NEC 2026 — Chapter 9, Tables 8 & 9 · Solve for Length · Current · Min. Conductor Size · K-Formula vs. AC Impedance · §210.19 (branch circuits) / §215.4(A)(2) (feeders)
Shared Inputs
Common settings for both methods
System Type
Conductor Material
Construction — Method 1 / Table 8
Number of Parallel Sets310.10(G)(1)ⓘNEC 310.10(G)(1): conductors run in parallel must each be the same length, material, size, insulation, and terminate together. Check Exceptions for control-circuit conductors and other special cases.
Voltage Source (V)
Conduit Type — Method 2
Method 1 — K-Formula
Chapter 9, Table 8 · DC-resistance basis · Quick field estimates
Solve For
Method 2 — Table 9 AC Impedance
Chapter 9, Table 9 · AC impedance basis · Preferred for large conductors & long runs
Solve For
Disclaimer — Important
NEC Chapter 9 Table 8 (K-Formula) and Table 9 (AC impedance, 3-phase, 60 Hz, 75°C). All rearranged formulas are algebraically equivalent to the forward VD calculation. Table 9 does not publish values above 1000 kcmil, or for 700/800/900 kcmil, or for 14 AWG aluminum — Method 2 shows “not available” for those combinations.
NEC §210.19 Informational Note & §215.4(A)(2) Informational Note: 3% branch circuit / 5% combined feeder + branch are recommendations, not requirements.
This calculator is an independent tool intended to assist electrical professionals for educational and estimation purposes only. Results must be verified by a licensed electrical engineer or electrician and reviewed against the full NEC text, equipment listings, and applicable local codes. NEC® and NFPA 70® are registered trademarks and copyrighted publications of the National Fire Protection Association.