Guide · NEC 2023
How to calculate voltage drop
Rule: enter one-way feet (panel → load). Basic formula VD = 2·K·I·L÷CM (√3 for 3φ), K = 12.9 Cu / 21.2 Al at 75°C. Target ≤3% branch, ≤5% total.
Example 1 — 120V kitchen, 20A, 100ft, #12 Cu
VD = 2 × 12.9 × 20 × 100 ÷ 6530 = 7.90V = 6.58% — fails 3% and 5%. #10: 4.97V (4.14%). #8: 3.13V (2.60%) ✓. Pull minimum #8 copper.
Example 2 — 240V EV charger, 48A, 100ft, #6 Cu
VD = 2 × 12.9 × 48 × 100 ÷ 26240 = 4.72V = 1.97% ✓. #8 would be 7.50V (3.12%) — just over 3%. Continuous loads also size ampacity at 125% (48A → 60A conductor) — check the wire-size calculator.
Example 3 — 480V 3φ feeder, 100A, 200ft, #2 Cu (Advanced)
Table 9 PVC: R 0.201, X 0.045. PF 0.85 → Zeff = 0.201×0.85 + 0.045×0.527 = 0.195 Ω/kft. VD = √3 × 100 × 0.195 × 200 ÷ 1000 = 6.75V = 1.41% ✓. K-method alone reads ~5.1V — Advanced is the honest number for motors and steel conduit.
Example 4 — 12V solar, 10A, 30ft, #10 Cu
VD = 2 × 12.9 × 10 × 30 ÷ 10380 = 0.75V = 6.2% — fails. #6: 0.29V (2.5%) ✓. On 12V every volt is 8.3%: keep runs short and fat, or step up to 24V/48V (same watts, ½/¼ the current).
The 3 mistakes that fail inspections
1) Entering round-trip feet (doubles the answer). 2) Using breaker amps instead of actual load. 3) Ignoring PF on motors — starting current is 5–8× running current; if lights dim on compressor start, inrush drop is the problem.