Methodology

Sources

• Ampacity: NEC 2023 Table 310.16, not more than three current-carrying conductors, 30 °C ambient, columns 60, 75 and 90 °C.
• Resistance: NEC Chapter 9 Table 8, uncoated copper and aluminum at 75 °C. Copper 14–10 AWG uses the solid-conductor value (as in NM-B cable); 8 AWG and larger use stranded values.
• Breaker limits: NEC 240.4(D): 14, 12, 10 AWG copper at 15, 20, 30 A; 12 and 10 AWG aluminum at 15 and 25 A.
• Gauge geometry: ASTM B258, d = 0.005 in × 92^((36 − n)/39), with 1/0 = 0, 2/0 = −1, 3/0 = −2, 4/0 = −3.

Formulas

VD = k × I × R × L ÷ 1000   k = 2 (single-phase, DC) or √3 (three-phase)
Continuous load: conductor ampacity ≥ 1.25 × load

Selection order

For a size recommendation we take the smallest conductor that (1) meets the required ampacity in the chosen temperature column, (2) is not limited below that current by 240.4(D), and (3) keeps the voltage drop at or below the chosen percentage. We report which of these set the size.

Assumptions and limits

• Unity power factor and DC resistance. Large AC feeders and low power-factor loads should be checked with Chapter 9 Table 9 impedance.
• No ambient correction or adjustment for more than three current-carrying conductors. Apply 310.15(B) and (C) separately where they apply.
• No 310.12 dwelling service reductions, no motor (Article 430) or HVAC nameplate sizing.

Testing

The calculation code has reference tests with hand-worked cases, for example 20 A on 100 ft of 12 AWG copper at 120 V = 7.72 V, and 50 A on NM-B = 6 AWG. The tests run before every build. Values are cross-checked against independent published calculators before a page goes live.

Reporting an error

Email [email protected]. Include the URL, your inputs and the result you expected.