Cable Conduit Fill Calculator
Calculate conduit fill percentage for a set of cables against the NEC's 1/2/3+ conductor fill limits (53%/31%/40%), or a custom allowed fill. Supports mixed cable sizes in one conduit, not just a single diameter.
Input
One line per cable group, as "diameter:qty" (or "diameter,qty") in the unit selected above — e.g. "6.5:3" for three 6.5mm cables. Mixed sizes in the same conduit are supported.
Output
| Metric | Value |
|---|---|
| No data yet | |
| Diameter | Qty | Group Area | % of Conduit Area |
|---|---|---|---|
| No data yet | |||
Guides
The Cable Conduit Fill Calculator works out what percentage of a conduit's interior a set of cables occupies, and checks that against the NEC's allowed fill limit (or a custom limit you set). Unlike a same-size-only fill calculator, it accepts multiple cable groups of different diameters in one conduit — the common real-world case of a raceway carrying, say, three 6.5mm control cables and two 9.2mm power cables together.
How to use it
- Pick your unit (millimeters or inches) — it applies to both the conduit diameter and every cable diameter you enter.
- Enter the conduit inner diameter.
- List your cables, one group per line, as
diameter:qty— e.g.6.5:3for three 6.5mm cables. Add as many lines as you have distinct cable sizes. - Choose the allowed fill: the NEC preset (53% for a single cable, 31% for two, 40% for three or more — NEC Chapter 9, Table 1) or a custom percentage.
The result table shows the conduit's cross-sectional area, total cable count and area, the resulting fill percentage, the allowed fill area for your chosen standard, and a pass/fail status with the spare (or over-limit) area. A second table breaks down each cable group's own area and share of the conduit.
The NEC fill rule
Conduit fill limits exist so cables can be pulled through without damage and so heat can dissipate. NEC Chapter 9, Table 1 caps fill based on the number of conductors in the raceway:
| Conductors | Max fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
Fill percentage is simply total cable cross-sectional area divided by conduit interior area, ×100. This calculator sums the individual cross-sectional area of every cable you list (π × (diameter ÷ 2)²) rather than assuming a single size, so mixed bundles don't need a manual side calculation.
Is this a substitute for code compliance?
No. This is a planning estimate based on the general NEC fill rule using cable outer diameters you supply. It doesn't replace your jurisdiction's conduit fill tables (which factor in conduit trade size and type — EMT, RMC, PVC — and standardized conductor dimensions), account for jam ratios on 3-conductor pulls, or substitute for a licensed electrician's sign-off. Always verify against the applicable electrical code before installing.
Everything runs in your browser; no inputs are sent to a server.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/cable-conduit-fill-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"unit": "mm",
"conduitDiameter": "21",
"cables": "6.5:3\n9.2:2",
"fillStandard": "nec",
"customFillPercent": "40"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `cable-conduit-fill-calculator` tool (Cable Conduit Fill Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.