Conductor Sizing Bench

Copper and aluminium, 30 AWG to 500 kcmil / 0.05 to 253 mm². DC, single-phase and three-phase.

Ampacity basis—

Circuit—

Section scale—

1 · Load & conditions

Application

Load

Volts · Amps · Watts — edit any two, the third follows.

One-way run length

Source to load. The return conductor is included automatically.

Circuit type

Allowable voltage drop

% custom

Ampacity basis

Conductor

Insulation rating

Ambient temperature °C

Current-carrying conductors bundled

Continuous load — on 3 hours or more. Adds the NEC 125 % factor.

Wire will flex, vibrate, or be disturbed in service.

—

AWG

—

V at load—

Drop—

Derated ampacity—

Loss in cable—

Loop resistance—

Thermal — current vs derated ampacity—

Voltage drop — used vs allowed—

2 · Conductor cross-section

One size either way

3 · How the size was decided

4 · Solid, stranded, or fine-stranded

All three carry the same current for the same cross-section — construction is a mechanical decision, not an electrical one. The one electrical caveat: stranding raises DC resistance about 2 % for standard Class B and 4–5 % for fine-stranded, because the strands spiral and so run slightly longer than the cable.

The termination trap. Fine-stranded cable (UL Class K/M, IEC Class 5/6) is listed only for lugs and terminals rated for fine-stranded conductors. Standard set-screw lugs are listed for Class B; fine strands squirm out from under the screw, and the joint loosens and heats. Ferrules and listed flex lugs are the compatible terminations.

5 · Reference tables

6 · Where the numbers come from

NEC Table 310.16 — allowable ampacities, not more than three current-carrying conductors in raceway, cable or earth, 30 °C ambient, 60/75/90 °C columns, copper and aluminium. 2020 edition. lugsdirect.com
NEC Table 310.15(B)(1) — ambient temperature correction factors, same source.
NEC Chapter 9, Table 8 — conductor properties: circular mils, Class B stranding, DC resistance at 75 °C for uncoated copper, coated copper and aluminium. buildmyowncabin.com
NEC 210.19(A) Informational Note — the 3 % branch / 5 % combined voltage-drop figures. These are recommendations for reasonable efficiency, not enforceable requirements, though many jurisdictions adopt them as local amendments.
ABYC E-11, Tables VI-A to VI-E, IX, X, XI — marine ampacity by insulation rating and bundling, inside and outside engine spaces; 3 % and 10 % voltage-drop tables; the CM = K·I·L/E formula with K = 10.75 for copper; conductor stranding Types 1/2/3; 16 AWG minimum; solid conductors prohibited. E-11 excerpts (PDF)
IEC 60364-5-52 Table B.52 — metric current-carrying capacity, copper, PVC 70 °C and XLPE 90 °C, installation methods A1 through F. The international parent of BS 7671 Appendix 4. ecalpro.com
IEC 60228 — conductor classes 1 (solid), 2 (stranded, fixed), 5 (flexible) and 6 (extra flexible), and maximum DC resistance per class. standard overview
Chassis-wiring and 700-circular-mils-per-amp ratings — from the Handbook of Electronic Tables and Formulas, together with AWG diameters, cross-sections, resistance and skin-depth frequencies. powerstream.com
Resistivity constants used in the engine — 10.371 Ω·cmil/ft for annealed copper at 20 °C (100 % IACS) and 17.00 for aluminium (61 % IACS), with a temperature coefficient of 0.00393 /°C. These reproduce the published NEC Table 8 values to within about 1 %.