Copper transfers electricity through free electrons. Each copper atom gives up one outer electron to a shared “sea” that can move through the metal. When a voltage is applied, those electrons drift slowly in one direction, and that drift is the current. Copper’s resistance to the drift is very low, which is why it wastes little energy as heat and is the reference metal for electrical conductivity.
Key Takeaways
- Copper conducts through a sea of free electrons.
- Electrons drift only about 0.4 mm/s at 32 A in 6 mm² copper.
- NIST defines 100% IACS copper at 1.7241 µΩ·cm.
- Heat loss = I² × R, and it rises as the cable warms.
Free Electrons and Drift Speed
The energy in a circuit moves at close to the speed of light along the wires, but the electrons themselves crawl. Drift speed equals current divided by (electron density × area × electron charge). Copper has roughly 8.5 × 10^28 free electrons per cubic metre. In a 6 mm² core carrying 32 A, that gives about 0.4 mm per second. In an alternating current charger, the electrons mostly jiggle back and forth 50 or 60 times a second rather than travelling along the cable.
Resistance: How Copper Is Measured
Conductivity is defined against copper. The International Annealed Copper Standard, published in NIST’s copper wire tables, sets annealed copper at 100% IACS, with a resistivity of 1.7241 microhm-centimetre at 20 °C. In practical terms, 1 m of 1 mm² copper has 0.017241 ohm of resistance.
| Quantity | Value for copper | What it means in a charging cable |
|---|---|---|
| Resistivity at 20 °C | 1.7241 µΩ·cm (100% IACS) | The baseline for every other conductor |
| Resistance of 1 m, 6 mm² | about 2.9 milliohm | About 22 milliohm per core over 7.5 m |
| Temperature coefficient | 0.00393 per °C | Resistance rises about 0.4% for every 1 °C |
| Density (NIST standard) | 8.89 g/cm³ | Used to convert core size into copper mass |
From Resistance to Heat
Every conductor turns some current into heat: power loss = current² × resistance. Take a 7.5 m single-phase cable with 6 mm² cores at 32 A:
- Each current-carrying core: 32² × 0.0216 Ω ≈ 22 W.
- Two current-carrying cores: about 44 W spread along the cable.
- At 70 °C instead of 20 °C, resistance rises about 20%, so loss rises to roughly 53 W.
Spread over 7.5 m, that is gentle warmth, not a hazard, when the cable is uncoiled and the contacts are clean. A loose or corroded pin adds contact resistance in one tiny spot, which concentrates heat. That is why plug ends are where heat problems usually start, as our article on whether EV charging cables get hot explains. Our guide to wire size for EV chargers covers why the supply circuit is sized for continuous load.
Why This Matters When You Choose a Charger
Lower resistance means less heat and less energy lost before the car’s battery. Undersized cores, long coiled cables and poor terminations all raise losses. When comparing products, check core size, cable length and certification on the datasheet, and see whether all EV charging cables are the same for what else differs. EVDANCE designs cables for continuous current, for example the 48 A EV-ACT5006T Level 2 wall charger and the 9.6 kW EV-AC04001U J1772 portable charger.
Frequently Asked Questions
Do electrons travel from the wall to the car?
No. They drift less than a millimetre per second and, with AC, mostly oscillate in place. Energy moves through the electromagnetic field around the conductors.
Why does a warm cable conduct slightly worse?
Copper resistance rises about 0.39% per °C, so a hot cable loses a little more energy than a cool one.
Is silver a better conductor than copper?
Yes, slightly, but it is far too costly for cables. It appears as plating on some contacts.
Related Reading
- Do EV Charging Cables Get Hot?
- What Size Wire Do You Need for an EV Charger?
- Are All EV Charging Cables the Same?
- Cold-Weather EV Charging: Chargers and Cables Below Freezing
