NEC Chapter 9, Table 8 — Annotated

The DC resistance data behind every voltage drop calculation on this site — uncoated stranded conductors at 75 °C — with the footnotes that actually matter.

Size	Circular Mils	Ω/kft Cu	Ω/km Cu	Ω/kft Al	Ω/km Al
14 AWG	4,110	3.07	10.072	5.06	16.601
12 AWG	6,530	1.93	6.332	3.18	10.433
10 AWG	10,380	1.21	3.970	2	6.562
8 AWG	16,510	0.764	2.507	1.26	4.134
6 AWG	26,240	0.491	1.611	0.808	2.651
4 AWG	41,740	0.308	1.010	0.508	1.667
3 AWG	52,620	0.245	0.804	0.403	1.322
2 AWG	66,360	0.194	0.636	0.319	1.047
1 AWG	83,690	0.154	0.505	0.253	0.830
1/0 AWG	105,600	0.122	0.400	0.201	0.659
2/0 AWG	133,100	0.0967	0.317	0.159	0.522
3/0 AWG	167,800	0.0766	0.251	0.126	0.413
4/0 AWG	211,600	0.0608	0.199	0.1	0.328
250 kcmil	250,000	0.0515	0.169	0.0847	0.278
300 kcmil	300,000	0.0429	0.141	0.0707	0.232
350 kcmil	350,000	0.0367	0.120	0.0605	0.198
400 kcmil	400,000	0.0321	0.105	0.0529	0.174
500 kcmil	500,000	0.0258	0.085	0.0424	0.139

What the table assumes

DC resistance. For AC circuits up to roughly 4/0, DC values are accurate within a couple of percent; beyond that, skin effect and raceway reactance grow — NEC Chapter 9 Table 9 carries the AC impedance values for those cases.
75 °C conductor temperature. Convert to any other temperature with R₂ = R₁ × (234.5 + T₂)/(234.5 + T₁) for copper (228.1 for aluminum). A conductor loafing at 30 °C has ~15% less resistance than tabulated — Table 8 is the conservative choice.
Uncoated copper. Coated (tinned) conductors run a few percent higher; the official table carries a separate coated column.
Stranded construction. Stranding makes the conductor path slightly longer than the cable; the table includes it. Solid wire of the same size reads marginally lower.

This data feeds the calculator directly, and the K-factor is its single-number summary. For current ratings rather than resistance, see Table 310.16.

Frequently Asked Questions

Why does my multimeter read lower than Table 8?

You're measuring a cool conductor — the table states resistance at 75 °C operating temperature. At 20 °C room temperature, expect about 82% of the tabulated value for copper.

Built & maintained by Murugan Vellaichamy · Every calculation verified against NEC Chapter 9 Table 8 & Table 310.16 published values · Informational reference — not engineering advice