Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of conduit fill. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
The limit exists so that conductors can be pulled without damage and so heat can escape. It is typically 40% for three or more conductors, with higher allowances for one or two.
Fill is calculated on the conductor's outside diameter including insulation, not on its cross-sectional area of copper. That is why the same copper area in thicker insulation may not fit.
Common misuse
Calculating conduit fill from the conductor's copper area. Fill is based on overall diameter with insulation.