Voltage drop and the NEC: what's required vs recommended

The 3% branch / 5% total figures in the NEC are Informational Notes (210.19(A), 215.2) — recommended, not enforceable, in most jurisdictions. Some local codes and specific rules (e.g., fire pumps, sensitive loads) make them binding.

DC/single-phase math: Vd = 2 × L × I × R ÷ 1000, with R from Chapter 9 Table 8 (75°C values — e.g., 10 AWG ≈ 1.24 Ω/kft stranded). Marine work (ABYC E-11) uses the circular-mil method with K = 10.75 and hard 3%/10% limits for critical/non-critical circuits.

Rule of thumb that survives checking: at 120 V, a 20 A load at 100 ft one-way on 12 AWG drops ≈6.6% — upsizing to 10 AWG brings it to ≈4.1%.