference between NEC Chapter 9 Table 8 and Table 9?+

Table 8 lists conductor properties including DC resistance per 1000 ft for bare copper, tinned copper, and aluminum at 75 °C. Table 9 lists AC resistance, reactance, and an "effective Z" at 0.85 power factor for three single conductors in PVC, aluminum, or steel conduit. Use Table 8 for DC and small AC conductors; Table 9 for larger AC feeders. Look up both free at nfpa.org/freeaccess.

Why is AC resistance higher than DC resistance?+

At 60 Hz, current crowds toward the outside of the conductor (skin effect) and is pushed around by nearby conductors (proximity effect), so less of the copper carries current. The effect is negligible in small sizes and grows in large conductors — and a steel raceway makes it worse.

What does "effective Z at 0.85 PF" mean?+

It is the combination R·cosθ + XL·sinθ at a power factor of 0.85 — a close approximation of line-to-neutral voltage drop per ampere per 1000 ft. At any other power factor, compute it yourself from the table’s R and XL columns.

What K factor do I use for voltage drop?+

The common values are about 12.9 ohm-circular-mils per foot for copper and 21.2 for aluminum at 75 °C. Voltage drop is then 2 × K × I × L ÷ circular mils for single-phase, or 1.732 × K × I × L ÷ circular mils for three-phase.

Can I just use the free voltage drop calculator?+

Yes — Field PM’s free voltage drop calculator does the math for copper or aluminum, single- or three-phase. For large AC feeders at a known power factor, check the result against Table 9 R and XL.

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