Free Electrical Conduit Fill calculator with clear step-by-step results.
Conduit fill compares the total cross-sectional area of the conductors against the internal area of the conduit. The permitted percentage depends on how many conductors are present - 53% for one, 31% for two and 40% for three or more - because the limits exist to allow pulling without damaging insulation.
Fill percentage
Fill % = conductors x pi x (OD/2)^2 / (pi x (ID/2)^2) x 100
Permitted fill
One conductor 53%, two conductors 31%, three or more 40%
Conduit fill, bend radius, derating and conductor selection are governed by electrical codes that differ by jurisdiction. Installations must be designed and inspected by a licensed electrician.
Two round conductors in a round conduit jam against each other and the wall in a way that makes pulling disproportionately hard, so the code allows less fill than for one or for three.
Yes, separately. More conductors in a raceway means mutual heating, which triggers ampacity derating on top of the geometric fill limit.