Conduit Fill Calculator | Kewo Engineering

Conduit

Type Trade size Inside diameterin Nipple, 24 in or less

Cables

Cable 1

Cable type Insulation Size Conductor Library Voltage class Insulation level Conductor Size Construction Insulation Shield and jacket Size Size Cable Outside diameterin Construction Fibers Fiber type Outside diameterin Description Outside diameterin Multiconductor Quantity Color in the drawing

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Report details

Location Prepared by Reference

Cross-section, to scale

Pulling checks

Cables

Fill by trade size

Maximum number of one cable

Basis

Fill limits from NEC Chapter 9 Table 1: 53 % for one conductor, 31 % for two, 40 % for more than two, and 60 % for nipples of 24 in or less (Note 4). Equipment grounding conductors are included (Note 3); a multiconductor cable counts as one conductor (Note 9).

Conduit areas from NEC Chapter 9 Table 4; HDPE duct from IPS outside diameter and SDR. Conductor areas from NEC Chapter 9 Tables 5, 5A and 8. Medium-voltage cable diameters from the cable model (ICEA thickness rules or the manufacturer or utility specification). Ethernet and fiber cable diameters are typical values from the data sheets named in the cable table; a duplex zipcord counts at its major axis (Note 9).

Jam ratio 1.05 D/d; jamming is possible between 2.8 and 3.2 (NEC Chapter 9 Table 1, Informational Note 2). Clearance for one cable or three cables of one size uses d' = 1.05 d.

Method

Conduit fill by NEC Chapter 9, with the jam ratio and clearance checks used for cable pulling. The drawing places the cables to scale; the fill calculation does not depend on it.

Fill

Percent fill: Fill = Σ n_i a_i / A, with a_i the area of one cable over its insulation or jacket and A the conduit area from NEC Chapter 9 Table 4.
Limits: NEC Chapter 9 Table 1: 53 % for one conductor, 31 % for two, 40 % for more than two. Nipples of 24 in or less may be filled to 60 % (Note 4).
What counts: Equipment grounding and bonding conductors are included (Note 3). A multiconductor cable, an Ethernet or optical fiber cable, or a three-conductor medium-voltage cable counts as one conductor at its overall diameter; an elliptical cable, such as a duplex zipcord, uses its major diameter (Note 9).
Ethernet and fiber: The NEC applies the Chapter 9 limits to optical fiber cable only where it shares a raceway with power conductors (770.110(B)), and not to communications cable (800.110(B)). TIA-569 and BICSI design communications conduit to 40 % fill for three or more cables; the calculator applies the Table 1 limits to every cable.
Maximum number: For conductors all the same size, n = 0.40 A / a, rounded up when the decimal is 0.8 or more (Note 7), checked against the one- and two-conductor limits. This is how NEC Annex C is built.
Smallest trade size: The smallest size of the selected conduit type whose fill is within the limit and whose inside diameter is larger than the largest cable.

Conduit

NEC Table 4 types: PVC Schedule 40 and HDPE, PVC Schedule 80, PVC Type A, PVC Type EB, EMT, RMC, IMC, FMC, LFMC, LFNC-A, LFNC-B and ENT: nominal inside diameter and area as tabulated.
HDPE SDR 9, 11, 13.5: IPS outside diameter D with minimum wall t = D/SDR; average inside diameter D − 2 × 1.06 t (ASTM F2160 average wall).
Other: Enter the inside diameter of any duct, for example DB-120 or a manufacturer’s HDPE.
Wall in the drawing: Outside diameters are the standard IPS, EMT and IMC values; flexible conduit walls are drawn at a typical thickness. Only the inside diameter enters the calculation.

