Trunk & Conduit Sizing Spreadsheet: Free Excel Download

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 Every electrical installation—from a single circuit in a home to a complex industrial plant—relies on conduit or trunking to protect and route cables. But if the conduit is undersized, cables overheat, pulling becomes impossible, and future upgrades are blocked. If it's oversized, you waste material and space.

The key calculation is the filling ratio—the percentage of the conduit's internal cross-sectional area that is occupied by cables. Getting this right ensures:

• Safe heat dissipation — cables don't overheat

• Easy cable pulling — installers can actually pull cables without damage

• Code compliance — IEC 60364 and NEC have specific filling ratio limits

• Future-proofing — some capacity for future additions

In this guide, you'll learn the filling ratio calculation methodology, understand the code limits, and get access to a free Excel spreadsheet that automates the entire process. We'll also connect you with electrical design courses that will make you an expert in cable management.

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Conduit and Trunking Sizing Fundamentals

The Filling Ratio Formula

The filling ratio is:

$$\text{<span style="font-family: verdana; text-align: justify;">Filling Ratio</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\frac{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\sum </span>{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">A</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">c</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">a</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">b</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">l</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">s</span>}}}{{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">A</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">c</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">n</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">d</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">u</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">i</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">t</span>}}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">100</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\%</span>}$$

Where:

• A_cables = Total cross-sectional area of all cables (mm²)

• A_conduit = Internal cross-sectional area of the conduit (mm²)

Code Limits for Filling Ratio

Standard	Conduit Filling Limit	Trunking Filling Limit
IEC 60364	40% (for 3+ cables)	45%
NEC (Chapter 9)	40% (for 3+ conductors)	40%
BS 7671	40%	45%

Rule of thumb: Never exceed 40% filling ratio for conduit with 3 or more cables.

Cable Cross-Sectional Areas

For sizing calculations, use the overall diameter of each cable (not just the conductor):

Cable Type	Overall Diameter (mm)	Area (mm²)
1.5 mm² 2-core	6.5	33
2.5 mm² 2-core	7.5	44
4 mm² 3-core	10	79
6 mm² 3-core	12	113
10 mm² 3-core	15	177

Always use the cable manufacturer's data for the most accurate sizing.

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Download Your Free Trunk & Conduit Sizing Spreadsheet

Skip the manual calculations and get instant sizing results:

📥 Download Free Trunk and Conduit Sizing Spreadsheet

What's Inside:

• ✅ Cable input table (type, size, quantity)

• ✅ Automatic cable area calculation

• ✅ Conduit/trunking cross-section calculator

• ✅ Filling ratio check (IEC/NEC)

• ✅ Recommended conduit/trunking size

• ✅ Print-ready summary report

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Step-by-Step Example: Sizing a Conduit Run

Given Data

Cable Type	Size	Quantity
2.5 mm² 2-core	7.5 mm dia	4
1.5 mm² 2-core	6.5 mm dia	2

Step 1: Calculate Total Cable Area

• 2.5 mm² cable: area = π × (7.5/2)² = 44.2 mm² each

• 4 × 44.2 = 176.8 mm²

• 1.5 mm² cable: area = π × (6.5/2)² = 33.2 mm² each

• 2 × 33.2 = 66.4 mm²

• Total cable area = 243.2 mm²

Step 2: Calculate Required Conduit Area

For 40% filling ratio:

$${\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">A</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">c</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">n</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">d</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">u</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">i</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">t</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\frac{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">243.2</span>}}{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">0.40</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">608</span>}\text{<span style="font-family: verdana; text-align: justify;">\hspace{0.17em}</span>}{\text{<span style="font-family: verdana; text-align: justify;">mm</span>}}^{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">2</span>}}$$

Step 3: Select Conduit Size

For a conduit with internal area ≥ 608 mm²:

• 25 mm conduit: area = π × (25/2)² = 491 mm² (too small)

• 32 mm conduit: area = π × (32/2)² = 804 mm² (adequate)

Select a 32 mm conduit.

Step 4: Verify Filling Ratio

$$\text{<span style="font-family: verdana; text-align: justify;">Filling Ratio</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\frac{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">243.2</span>}}{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">804</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">100</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">30.2</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\%</span>}$$

This is below the 40% limit—good.

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Common Conduit Sizing Mistakes

❌ Mistake #1: Using Conductor Area Instead of Cable Area

Fix: Use the overall cable diameter (including insulation and sheath), not just the conductor cross-section.

❌ Mistake #2: Ignoring the 40% Limit

Fix: IEC and NEC both limit conduit filling to 40% for 3+ cables. Never exceed this—overheating and pulling difficulties result.

❌ Mistake #3: Forgetting Future Capacity

Fix: Leave some spare capacity (e.g., 30% filling instead of 40%) for future additions.

❌ Mistake #4: Mixing Cable Types

Fix: Different cable types have different diameters. Calculate each type separately and sum the areas.

❌ Mistake #5: Not Checking Bend Radius

Fix: Conduit sizing must also consider bend radius. Small conduits with tight bends may not allow cable pulling.

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FAQ

Q1: What is the maximum filling ratio for conduit?

A: 40% for 3 or more cables per IEC 60364 and NEC Chapter 9. For 1–2 cables, higher filling ratios may be allowed.

Q2: How do I calculate conduit size?

A: Sum the cross-sectional areas of all cables, divide by the filling ratio limit (0.40), and select a conduit with internal area greater than the result.

Q3: What is the difference between conduit and trunking?

A: Conduit is a round tube for individual cables. Trunking is a rectangular enclosure for multiple cables with removable covers.

Q4: Can I use the same spreadsheet for trunking?

A: Yes—the calculation method is the same. The filling ratio limit for trunking is typically 45% per IEC.

Q5: What about cable trays?

A: Cable trays are sized by cable count and weight, not filling ratio. Use the cable tray sizing spreadsheet for trays.

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Conclusion

Conduit and trunking sizing is a straightforward calculation—but getting it right is essential for safety, code compliance, and installation efficiency. Download the free Excel spreadsheet, apply the filling ratio method explained in this guide, and pair it with professional electrical design training to master cable management.

📥 Download Free Trunk & Conduit Sizing Spreadsheet

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