By Murugan Vellaichamy · 2026-03-12 · 5 min read
Ask two electricians to size the same circuit and you may watch two different calculations produce the same wire. One used Table 8 resistance; the other used K = 12.9. Both are right — here is the relationship.
NEC Chapter 9, Table 8 lists DC resistance per 1000 ft for every conductor at 75 °C. The drop formula uses it directly: Vd = 2 × I × L × R / 1000. It is the more precise method and the one this site's calculator uses — every size carries its own measured value.
K asks: what is the resistance of a conductor 1 foot long and 1 circular mil in area? For copper at 75 °C, K ≈ 12.9 Ω·cmil/ft (21.2 for aluminum). Then Vd = 2 × K × I × L / CM — one constant, one formula, any size, and crucially it inverts cleanly for sizing: CM = 2 × K × I × L / Vd, then round up to the next size in the chart. That algebraic inversion is why exam prep and field work love it.
Resistance per cmil is not perfectly constant across sizes — stranding and skin-area effects drift the true value slightly, and 12.9 is a representative rounding. On 12 AWG at 20 A and 100 ft, Table 8 gives 7.72 V; K gives 7.90 V. The discrepancy almost never changes the chosen size, and when a result lands within a few percent of a budget boundary, the honest move is the next size up regardless of method.
Use K when solving by hand for a size; use Table 8 (or this site) when verifying a design. Both beat guessing by a country mile.