Compliance Notice:
Overfilling conduit causes severe installation friction, tears outer cable sheathing, and traps heat. AS/NZS 3000 Appendix C caps maximum conduit fill at 40% for 3 or more cables. After verifying conduit capacity, calculate feeder sizes with our AS/NZS 3008 Cable Size Calculator, check line drop with our Voltage Drop Calculator, calculate peak site load with our Maximum Demand Calculator, and match MCBs with our Circuit Breaker Sizing Tool. Explore all tools on our Electrician Calculators Hub.
AS/NZS 3000 Conduit Cable Capacity Quick Reference Chart
Under AS/NZS 3000 Appendix C, maximum conduit fill is capped at 40% for three or more cables (or 31% for two cables) to prevent sheathing damage and trapped heat. In standard Australian Medium Duty (MD) PVC conduit, a 20mm conduit fits maximum 1 × 2.5mm² TPS cable, a 25mm conduit fits up to 2 × 2.5mm² TPS cables, and a 32mm conduit accommodates up to 4 × 2.5mm² TPS cables.
| Cable Type & Conductor Size | 20mm MD Conduit | 25mm MD Conduit | 32mm MD Conduit | 40mm MD Conduit | Recommended Minimum |
|---|---|---|---|---|---|
| 1.5 mm² TPS Flat (2C+E Lighting) | 2 cables (30.8%) | 4 cables (34.0%) | 7 cables (35.6%) | 11 cables (37.1%) | 20mm (1–2 runs) / 25mm (multi) |
| 2.5 mm² TPS Flat (2C+E Power/GPO) | 1 cable (28.1%) | 2 cables (30.8%) | 4 cables (34.0%) | 7 cables (39.5%) | 25mm for 2 runs |
| 4.0 mm² TPS Flat (2C+E Air-Con/Oven) | 1 cable (tight 41%) | 1 cable (24.7%) | 3 cables (39.8%) | 5 cables (36.9%) | 25mm (single) / 32mm (multi) |
| 6.0 mm² TPS Flat (2C+E Cooktop/EV) | 1 cable (48.4%) | 1 cable (29.2%) | 2 cables (31.0%) | 4 cables (34.0%) | 25mm (single) / 32mm (2 runs) |
| 10.0 mm² TPS Flat (2C+E Sub-mains) | Fails (>53% limit) | 1 cable (45.1%) | 1 cable (21.2%) | 2 cables (29.7%) | 32mm minimum |
| 16.0 mm² Single-Core (Mains Feeder) | 1 single (27.2%) | 2 singles (31.0%) | 4 singles (33.1%) | 6 singles (32.8%) | 32mm / 40mm |
What is the Conduit Space Factor?
The space factor is the maximum percentage of a conduit's internal cross-sectional area that can be occupied by cables. It addresses three critical installation requirements:
- Physical Pulling: Cables require space to slide freely past each other. Overfilled conduit creates high friction, jamming cables halfway and damaging protective sheathing.
- Heat Dissipation: Current-carrying cables generate heat. Tight packing restricts airflow, forcing conductors to operate at higher temperatures and requiring heavy derating under AS/NZS 3008.
- Future Spare Capacity: Leaving space factor margin ensures room for future circuit additions without replacing existing conduit runs.
AS/NZS 3000 Appendix C mandates three specific limits:
- 1 Cable alone in conduit: 53% max fill.
- 2 Cables in conduit: 31% max fill (cables rest side-by-side and restrict movement).
- 3 or more Cables in conduit: 40% max fill (the standard benchmark for multi-circuit pulls).
