NEC conduit fill is the ratio of total wire cross-sectional area to the conduit's internal area, compared against the applicable percentage limit based on wire count.
Wire areas are from NEC 2023 Table 5 for THHN/THWN-2 conductors. Conduit internal areas are from Table 4. Other insulation types have different areas: check Table 5 for XHHW, RHH, and other types.
Rick needs to run 3 circuits in a single 1" EMT conduit: 3 × #12 AWG for one circuit, 3 × #10 AWG for a second, and 1 × #12 AWG ground. Total: 7 conductors.
NEC Chapter 9, Table 1 sets three distinct fill limits, and using the wrong one is the most common calculation error in conduit sizing. A single conductor running alone can occupy up to 53% of the conduit's internal area because there is no heat buildup from adjacent conductors and pulling friction is minimal. Two conductors drop to 31%, a counterintuitive step down, because side-by-side contact reduces the effective circular cross-section available to each. Three or more conductors level off at 40%, which accounts for how round conductors pack inside a circular conduit. When running conduit alongside drywall installations that will carry a ground wire, always count the EGC as a conductor; it occupies physical space regardless of whether it carries fault current at the time of inspection.
A practical way to think about these limits: if you are pulling a single 500 kcmil feeder, you can use a much smaller conduit than if you are pulling three #1/0 AWG conductors of the same total area, because the single-conductor limit is 53% versus the three-conductor limit of 40%. When calculating room dimensions to estimate panel home run lengths, keep this asymmetry in mind; a long run with many conductors may need a conduit upsized specifically to meet the 40% limit even when the wire area calculates close on paper.
EMT (Electrical Metallic Tubing) is the default for commercial interiors and exposed accessible locations because it is lightweight, easy to bend, and has the largest internal area per trade size. IMC (Intermediate Metal Conduit) offers a thicker wall for industrial settings with moderate mechanical risk. RMC (Rigid Metal Conduit) is the heaviest option and is specified for outdoor exposed locations, hazardous areas, and high-abuse environments where EMT's thinner wall would not survive long-term. PVC Schedule 40 is used underground and in corrosive environments because metal conduit corrodes in direct burial and chemical exposure. Note that PVC internal areas are slightly smaller than EMT for the same trade size; always check the conduit-type-specific Table 4 values rather than assuming the areas are interchangeable.
The choice of conduit type also affects how you calculate runs. When estimating conduit needed for a rooftop installation, RMC is typically required for the first 6 feet from the roof surface under NEC 358.12, which means your fill calculation may involve mixing conduit types along the same run, so calculate each section separately using the appropriate internal area for that conduit type.
When fill is close to the limit, switch to the next trade size. Going from 3/4" to 1" EMT adds 0.331 in² of internal area at minimal material cost, far cheaper than a failed inspection and conduit replacement.