NEC NFPA 70 Cable / Wire Sizing — Method
NEC Article 310 (NFPA 70) governs wire sizing in the United States, Canada (CSA-modified), Mexico (NOM) and many countries that adopted the US system. Wire is sized by reading Table 310.16 (75°C copper is most common), then derating for ambient temperature (Table 310.15(B)(2)(a)) and the number of current-carrying conductors in a single raceway (Table 310.15(B)(3)(a)). The 80 % continuous-load rule (Article 210.19/215.2) applies to feeders and branch circuits.
Where:
- Ib — design current of the circuit (A), from the load calculation
- Ca — ambient temperature correction (1.00 at 30 °C reference)
- Cg — grouping / bunching factor (1.00 for a single circuit)
- Ci — thermal-insulation factor (1.00 if the cable is in free air; 0.50 if fully buried in insulation)
Then pick the smallest cable cross-section in NEC Table 310.16 (75°C copper column) whose tabulated ampacity Iz ≥ It.
Related cable sizing calculators
Other standard- and method-specific cable-sizing calculators in the same series — same procedure, different reference tables and defaults:
- Cable Sizing Calculator (universal) — the seed page covering all standards in one tool
- IEC 60364 Cable Sizing Calculator — IEC 60364-5-52 · International
- BS 7671 Cable Sizing Calculator — BS 7671:2018+A2 · UK / NICEIC
- Single-Phase Cable Sizing Calculator — Single-Phase · 230 V / 240 V
- All Electrical Engineering Calculators →
Frequently Asked Questions
NEC Table 310.16 is the standard for conductors in conduit or cable. Use the 60 °C column for #14–#10 AWG NM cable, the 75 °C column for typical THHN/THWN-2 in conduit, and the 90 °C column only when terminations are 90 °C-rated (most are 75 °C). For free-air conductors use Table 310.17 instead — values are ~1.5× higher.
Multiply the table ampacity by the temperature correction factor (TCF) from the bottom of Table 310.16. Examples for 75 °C conductors: TCF = 1.05 at 21–25 °C, 1.00 at 26–30 °C, 0.94 at 31–35 °C, 0.88 at 36–40 °C, 0.82 at 41–45 °C. Below freezing TCF can rise above 1.10 — useful in cold storage.
NEC 210.19 (branch circuits) and 215.2 (feeders) require: conductor and overcurrent device shall be sized for 125 % of the continuous load + 100 % of non-continuous. Equivalently, the rated ampacity of the conductor must be ≥ continuous load ÷ 0.80. A continuous load is one expected to run ≥ 3 hours (lighting, HVAC, EV charging).
Table 310.15(B)(3)(a) reduces ampacity when 4–6 current-carrying conductors share a raceway (×0.80), 7–9 (×0.70), 10–20 (×0.50), 21–30 (×0.45). Neutral, ground and equipment-grounding conductors do not count unless they carry harmonic current — see 310.15(B)(5).
Approximate equivalents: 14 AWG ≈ 2.08 mm² (use 2.5 mm²), 12 AWG ≈ 3.31 mm² (use 4 mm²), 10 AWG ≈ 5.26 mm² (use 6 mm²), 8 AWG ≈ 8.37 mm² (use 10 mm²), 6 AWG ≈ 13.3 mm² (use 16 mm²), 4 AWG ≈ 21.2 mm² (use 25 mm²), 2 AWG ≈ 33.6 mm² (use 35 mm²), 1/0 AWG ≈ 53.5 mm² (use 50 mm²), 4/0 AWG ≈ 107 mm² (use 95 mm²), 250 kcmil ≈ 127 mm² (use 120 mm²).
Generally similar within 10 %. NEC 75 °C copper Table 310.16 #12 AWG = 25 A vs IEC 4 mm² Method B1 = 36 A (XLPE 90 °C). The difference comes from insulation-rating temperature, the 80 % continuous-load adjustment (NEC) vs the Ca/Cg derating (IEC), and free-air vs raceway assumptions. Always design to the local code, never mix tables.