Conduit Sizing Quick Reference Charts
Below are standard reference tables mapping nominal conduit sizes to internal dimensions and maximum cable capacities:
Table 1: Conduit Internal Diameters (mm)
| Nominal Size | Medium Duty PVC (MD) | Heavy Duty PVC (HD) | Steel Conduit |
|---|---|---|---|
| 16mm | 15.0 mm | 14.4 mm | 15.8 mm |
| 20mm | 19.8 mm | 18.8 mm | 21.0 mm |
| 25mm | 24.4 mm | 23.6 mm | 26.6 mm |
| 32mm | 31.2 mm | 30.0 mm | 35.1 mm |
| 40mm | 38.4 mm | 37.2 mm | 40.9 mm |
| 50mm | 48.0 mm | 46.8 mm | 52.5 mm |
| 63mm | 60.2 mm | — | — |
Table 2: Max Cables in 25mm Medium Duty PVC Conduit (40% Limit)
| Cable Type & Size | Outer Diameter (Ø) | Max Allowable Quantity |
|---|---|---|
| 1.5mm² TPS 2C+E | 9.0 mm | 7 cables |
| 2.5mm² TPS 2C+E | 10.5 mm | 5 cables |
| 4.0mm² TPS 2C+E | 11.5 mm | 4 cables |
| 6.0mm² TPS 2C+E | 13.0 mm | 3 cables |
| 1.5mm² Single Core (V-90) | 4.5 mm | 29 cables |
| 2.5mm² Single Core (V-90) | 5.2 mm | 21 cables |
How to Calculate Conduit Fill
Conduit fill is calculated by comparing total cable cross-sectional area against internal conduit area:
- Single Cable Area:
Area = π × (Outer Diameter ÷ 2)² - Total Cable Area: Sum the individual areas of all cables in the pull.
- Internal Conduit Area:
Conduit Area = π × (Internal Diameter ÷ 2)² - Fill Percentage:
Fill % = (Total Cable Area ÷ Internal Conduit Area) × 100
Worked Example — 4 Lighting Circuits in 25mm Conduit
Consider pulling 4 separate 1.5mm² TPS 2C+E cables (outer diameter 9.0mm) through a 25mm Medium Duty PVC conduit:
Step 1 — Cable Area
1 Cable Area = π × (9.0 ÷ 2)² = 63.62 mm²
4 Cables Area = 63.62 × 4 = 254.47 mm²
Step 2 — Conduit Area (25mm MD PVC)
Internal Diameter = 24.4 mm
Internal Area = π × (24.4 ÷ 2)² = 467.59 mm²
Step 3 — Fill Ratio Check
Result: 54.4% — Exceeds the 40% space factor limit. Four 1.5mm² TPS cables cannot be pulled through 25mm MD conduit.
Solution: Upgrade to 32mm MD PVC conduit (31.2mm ID = 764.54 mm² area):
Handling Flat TPS Cables in Round Conduit
TPS (Thermoplastic Sheathed) building cable is flat. As flat cables are pulled through round conduit, they tumble and rotate unpredictably.
Under standard Australian electrical practice, flat cable is evaluated as an equivalent circle using its major axis (widest dimension) as the effective diameter. This builds in a safe margin for rotation inside the bore.
Thermal Derating Impact (AS/NZS 3008 Table 22)
It is crucial to remember that space factor and current-carrying capacity are dual constraints. While a 32mm conduit can physically fit four 2.5mm² TPS cables within the 40% fill limit, grouping multiple circuits inside the same conduit triggers AS/NZS 3008 Table 22 derating factors.
For example, grouping 4 circuits in conduit applies a 0.65 derating factor (35% rating reduction) to every cable in that run. A 2.5mm² cable rated at 23A in open air drops to only 14.9A effective capacity in conduit — which may require stepping up to 4mm² cable.
Mixed Cable Sizes in One Conduit
Pulls combining different cable types (e.g., 2.5mm² TPS power, 1.5mm² lighting, and 6mm² sub-main) require calculating each cable's area individually before summing. Our interactive calculator handles mixed cable rows automatically.
Critical Mistakes Sparkies Make with Conduit Fill
- Confusing Copper Size with Outer Diameter: A 2.5mm² cable has 2.5mm² copper cross-section, but its outer TPS sheath is 10.5mm across. Always use total outer diameter.
- Ignoring Conduit Wall Thickness: Medium Duty (MD) and Heavy Duty (HD) conduits have different internal diameters. HD PVC has a thicker wall, reducing internal bore area.
- Neglecting Pulling Bends: Multiple 90° bends dramatically increase friction. Reduce fill to 30-35% on runs with 3+ bends for smooth pulling.