What is conduit fill?
Conduit fill is the ratio of total cable cross-sectional area to the conduit’s interior area, expressed as a percentage. Electrical codes cap this ratio because overfilled conduits are harder to pull, generate more heat (the cables can’t dissipate it), and can damage insulation during installation. This calculator applies fill limits from either the NEC (US) or IEC / BS 7671 practice (Europe) automatically so you don’t have to look them up.
Choose your standard first
The Standard dropdown at the top switches everything: conduit types, sizes, cable types, and the fill limits themselves. Pick the one that matches your project:
- NEC (US) — imperial trade sizes (½”, ¾”, 1″…), AWG conductors, EMT/RMC/PVC conduits, fill limits per NEC Chapter 9 Table 1 (53% / 31% / 40%).
- IEC / EU — metric tube sizes (16, 20, 25, 32, 40, 50, 63 mm) per EN 61386, mm² cable sizes, steel or rigid PVC tubes, 40% flat fill limit per common BS 7671 practice.
How to use this calculator
Three modes cover the questions electricians actually ask:
- Max cables per conduit — you already have a conduit and want to know how many of a given cable size will fit.
- Min conduit for cables — you know how many cables you need to pull and want the smallest legal conduit size.
- Mixed cable sizes — real-world runs almost always mix sizes (branch circuit conductors + a smaller equipment grounding conductor, for example). Add each size and quantity, and the calculator finds the smallest conduit that accommodates the whole pull.
NEC fill limits (US)
NEC Chapter 9 sets three tiers based on the number of conductors:
- 53% — for a single conductor
- 31% — for exactly two conductors (this is intentionally tighter because two round wires can bind against the conduit wall)
- 40% — for three or more conductors (the most common case)
IEC / European fill limits
European practice under BS 7671 and general IEC 60364 uses a simpler flat limit: typically 40% for straight runs, with tighter factors (down to around 30%) applied to runs with bends. Different national codes may specify slightly different values — this calculator uses 40% as a common baseline. Also note that the European approach measures the overall cable diameter (insulation and all), rather than the NEC-style conductor cross-section from Chapter 9 Table 5.
The formula
The math is simple — the tables do the heavy lifting:
Fill % = (Σ cable areas / conduit area) × 100
For the NEC path, cable “area” comes from NEC Chapter 9 Table 5 (insulated conductor cross-section) and depends on insulation type. For the IEC path, cable “area” is calculated from the overall diameter using π·d²/4 — because the outer diameter is what actually fills the tube.
Which cable to pick
NEC insulation types:
- THHN / THWN-2 — the workhorse of North American installations. Thin insulation, high fill count.
- XHHW-2 — slightly thicker insulation, common in commercial and industrial work.
- TW / RHW — older, thicker insulation types.
IEC cable types:
- H07V-K — flexible fine-strand PVC single-core, 450/750 V. Common in switchgear and building installations.
- H07V-U — rigid single-strand PVC single-core, 450/750 V. Common in fixed installations up to 10 mm².
- N2XH / XLPE single-core — cross-linked polyethylene insulation, thicker outer diameter, used for higher-rated feeders.
Important limitations
This calculator implements code-based fill percentages using NEC Chapter 9 Tables 4 & 5 data (imperial) and typical EN 61386 tube dimensions with published cable overall diameters (metric). It does not apply:
- The NEC nipple exception (short conduit sections under 24″ allow 60% fill)
- Bundling / ampacity derating (that’s a separate calculation)
- Pull box and bend radius requirements
- Manufacturer-specific tolerances on cable overall diameter (typically ±5–10% for IEC cables)
Always verify final designs against the current NEC edition, IEC 60364, BS 7671, or LST EN 60364 as applicable, and have installations checked by a licensed electrician.