Conduit Fill Calculator
Fill percentage of a conduit for a number of equal cables, with the commonly used fill limit. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates fill, common fill limit and most cables at the limit from conduit inside diameter, cable outside diameter and number of cables. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Pulling tension, bends and the length of run can require a lower fill than the limit.
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: NEC-style fill convention, not a universal rule.
What this calculator checks
- Fill percentage against the 53 / 31 / 40% NEC-style limits.
What remains unverified
- Whether that rule applies in your code (it is not a universal IEC or IS rule).
- Pulling tension, bends, run length and cable derating inside the conduit.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | NFPA 70, 2026 edition, Chapter 9 Table 1 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | The calculator uses the NEC conduit/conductor fill percentages (53%, 31%, 40%). This is applicable only where the relevant NFPA 70 edition and Chapter 9 rules are adopted; cable types and local amendments must be checked. |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Conduit inside diameter | 50 mm |
| Cable outside diameter | 12 mm |
| Number of cables | 6 |
- fill = 6 × (12 / 50)² × 100 = 34.56 %
- Limit for 6 cables = 40 %
Assumptions
- Round, equal cables
- Limits are the widely used general rule; your code may differ
Pulling tension, bends and the length of run can require a lower fill than the limit.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Conduit inside diameter | mm | greater than 0 mm | |
| Cable outside diameter | mm | greater than 0 mm | |
| Number of cables | (number) | 1 or more, up to 500 |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Fill (%) | (number) | Shown to 2 decimal places | |
| Common fill limit (%) | (number) | Shown to 0 decimal places | |
| Most cables at the limit | (number) | Shown to 0 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with conduit inside diameter
| Conduit inside diameter (mm) | Fill | Common fill limit | Most cables at the limit |
|---|---|---|---|
| 5 | 3,456 | 40 | 0 |
| 12.5 | 552.96 | 40 | 0 |
| 25 | 138.24 | 40 | 1 |
| 50 | 34.56 | 40 | 6 |
| 100 | 8.64 | 40 | 27 |
| 200 | 2.16 | 40 | 111 |
| 500 | 0.35 | 40 | 694 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60364-5-52 (selection and erection of wiring systems, current-carrying capacity)
- IEC 60364-4-43 (protection against overcurrent, short-circuit withstand k factors)
- IEC 60228 (conductor sizes)
- IEC 60287 (calculation of cable current rating)
Ampacity and grouping factors come from your cable data sheet or your licensed copy of the standard; see STANDARDS.md for the method constants Elecmetric does use. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter conduit inside diameter, cable outside diameter and number of cables. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Round, equal cables. Limits are the widely used general rule; your code may differ.
Which calculators should I use next?
Common next steps are Cable Derating and Ampacity and Parallel Cable Sets. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Conduit Fill Formulas: Percent Fill and Size: Conduit fill formulas: cable area from diameter, percent fill, the smallest conduit for a set of cables
- Cable Tray Fill Formulas: Area and Cable Count: Cable tray fill formulas: cable area from diameter, tray cross-section area
- Cable Pulling Tension Formulas: Cable pulling tension formulas: tension on a straight run from weight, length and friction, the increase around a bend
More Cable Engineering calculatorsAll engineering calculatorsConduit Fill in Elecmetric