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.

AVAILABLEAUTOMATED CHECKS ONLYSTANDARDS-AWAREBEGINNERNECWHAT DO THESE STATUSES MEAN?

NEC • CH. 97 RACEWAY TYPES5 INSULATION TYPESAUTO CONDUIT SIZING

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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

Example data — not a real field report

Dashboard with fill gauge, cross-section, and quick metrics — Dashboard — pick a raceway, add conductors on the left, and the fill gauge, cross-section, quick metrics, and insulation-type comparison update live for the selected NEC edition.

Results tab with allowable fill, percentage used, and pass/fail — Results — total conductor area, allowable fill area, percentage used, and pass/fail against the applicable 53% / 31% / 40% limit from NEC Chapter 9, Table 1.

Calc Trace showing per-conductor area lookups — Calc Trace — every conductor is traced back to the correct NEC Chapter 9 annex table with its cross-section area and cumulative fill so the number is auditable, not a black box.

Design Challenge with fill-percentage guess slider — Design Challenge — a random raceway/conductor scenario asks you to guess the fill percentage and grades the answer against the calculated value.

Code References tab listing NEC Chapter 9 tables — Code References — the Chapter 9 tables actually used in the current calculation (Table 1 fill limits, Tables 4, 5, 5A, 8) are surfaced with edition-specific notes.

Learn Mode covering Chapter 9 theory and common mistakes — Learn Mode — Chapter 9 theory, the rationale behind the 53% / 31% / 40% limits, and the common mistakes that produce non-compliant designs.

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

What this lab is not

Not a design document, stamped calculation, or approval record.
Not a substitute for engineering analysis by a licensed professional.
Not a reproduction of NEC tables — values are computed and cited, and you must verify against the code edition enforced in your jurisdiction.
Not connected to live equipment and not a safety device.

Calculation basis

References NEC. Results are computed from the entered inputs and cited back to the referencing clause or table number — no code text or table is reproduced verbatim.

Simplifying assumptions are stated in the lab itself; where a model is intentionally conceptual, the lab says so rather than implying study-grade accuracy.

Standards basis: Standards-aware. Validation state: Automated checks only (last checked 2026-08-15).