Cables

Building wire: NEC Chapter 9 Table 5 (THHN/THWN-2, XHHW-2, RHH/RHW/RHW-2, TW/THW/THW-2), Table 5A (compact aluminum) and Table 8 (bare stranded).
PV wire, 2 kV: Southwire SPEC 45411, 2000 V copper XLPE, Type RHH/RHW-2/PV.
Airport lighting, 5 kV: FAA L-824 Type C, 110 mil insulation.
Medium voltage: Generic constructions from ICEA thickness rules: insulation for the voltage class and level (5, 8, 15, 25, 28 and 35 kV; 100 % and 133 %), semiconducting shields, concentric neutral with an LLDPE encapsulating jacket or copper tape shield with a PVC jacket. LADWP, Southwire and TO29 library cables use their specification dimensions. These are the same cable models as the Cable Ampacity Calculator.
Ethernet: Four-pair 100 Ω cable, Category 5e, 6 and 6A: U/UTP and F/UTP in riser (CMR), plenum (CMP) and outdoor (CMX) jackets, and gel-filled outside plant cable. Nominal diameters from the data sheet of the cable named (Belden 1583A, 1585A, 2412, 2413, 7851A (discontinued), 10GXS12, 2142A and 2148A; Mohawk GigaLAN 10 XSD M59234; Siemon gel-filled Cat6A F/UTP). The diameter can be overwritten.
Fiber optic: Tight-buffered riser cable, 2 to 24 fibers (Corning MIC); tight-buffered cable under interlocking aluminum armor, 6 to 24 fibers (Vertical Cable); outdoor gel-free loose tube, 6 to 432 fibers (Corning ALTOS); outdoor loose tube under corrugated steel armor, 12 to 216 fibers (Belden FSSL0126G, Leviton Berk-Tek OPAD); duplex zipcord, 2.0 by 4.0 mm and 2.9 by 6.0 mm (AFL), counted at the major axis. Fiber type sets the jacket color only (TIA-598). Diameters convert from millimeters and can be overwritten.
Other: Enter the outside diameter from the manufacturer’s data sheet. Innerduct can be entered this way at its outside diameter; check the cables inside it as a separate conduit with the innerduct inside diameter.

Pulling checks

Jam ratio: J = 1.05 D/d, D the conduit inside diameter and d the cable diameter; 1.05 allows for the oval section in bends. Between 2.8 and 3.2, three cables can wedge (NEC Chapter 9 Table 1, Informational Note 2). Shown for three or more cables, using a size present three or more times.
Clearance: Distance from the top of the highest cable to the top of the conduit, with d′ = 1.05 d. One cable: C = D − d′. Three cables triangular (triplexed): C = D/2 − 1.366 d′ + (D − d′)/2 √(1 − (d′/(D − d′))²). Three cables cradled: C = (D − d′)(1 − (d′/(D − d′))²), the side cables being highest. 0.5 in is the usual minimum; 1 in for long pulls with bends.
Weight correction: For pulling tension. Triangular: 1/√(1 − (d/(D − d))²). Cradled: 1 + (4/3)(d/(D − d))². One cable: 1.

Drawing

Cables settle one at a time, largest first, at the lowest point where they touch the conduit or other cables. Three cables of one size can be shown triangular or cradled. Zoom shows the cable layers and conductor strands. Ethernet cables show the four pairs, pair separator, foil and drain wire; fiber cables show the buffers or tubes, aramid yarn, central strength member, fillers and armor of a typical construction, with the fibers in TIA-598 colors. Only the overall diameter enters the calculation.

The 3D view under the cable editor shows the selected cable, built from the same dimensions and shown at a common size. The front cable has its layers cut back in steps and numbered to match the list below it; the others are cut square and stacked to show the quantity, up to 36 (fewer for large constructions; the caption says how many are drawn). The insulation or jacket takes the color chosen for the drawing; medium-voltage jackets are black with three stripes in that color (red when black is chosen). A cable entered by outside diameter is drawn as a jacket over a core, or as three conductors with fillers when marked multiconductor; its construction is not known to the calculator.

Checks

Each case is solved live by this calculator.

Case	Reference	Expected	Calculated

Limitations

NEC fill limits apply to raceways; utility duct banks may follow the owner’s standard. Pulling tension and sidewall pressure are not calculated.
Medium-voltage generic diameters are typical. Confirm against the manufacturer’s data before final design.
Ampacity adjustment for more than three current-carrying conductors (NEC 310.15(C)(1)) is not included.
Ethernet and fiber diameters are typical of the cables named; other makes differ. Four-pair cable is limited to 110 N (25 lbf) pulling tension (TIA-568); fiber cable to its rated installation load.