Conduit Fill Calculator

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 accounts for three practical problems:

1. Physical pulling: Cables need room to slide past each other during installation. If the fill is too tight, the friction is enormous and the sheathing gets damaged as you pull.
2. Heat dissipation: Cables generate heat when carrying current. Packing them tightly reduces airflow inside the conduit and causes all cables to run hotter, reducing their effective current-carrying capacity.
3. Future maintenance: A conduit that is filled to the absolute maximum leaves no room to pull an additional circuit later. Good practice leaves some spare capacity for future work.

AS/NZS 3000 Appendix C defines the limits:

• One cable alone in conduit: 53% fill is the maximum (the cable can sit anywhere inside)
• Two cables: 31% fill maximum (they sit side by side and restrict each other)
• Three or more cables: 40% fill maximum (this is the number most sparkies remember)

The 40% number is the one you will use 90% of the time, because most conduit runs contain three or more individual cables.

How do you calculate conduit fill?

It is pure geometry. Every cable has a circular cross-section — you calculate the area of each cable (π × radius²), add up all the cable areas, and compare the total to the internal area of the conduit.

The tricky part is knowing the correct outer diameter of each cable. The conductor size (1.5mm², 2.5mm²) is the cross-section of the copper inside — the overall outer diameter of the finished cable (including insulation and sheathing) is much larger.

A 2.5mm² TPS twin-and-earth cable has a copper cross-section of 2.5mm², but the complete cable with its PVC insulation and outer sheath has an outer dimension of roughly 10.5mm (flat cable — you use the major axis as the equivalent circular diameter). This is the number that matters for conduit fill.

Worked example — 4 circuits through 25mm conduit

You are running four separate lighting circuits from a switchboard to a junction area on a commercial fitout. Each circuit uses 1.5mm² TPS 2C+E cable (outer diameter 9.0mm).

Fix: Step up to 32mm MD PVC (internal diameter 31.2mm):

32mm conduit passes comfortably with room to spare for a fifth circuit later.

Flat cables in round conduit — how do you handle the shape?

TPS (Thermoplastic Sheathed) cable is flat, not round. It has a rectangular cross-section with a major axis and a minor axis. When pulled through round conduit, the flat cable does not pack neatly — it tumbles and twists unpredictably inside the conduit.

The standard approach (and the one used by this calculator) treats flat cable as a circle with a diameter equal to the cable's major axis dimension. This overestimates the cable's area slightly, which builds in a safety margin. Some manufacturers publish an "equivalent circular diameter" for their flat cables, which is the value this calculator uses.

Do not try to calculate flat cable area as width × height and use that smaller value. The cable does not sit flat inside round conduit — it rotates and takes up more space than the rectangular cross-section suggests.

What about mixed cable sizes in one conduit?

This is where manual calculation gets tedious and mistakes happen. If you are pulling two 2.5mm² TPS circuits, one 4mm² TPS circuit, and three 1.5mm² single-core cables (for a light switching arrangement) through the same conduit, you need to calculate each cable's area separately, multiply by quantity, and add them all together.

This calculator handles mixed cables natively — add as many cable rows as you need, set the quantity for each, and it totals everything automatically.

Common mistakes with conduit fill

• Using conductor size instead of outer diameter: A 2.5mm² cable does not have a 2.5mm diameter. The 2.5mm² refers to the copper cross-section, not the overall cable dimension. The finished cable is roughly 10.5mm across.
• Ignoring the earth conductor: TPS 2C+E means two active cores plus an earth. All three conductors contribute to the cable's outer dimensions. Some people mistakenly use 2-core cable dimensions when they are actually using 2C+E.
• Forgetting bends: A straight conduit run can theoretically be filled to the space factor limit. But if the run has multiple bends (especially tight 90° bends), pulling friction increases dramatically. Many experienced sparkies derate their fill by an extra 5-10% if the run has more than two bends.
• Mixing conduit types: Medium Duty and Heavy Duty PVC conduit have different internal diameters at the same nominal size. A 25mm HD conduit has a smaller bore than a 25mm MD conduit because the wall is thicker. Always check the actual internal diameter for your conduit type.