Inductance is the property of a conductor that causes it to oppose changes in current. Whenever current flows through a conductor, it creates a magnetic field around it. If that current changes, the magnetic field also changes, which induces a voltage that resists the change in current.

This effect is described by Lenz’s law and is quantified as inductance, measured in henries (H).

In simple terms:

• – A cable with high inductance will resist fast changes in current more strongly.
• – A cable with lower inductance will allow current to change more freely.

Inductance isn’t something we can see directly, but its effects become clear in certain situations — such as voltage spikes during switching, slower current response, or distortions in signals travelling along cables.

Self and Mutual Inductance

There are two flavours of inductance that matter when we talk about cables:

1. 1. Self-inductance: This is when a conductor resists changes to its own current flow. For example, a long single wire carrying current has self-inductance.
2. 2. Mutual inductance: This is when the magnetic field from one conductor induces voltage in another conductor nearby. In multi-core cables, parallel runs, or even busbars in a switchboard, mutual inductance is always at play.

Both self and mutual inductance influence how cables behave in real installations.

Why Cable Inductance Matters

You might be wondering — does inductance really affect day-to-day work with power cables? The short answer is yes, and here’s why.

1. Voltage drop and impedance

When current changes rapidly, the inductance in a cable creates inductive reactance. This is given by the formula:

XL = 2πfL

At 50 Hz (the standard frequency in Australia), the reactance is usually quite small, but in longer runs of cable or in higher frequency applications like variable speed drives or communication cables, it becomes noticeable.

2. Signal integrity

Inductance can distort signals travelling down data or control cables. This is why Ethernet cables use twisted pairs — twisting reduces inductance and balances the mutual inductance between conductors.

3. Switching and transients

In power systems, when a circuit is suddenly opened or closed, the inductance of the cables resists the sudden change in current. This can lead to voltage spikes, which can damage insulation or sensitive equipment.

4. Energy storage

Because inductance is tied to magnetic fields, it stores energy. That stored energy has to go somewhere when switching occurs, which again shows up as transients or oscillations in the system.

Factors That Affect Cable Inductance

The inductance of a cable isn’t fixed — it depends on several factors. Knowing these helps you select the right cable and installation method.

1. Cable length

Inductance increases with length. Longer cables simply have more conductor, and therefore more magnetic field to deal with. This is why long underground feeders have noticeable inductance effects.

2. Conductor spacing

The distance between conductors and their return path matters. A single conductor with its return path far away will have high inductance because the loop area is large. Bring the conductors closer together — as in a twin active-neutral pair — and the loop area shrinks, reducing inductance.

3. Conductor arrangement

The physical layout of cables affects mutual inductance. Twisted pairs, trefoil formations, and tightly grouped conductors all help reduce inductance. Spread them out and inductance goes up.

4. Surrounding materials

Magnetic materials around the cable can increase inductance, since they alter the magnetic field. Most electrical cables in Australia are insulated with non-magnetic materials, so this effect is usually small, unless you’re running near steel structures or using cables with magnetic armouring.

5. Frequency of operation

At higher frequencies, current crowds towards the surface of the conductor (skin effect). This changes the distribution of the magnetic field and modifies the effective inductance of the cable.

Typical Values of Cable Inductance

In practice, the inductance of cables is often expressed per kilometre. Some rough figures are:

• – Low-voltage power cables: around 0.3 to 1.0 mH/km, depending on spacing and arrangement.
• – Twisted pair cables: designed to have balanced inductance, often in the range of 400–600 nH/m.
• – Coaxial cables: typically around 200–300 nH/m, depending on geometry.

These values may look small, but when you scale them up over long distances or at higher frequencies, their effects become significant.

Applications

Power distribution

In medium-voltage feeders and long low-voltage circuits, inductance contributes to voltage drop and can interact with system capacitance. Engineers account for it when modelling fault currents, short-circuit performance, and harmonic behaviour.

Switchboards

When designing main switchboards or distribution boards, the arrangement of busbars affects inductance. Busbars placed in close proximity (such as in a sandwich arrangement) have lower inductance compared to widely spaced bars. This improves performance during fault conditions by reducing electromechanical forces.

Renewable energy and EV infrastructure

With the growth of solar farms, wind generation, and electric vehicle charging in Australia, cable inductance is more relevant than ever. High currents and switching electronics mean inductance directly affects power quality, harmonic distortion, and electromagnetic interference.

Data and communication networks

From NBN cabling to control wiring in industrial plants, inductance has to be carefully managed. That’s why twisted pairs and shielding are standard in these cables — to keep inductance balanced and reduce noise coupling.

How to Manage Cable Inductance

While you can’t eliminate inductance, you can reduce its impact through smart design and installation practices.

1. 1. Keep conductors close together: Always run active and neutral in the same conduit or tray. This minimises loop area and reduces inductance.
2. 2. Use twisted pairs for signals: For data or control wiring, twisted pairs help cancel magnetic fields and balance inductance.
3. 3. Consider trefoil formations for three-phase cables: Trefoil reduces inductance compared to flat arrangements, especially for high-current circuits.
4. 4. Be mindful of long runs: On long feeders, factor in both resistance and inductance when calculating voltage drop and system behaviour.
5. 5. Use shielding when necessary: Shields can control mutual inductance and reduce interference in sensitive circuits.

Key Takeaways

• – Inductance is the property of a conductor that resists changes in current, caused by the magnetic field around it.
• – Both self-inductance and mutual inductance affect how cables behave in power and communication systems.
• – Inductance matters in real-world applications, from voltage drop and transients in power cables to crosstalk in data cables.
• – Factors such as length, spacing, arrangement, and frequency determine the inductance of a cable.
• – Practical installation methods — like running conductors together, using trefoil arrangements, and selecting the right cable design — help reduce unwanted inductance.