Cable resistance is the opposition that a conductor gives to the flow of electric current. No matter how “good” the conductor material is, it will never be perfect—there will always be some level of resistance.

This resistance has three major effects:

1. 1. It causes voltage drop along the cable.
2. 2. It generates heat losses, which can reduce cable life and efficiency.
3. 3. It affects accuracy and performance of devices such as current transformers (CTs) and protective relays.

That’s why resistance is not just a theoretical concept from textbooks—it’s something we need to consider in real-world design and installation work.

The Physics Behind Resistance

The formula that governs resistance is:

R = ρ × (L / A)

where:
– R = resistance (in ohms, Ω)
– ρ = resistivity of the material (Ω·m)
– L = length of the conductor (m)
– A = cross-sectional area of the conductor (m²)

This tells us that resistance depends on three main factors:

• – The material (copper or aluminium)
• – The length of the run
• – The cross-sectional area of the cable

Conductor Materials: Copper vs Aluminium

In Australia, the most common conductor materials are copper and aluminium.

• – Copper: Lower resistivity, better conductor, higher cost. Most domestic wiring (TPS, flexible cables) is copper.
• – Aluminium: Higher resistivity, cheaper, lighter, used more in large feeders, HV distribution, and overhead lines.

Typical resistivity values at 20°C are:
– Copper: ~0.0172 Ω·mm²/m
– Aluminium: ~0.0282 Ω·mm²/m

So, for the same cross-sectional area, aluminium has about 65% higher resistance than copper.

Cable Length

Resistance is directly proportional to length. Double the length, and you double the resistance.

This is why long runs in large buildings, factories, or solar farms require special attention. Even if the cable size looks big enough for the current, resistance over long distances may cause unacceptable voltage drop.

Cross-sectional Area

Resistance is inversely proportional to area. A bigger cross-section means less resistance. That’s why we often upsize cables—not just for current capacity, but to keep the resistance (and therefore voltage drop) under control.

Temperature Effects

Here’s something that often gets overlooked: temperature affects resistance.

As the conductor gets hotter, its resistance increases. For copper, resistance rises by about 0.39% per °C above 20°C. In Australian conditions, where cables often run through roof cavities that can reach 50–60°C, this is a real issue.

For example, a copper cable that measures 1.0 Ω at 20°C could be around 1.15 Ω at 60°C. That might not sound like much, but in long runs or heavily loaded cables, it adds up.

AC Resistance vs DC Resistance

When we deal with alternating current (AC), the situation gets a bit more complicated.

At Australia’s standard 50 Hz supply frequency, two effects come into play:

• – Skin effect: Current tends to flow near the surface of the conductor, reducing the effective cross-sectional area.
• – Proximity effect: Magnetic fields from nearby conductors distort current flow.

Both effects increase the effective resistance compared to the DC value. For small cables at 50 Hz, the difference is minimal. But in larger conductors, especially those above 25 mm², the AC resistance is noticeably higher.

Why Does Cable Resistance Matter?

Now let’s step away from the formulas and look at the practical reasons we care about resistance in Australia.

1. Voltage Drop

Too much resistance means the voltage at the load end is lower than at the supply.

The Wiring Rules (AS/NZS 3000) generally recommend that voltage drop should not exceed 5% of the nominal voltage for final subcircuits. For a 230 V supply, that’s about 11.5 V.

If voltage drop is too high, equipment may underperform, motors may overheat, and sensitive electronics may malfunction.

2. Heating and Cable Life

The power lost as heat in a cable is:

P = I²R

This means that if you double the current, the heating effect increases fourfold. High resistance accelerates insulation breakdown, increases derating requirements, and reduces overall system efficiency.

3. CT and Metering Accuracy

In current transformer (CT) circuits, the resistance of the secondary wiring adds to the burden. If resistance is too high, it can cause measurement errors and even prevent protection relays from tripping correctly.

For this reason, CT cabling must be kept short and appropriately sized to minimise resistance.

4. Energy Efficiency

Across Australia, energy efficiency is becoming a priority, especially in solar, battery, and EV systems. Lower resistance means fewer losses, better system efficiency, and more savings over time.

Example Calculation

Let’s run through a practical example.

Suppose you have a 50 m run of 10 mm² copper cable, carrying 20 A.

From AS/NZS 3008 tables, the DC resistance at 20°C is about 1.83 mΩ/m.

• – Total resistance (active + neutral):
  

  R = 0.00183 × 50 × 2 = 0.183 Ω

• – Voltage drop:
  

  Vd = I × R = 20 × 0.183 = 3.66 V

• – As a percentage of 230 V:
  

  (3.66 / 230) × 100 ≈ 1.6%

This is well within the 5% rule, so the cable size is acceptable.

Typical Cable Resistance Values

Here’s a quick reference for copper cables at 20°C, DC resistance per km:

• – 1.5 mm² → ~12.1 Ω/km
• – 2.5 mm² → ~7.41 Ω/km
• – 4 mm² → ~4.61 Ω/km
• – 6 mm² → ~3.08 Ω/km
• – 10 mm² → ~1.83 Ω/km
• – 16 mm² → ~1.15 Ω/km

Remember, in real design work, you should always use the resistance values from AS/NZS 3008, since these account for AC effects, operating temperatures, and installation conditions.

Key Takeaways

• – Cable resistance is unavoidable but manageable.
• – It depends on material, length, cross-sectional area, and temperature.
• – AC resistance is slightly higher than DC due to skin and proximity effects.
• – High resistance leads to voltage drop, heating, and inefficiency.
• – In Australia, resistance values and voltage drop limits are covered by AS/NZS 3008 and AS/NZS 3000.