Conduit Fill

The formula

Exactly what the calculator computes, in the order it computes it. Highlighted terms are your inputs.

// Cross-sectional area of each round cable
cableArea = π × (outsideDiameter ÷ 2)²
totalArea = Σ ( quantity × cableArea )

// Fill against the conduit internal area (NEC Chapter 9, Table 4)
fill% = totalArea ÷ conduitInternalArea × 100

// Permitted fill (NEC Chapter 9, Table 1)
  1 conductor  → 53%
  2 conductors → 31%
  3 or more    → 40%

PASS when fill% ≤ limit

// Spare capacity for additional cable
spareArea = (conduitInternalArea × limit) − totalArea

What it assumes

Each of these is a way the number can be wrong for your job. Read them before you buy hardware.

1. NEC Chapter 9 areas and limits. Internal areas follow Table 4 and the fill percentages follow Table 1. These tables are revised between editions — verify against the NEC edition your AHJ has adopted, and note that some jurisdictions amend them.
2. The two-conductor 31% limit is easy to miss. It is lower than both the single-conductor and the three-or-more limit. It exists because two round cables pack poorly, not as an arbitrary rule.
3. Round cables assumed. Area is calculated from a circular cross-section. Flat, oval, or irregular cable will not pack as the arithmetic suggests — use the manufacturer's stated cross-sectional area for anything non-round.
4. Published cable ODs are nominal. Real diameters vary by manufacturer, jacket rating, and production lot. Plenum-rated cable is often a different diameter from riser-rated cable of the same type. Measure the cable you are actually pulling for anything close to the limit.
5. Fill is not the only limit. A run that passes fill can still be unpullable. Bend count and radius, run length, pull tension, jam ratio, and lubrication all matter. NEC limits total bends to 360° between pull points, and long runs with multiple bends need derating well below the fill limit.
6. Jam ratio is not checked. Three cables of similar diameter in a conduit whose inner diameter is 2.8–3.2 times the cable OD can jam regardless of fill percentage. This is a common failure on three-cable pulls.
7. Nipples are exempt. Conduit nipples 24 inches or shorter between boxes are permitted 60% fill under NEC Chapter 9 Note 4. This calculator does not apply that exception.
8. Derating and separation are separate questions. Fill says nothing about ampacity derating for current-carrying conductors, nor about required separation between power and communications cabling. Both may govern before fill does.
9. Design below the limit. Filling to exactly 40% leaves nothing for the moves, adds, and changes that always come. Common practice targets 25–30% on new work so the next technician has somewhere to put a cable.