Wire Bundle Diameter, Conduit Fill and Heat-Shrink Tubing Size Calculator
Bundle diameter from wire count and OD, conduit fill percentage against the NEC limit, and heat-shrink size range.
Bundle diameter
Conduit fill
Heat-shrink tubing
Bundle diameter = d · √(N / φ); real bundles depend on stranding, twisting and how tightly they are tied. Conduit fill compares the total conductor-plus-insulation area with the conduit area and the NEC Chapter 9 Table 1 limits (53 % for one wire, 31 % for two, 40 % for more than two); conduit derating and the exact conduit internal area still follow the NEC tables. Heat-shrink: the tubing must slip over the part (inside diameter ≥ diameter + clearance) and shrink tight (inside diameter ÷ ratio < diameter); check the tubing datasheet for its supplied and recovered sizes.
For preliminary assessment only — the final decision must follow the manufacturer’s datasheet and the applicable NEC / IEC / UL standards. Results are calculated from the standard formulas stated on this page and are for design estimates only. They are not a rating of any specific part — always confirm current, voltage, temperature and environment limits against the manufacturer’s datasheet for the part you intend to use.
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