Pick a calculation mode, choose your conduit type and fill standard, add your conductors, and click Calculate.
Total conductor area compared against the maximum allowed fill area and the conduit's full internal area for the selected size.
Fill percentage for your current conductor set across every trade size of the selected conduit type. The size matching your Conduit Trade Size selection is highlighted.
| Trade Size | Total Area | Max Allowed Area | Fill % | Status |
|---|
Results are engineering estimates based on published NEC Chapter 9 fill percentages (Table 1), conduit internal areas (Table 4), and conductor areas (Table 5). Conduit area values are provided for EMT, PVC Schedule 40/80, RMC, and IMC and have been cross-checked against independent published reproductions — FMC, LFMC, and LFNC-B are not yet included pending verification. Actual sizing varies with installation conditions and the adopted NEC edition. Verify with a licensed electrician before any electrical work.
Estimates pulling tension and sidewall pressure using standard NECA-published formulas. Conductor weight is auto-filled from the main calculator above (editable).
300 lb/ft is a commonly published general limit — override if your cable's manufacturer spec lists a different value.
Results are estimates using standard NECA-published pulling formulas (T = W × L × f for a straight pull; the capstan equation T₂ = T₁ × e^(f·θ) for bends; sidewall pressure SWP = T ÷ R). Actual safe pulling tension depends on lubricant, exact cable jacket/construction, and real installation geometry — verify against the cable manufacturer's published maximum tension and sidewall pressure rating before pulling.
MV cable outer diameter varies by manufacturer and construction. Typical diameters shown are estimates from NEC Article 315.10(B) minimum insulation levels — use Custom OD for an exact product.
Typical MV cable diameters are estimates derived from NEC Article 315.10(B) minimum insulation-thickness levels by voltage class, not any specific manufacturer's product. Actual cable OD varies with shielding and jacket construction. Enter the exact OD from the cable's spec sheet using the field above for an installation-ready result.
NEC Chapter 9, Table 1 maximum fill percentages by conductor count.
| Conductor Count | Max Fill |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| 3 or more conductors | 40% |
| Nipple, ≤24 inches (any count) | 60% |
Equipment grounding conductors are excluded from the count above but their area still counts toward fill (Table 1, Note 2). Utility fill practice commonly applies a flat 40% regardless of count — confirm with the utility.
Two real-world scenarios worked by hand — run them yourself in the calculator above.
Why fill percentage limits exist
NEC Chapter 9, Table 1 limits how much of a conduit's cross-section conductors can occupy so cables can be pulled without jacket damage and heat can dissipate during operation. A single conductor is allowed the highest fill (53%) because it can't tangle with anything else; two conductors get the lowest allowance (31%) because they twist around each other during a pull; three or more form a bundle that moves together, allowing 40%.
Ground wires still count toward area
Equipment grounding and bonding conductors are excluded from the conductor count used to pick the fill percentage category, but their cross-sectional area is still added to the total conductor area per NEC Chapter 9, Table 1, Note 2. Skipping this is one of the most common conduit fill mistakes.
Jam probability
When exactly three same-size cables are pulled into a conduit, NEC Informational Note No. 2 flags a jamming risk if the conduit's inside diameter to cable outside diameter ratio falls between roughly 2.8 and 3.2 — a geometric condition where the three cables can wedge against each other and the conduit wall rather than sliding through. Changing the conduit size shifts this ratio out of the jam-risk range.
NEC versus utility fill practice
NEC Table 1's 53%/31%/40% scale applies to conductors and cables under NEC jurisdiction. Utility companies often apply their own fill practice to utility-owned primary and secondary conductors — commonly a flat percentage regardless of conductor count — which is a company standard, not an NEC requirement. Always confirm the applicable standard with the utility for utility-owned conduit runs.
Cable-pulling tension and sidewall pressure
Beyond fitting inside the conduit, conductors must survive the pull itself. Tension builds along a straight run as friction resists the cable's weight (T = W × L × f), and compounds at each bend under the capstan equation (T₂ = T₁ × e^(f·θ)). At bends, that tension translates into sidewall pressure against the conduit wall (SWP = T ÷ R) — exceeding a cable's rated sidewall pressure can damage insulation or shielding even if the fill percentage itself is well within code.
Medium-voltage cable sizing
Cable rated 2,001V–35,000V under NEC Article 315 uses the same fill-percentage math as low-voltage conductors, but outer diameter isn't standardized in NEC Chapter 9 the way THHN or XHHW is — it depends on the manufacturer's shielding and jacket construction. NEC 315.10(B) publishes minimum insulation thickness by voltage class, which gives a reasonable typical-diameter estimate, but an installation-ready fill calculation needs the exact OD from the cable's spec sheet.