A conduit run can look roomy until you add up the insulated conductors inside it. The answer depends not just on wire gauge, but on insulation, conduit type and size, and the number of conductors. A chart can help, but mixed conductors make lookup work less straightforward. WireFillChart is a browser-based calculator aimed at that specific task: estimating conduit fill from supported NEC 2023 Chapter 9 table data.
From a conduit choice to a fill result
The workflow starts by choosing a conduit type, such as EMT, IMC, RMC, PVC, flexible metal, or ENT. You then add conductor rows, selecting an insulation type and wire size, and set the quantities. The page updates the result as those selections change. Its stated output includes the actual fill percentage and an NEC status callout. When a proposed combination is over the limit, it can recommend the next trade size.
That is useful when a job involves several conductor sizes or insulation types and a single chart row will not answer the question. Instead of manually summing conductor areas and comparing them with the conduit area, the calculator is meant to do that arithmetic from its supported table values. The site also presents fill charts for reference, with an EMT chart shown on the landing page.
The calculations are based on the site’s transcription of NEC 2023 Chapter 9 Tables 1, 4, and 5. Table 4 provides conduit areas, while Table 5 supplies conductor areas. Table 1 sets base fill budgets of 53 percent for one conductor, 31 percent for two, and 40 percent for three or more. For equal-area conductors, the site says it applies Note 7’s rounding rule, so an accepted result can have an actual percentage above the base tier. Mixed conductor areas are checked against the area budget.
Useful for arithmetic, not a full installation review
The distinction between a base percentage and an accepted result matters. A displayed fill percentage is an area calculation, not a complete judgment about whether a particular installation is suitable. WireFillChart explicitly says it does not assess ampacity, product suitability, pulling feasibility, or installation approval. Local code adoption and requirements still need to be checked by the person responsible for the work.
The jamming indicator is narrower still. The site describes it as a geometry-only hint for exactly three identical round conductors, based on the ratio between conduit inside diameter and wire outside diameter. It does not model bends, friction, tolerances, or pulling tension, and it does not assess other conductor configurations. Treat it as a limited screening signal, not a probability or a promise that a pull will work.
There are other boundaries. The page says nipple mode is not implemented, and it does not provide XHHW or XHHW-2 data. Its supported conductor and conduit combinations are a subset of the possible materials and real-world arrangements an electrician may encounter. Cable assemblies such as MC and NM-B also require their own applicable method and dimensions rather than being treated as ordinary individual conductors by default.
Check the assumptions behind the answer
The developer says the calculator runs in the browser and requires no account. It is free, which makes it convenient for a quick check without setup. But convenience does not remove the need to verify the source values. The landing page notes that the table data is transcribed and that automated checks cover selected outputs and consistency, not independent verification of every cell. That is a meaningful caveat for a tool whose answer depends on reference data as well as arithmetic.
For that reason, it makes sense as a calculation aid and a way to inspect how conductor count affects fill, rather than as the final authority for a job. The result can help identify an obvious overfill and make a supported combination easier to reason about. It cannot validate inputs the tool does not cover, or settle questions outside area calculations.
If that boundary is clear, WireFillChart offers a direct workflow: choose a supported conduit, add conductors, and read the computed fill and status. It is aimed at US electricians, inspectors, and engineers who need a quick NEC 2023 conduit-fill check. It is not a fit for someone who needs a complete installation decision, unsupported wire data, or an assessment of pulling conditions.
Wire Fill Chart & Conduit Fill Calculator — NEC 2023