BendCraft Pro
Conduit Fill Calculator
Pick your conduit and add each cable group to check fill percentage against the NEC Chapter 9 Table 1 limit — 53% for one conductor, 31% for two, 40% for three or more.
Results are estimates. Verify against your local jurisdiction and the NEC 2026 (NFPA 70). BendCraft Pro is not affiliated with the NFPA. Conductor outside diameters are THHN/THWN-2; other insulation types will differ.
Why the fill limit isn't a single number
NEC Chapter 9, Table 1 sets three different fill limits depending on how many conductors share the conduit: 53% for a single conductor, 31% for exactly two, and 40% for three or more. The two-conductor limit is lower than the one-conductor limit because two round cables jam against each other more easily while being pulled than either one alone or three-plus, which settle into a more stable triangular pack.
Fill percentage itself is simple geometry — total cable cross-sectional area divided by the conduit's inside area — but picking the right limit depends on getting the conductor count right, including any equipment grounding conductors you're pulling in the same conduit.
Frequently asked questions
Do I count the ground wire in my conductor total?
Yes — every current-carrying conductor and equipment grounding conductor in the conduit counts toward the total used to pick the 40/31/53% limit, even though the ground wire doesn't carry load current in normal operation.
Does insulation type change the fill calculation?
Yes — this calculator uses THHN/THWN-2 outside diameters, the most common building wire insulation. Other types (XHHW, USE, etc.) have different overall diameters and would shift the result slightly.
What if my cable combination doesn't fit any conduit size?
Split the conductors across two conduits, or move up to a different conduit type — RMC and IMC have slightly different inside diameters than EMT at the same trade size.
Need voltage drop too?
BendCraft Pro for iOS pairs conduit fill with voltage drop, all 10 bend types, and PDF reports you can hand off on-site.