Begin with listed dimensions instead of nominal trade names
The workbench treats the raceway and each insulated conductor or cable as circles. It converts the entered raceway inside diameter to area with πd²/4, converts every entered conductor outside diameter the same way, adds occupied areas, and divides that sum by raceway area. Because diameter is squared, a casual diameter estimate can move the final percentage much more than expected. The independent area calculator is useful for checking one circular cross section, but the conduit result must include every repeated conductor or cable in the run.
Nominal conduit trade size is not an inside diameter, and a conductor's copper area is not its insulated outside area. Raceway material, schedule, wall construction, cable jacket, insulation type, and manufacturer design all affect the dimensions that belong in the calculation. Read them from the exact listed product data or approved drawing. Do not infer a diameter from a similar catalog family, and do not use the outside diameter of the conduit as though it described the open space available inside.
Separate the published NEC from the rule adopted on site
As of August 2026, NFPA publishes the 2026 edition of NFPA 70, while the enforceable rule for a project can be an earlier edition with state or local amendments.
Published model-code reference
NFPA 70 Chapter 9 contains the raceway-fill tables, percentage table, notes, and dimension references. NFPA's document-information record also carries edition-specific errata and interim amendments. That current publication is the correct research starting point, not evidence that a particular jurisdiction has adopted it unchanged.
Locally enforceable project rule
The authority having jurisdiction, permit documents, project specifications, approved drawings, and listed equipment instructions establish what applies at the installation. Confirm the adopted edition and amendments before treating any selected limit as relevant. An inspector or qualified electrical professional, not this page, resolves code classification and applicability.
Treat every percentage choice as a declared scenario
The familiar percentages do not license the calculator to decide which conductors, raceway article, or exception governs the work.
- Count what actually occupies the raceway
Enter each cable or insulated conductor, including repeats, with its applicable outside diameter. Verify how the adopted rule treats multiconductor cable, grounding and bonding conductors, bare conductors, and noncircular products before translating them into an area ledger.
- Select the standard-run case deliberately
The workbench exposes 53% for one conductor, 31% for two, and 40% for more than two as the Chapter 9 Table 1 standard-run scenarios. It performs the selected comparison; it does not establish that Table 1 is the complete rule for the raceway and wiring method entered.
- Confirm a nipple condition before using 60%
Choose the short-nipple option only when the physical segment, adopted edition, relevant Chapter 9 note, raceway article, and project details actually qualify. A length entered from memory or a convenient label is not confirmation of an exception.
A passing area ratio says nothing about the whole pull
Southwire's engineering tools treat fill, jam probability, pulling tension, and sidewall pressure as related but separate checks. Three similar cables can wedge when the conduit-to-cable diameter ratio falls in an unfavorable range. Bend count and radius, pull length, lubricant, pulling equipment, conductor construction, temperature, and route geometry also affect whether a pull can be completed without damage. This calculator computes none of those conditions and must not label a below-limit result as pullable or safe.
Fill is also separate from circuit performance. Use the voltage-drop calculator to examine a declared conductor run's resistive drop, then verify conductor resistance, temperature, ampacity, bundling adjustment, terminations, and protection with the applicable documents. When a load relationship is still being established, the volt-amps calculator can audit apparent power arithmetic without pretending to size conductors or raceways.
Reproduce three geometry checks before relying on a result
These examples use declared circular diameters only. They demonstrate the calculator branches; they are not product selections or installation approvals.
Simple metric hand-check
Enter a 25 mm raceway inside diameter and three 5 mm conductor outside diameters in a standard run. Diameter-squared ratios cancel π/4: 3 × 5² ÷ 25² × 100 = 12%. The three-conductor comparison limit is 40%, so the geometric margin is positive.
One-conductor threshold
Enter a 1 in raceway and one conductor whose outside diameter is √0.53 in (about 0.728011 in). Its circular area is exactly 53% of the raceway area, so the unrounded result lands on the one-conductor threshold and remains within it.
Obvious overfill warning
Enter a 1 in raceway and one 0.8 in cable. The occupied-area ratio is 0.8² ÷ 1² = 64%, which exceeds the selected 53% one-conductor comparison. That warning still says nothing about product listing, pullability, ampacity, or local approval.
Hand the reviewer an auditable raceway schedule
A useful result is a documented input set, not a screenshot of one green status line.
- Record manufacturer and catalog identifiers, raceway type and actual inside diameter, every conductor or cable description and outside diameter, quantity, unit, route, and source-document revision. Preserve full precision through the area calculation; if display rounding needs review, use the rounding calculator only after the unrounded fill value is retained.
- Identify the NFPA 70 edition researched, the edition and amendments adopted by the jurisdiction, the table or note consulted, and the reason the selected standard-run or nipple scenario applies. Flag any uncertainty rather than changing the percentage to force a pass.
- Submit the schedule with the electrical plans and product data to the responsible designer, electrician, permit reviewer, or inspector. A favorable entered-diameter comparison does not verify listing, ampacity, derating, grounding, bonding, firestopping, hazardous-location rules, accessibility, workmanship, or inspection acceptance.