The electrons that miss your battery did not vanish—they became heat in conductors obeying $P = I^2 R$. EV owners see the symptom as a warm cable or sluggish charge; engineers see resistance budget violated somewhere between the breaker and the J1772 or Type 2 plug.
Joule’s law on the charging path
For each amp through resistance $R$:
P_loss = I² × R (watts)
$R$ scales with length and inversely with cross-sectional area for copper:
R_one-way ≈ 0.0175 Ω·mm²/m × L / A
R_round-trip ≈ 2 × R_one-way (out + return conductor)
At 32 A and 0.05 Ω round-trip, you dissipate 51 W continuously—like leaving a bright incandescent bulb inside the cable jacket for the whole session.
Why “a little warm” is a signal
Insulation is rated for temperature rise. Chronic overheating:
- Accelerates jacket aging and copper oxidation
- Increases contact resistance at strained plugs
- Can push the EVSE to limit current when voltage sag triggers under-voltage protection
mm² vs. amps — picking quality home cable
European installs often label 2.5, 4, 6, 10 mm² copper. North American portable EVSEs cite AWG—convert mentally or use the mm² stamped on the cord jacket.
| mm² (copper) | Typical portable / fixed use |
|---|---|
| 2.5 | Light 16 A extension—avoid long runs at 32 A |
| 4 | 16–24 A sustained with short runs |
| 6 | Common OEM 32 A jumper, < 15 m |
| 10 | Long driveway runs, outdoor heat soak margin |
Quality means more than copper area:
- TPE / XLPE jacket rated for outdoor UV and coil flex
- Silver-plated contacts on replaceable ends (industrial cords)
- Stranded copper with proper strand count for flex cycles
- No household flat triple-tap extensions on EV duty
Buy EV-rated assemblies with temperature monitoring in the plug where available.
Home station checklist
- Measure one-way cable path length (not coil diameter).
- Read sustained amps from the EVSE screen during a full session.
- Enter mm² from jacket printing into the loss tool.
- If loss watts exceed ~30–40 W, upsize or relocate the wallbox closer to the parking spot.
Fixed wiring vs. portable cord
Premises wiring (6 mm² back to the panel) plus a short 6 mm² EV jumper usually wins over a 15 m 2.5 mm² “convenience” cord from a distant socket. The calculator models one continuous copper path—sum segments separately if you chain lengths.
Cable engineering is not accessory shopping—it is loss budgeting. Respect $I^2R$, size copper for the longest amp-heavy hour, and keep heat out of the plug.