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For three or more conductors, the NEC caps fill at 40% of the conduit's interior cross-sectional area, so a compliant run stays at or below 40%. Two conductors are limited to 31% and a single conductor to 53%. Staying under these ratios leaves room for heat dissipation and reduces pulling tension.

None. Conduit fill ratio, fill factor, fill percentage, and fill rate all describe the same value: total conductor cross-sectional area divided by the conduit's usable interior area, expressed as a percentage. The NEC does not distinguish between the terms; they are used interchangeably in the field.

One conductor is permitted 53% because there is no bundling friction between conductors during the pull and heat dissipates freely to the raceway wall. As conductor count rises, mutual friction and heat increase, so the NEC tightens the limit to 31% for two conductors and 40% for three or more.

Sum the cross-sectional area of every conductor, regardless of size, then divide by the conduit's interior area. Mixed sizes use the same method as identical sizes: add all areas and compare the total against the applicable NEC limit (40% for three or more conductors).

Per NEC Chapter 9, Table 1, the maximum is 53% for one conductor, 31% for two, and 40% for three or more conductors. These limits apply to all raceway types, including EMT, PVC, RMC, and ENT.

Yes. Fill rate is another informal name for the same occupied-area-to-available-area percentage. Whichever term a spec or inspector uses, the calculation and NEC limits are identical.