Conduit Fill Calculator
Add conductors by wire type, AWG, and insulation to any of seven raceway types — get live NEC Chapter 9 fill percentage, pass/fail status, and auto-select the smallest compliant conduit size, with a step-by-step trace and design challenge mode.
Overview
Conduit fill is one of the most routine NEC calculations in design and installation — and one of the most error-prone because the correct conductor area must be pulled from the right NEC table for each insulation type. This calculator eliminates the table-lookup friction: select a raceway type (EMT, IMC, RMC, PVC Schedule 40, PVC Schedule 80, FMC, or LFMC), add conductors one at a time by wire type (THHN/THWN-2, XHHW/XHHW-2, THW/THW-2, TW, RHH/RHW with cover, or bare from Table 8), conductor material (copper or aluminum), and AWG or kcmil size, and the fill percentage and pass/fail status update instantly. Conduit size can be set manually or left on Auto to return the smallest compliant trade size. The fill percentage limit — 53% for one conductor, 31% for two, 40% for three or more — is applied automatically from NEC Chapter 9, Table 1. The Results tab shows total wire area, allowable fill area, percentage used, and pass/fail per NEC Chapter 9. The Trace tab breaks down each conductor's cross-section area and shows the cumulative fill calculation with table citations. A Design Challenge mode presents fill scenarios with a fill-percentage guess slider and graded result. The Learn tab covers Chapter 9 theory, fill limit rationale, and common mistakes. Supports NEC 2017 and 2020 editions.
App preview
A live look at the app you'll launch — every control on the left drives the visualizations on the right in real time.
Inside the lab
What you'll do
- 01Apply the NEC Chapter 9, Table 1 fill percentage limits — 53% for one conductor, 31% for two, 40% for three or more — to a raceway fill calculation
- 02Look up conductor cross-section area from the correct NEC Chapter 9 annex table for a given insulation type and AWG size
- 03Explain why fill limits differ for one, two, and three-or-more conductors and what physical condition the limits protect against
- 04Select the smallest compliant trade size for a given conductor set using the Auto sizing mode and verify the result manually
- 05Distinguish the seven supported raceway types (EMT, IMC, RMC, PVC 40, PVC 80, FMC, LFMC) and identify when each is appropriate
- 06Trace a multi-conductor fill calculation to the correct NEC Chapter 9 table for each conductor and verify the cumulative fill percentage
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