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● ISO 6722 Guide

EV Cable Sizing: What ISO 6722 Says About Ampacity

A plain-language guide to sizing EV powertrain cable to ISO 6722: how the insulation temperature class sets ampacity, why it comes from a heat balance rather than a table, and how bundling derates current.

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What ISO 6722 is

ISO 6722 is the standard for single-core road-vehicle cables — the insulated wire used throughout a vehicle's electrical system, including the high-current battery, inverter and motor cabling in an EV powertrain. Its most-cited role in sizing is defining temperature classes: it grades a cable's insulation by the maximum continuous temperature it can withstand, from Class A (85 °C) up to Class H (250 °C).

Why the temperature class, not a current table, is what matters

The instinct from building wiring is to look up an ampacity table (like NEC Table 310). That's the wrong model for EV cabling. What physically limits the current is simple: the conductor heats up as current flows, and the insulation must not exceed its rated temperature. So the temperature class is the real constraint — it sets the ceiling, and the ampacity is whatever current brings the conductor up to that ceiling under the actual operating conditions.

That's why a serious EV cable calculation comes from a heat balance, not a table lookup: at the rated current, the resistive heat generated in the conductor equals the heat the cable can shed to its surroundings. Balance the two and you get the steady-state conductor temperature; find the current that lands it exactly at the insulation's limit and you have the ampacity.

What goes into the heat balance

  • Conductor heat generated — I²R, using the AC resistance (skin effect raises the effective resistance at the motor drive's fundamental frequency; the standard IEC 60287-1-1 skin-effect formula handles this).
  • Heat conducted out through the insulation wall (a thermal resistance set by the wall thickness and the insulation's conductivity).
  • Heat shed from the surface by natural convection and radiation. For a round cable the correct convection model is a horizontal cylinder (the Churchill–Chu correlation) — flat-plate correlations used for busbars don't apply to round wire.

A note on the standard itself: ISO 6722's own numeric current-rating tables sit behind the paywalled standard text and aren't publicly reproducible, so a good tool computes ampacity from this physics rather than transcribing an unverifiable table — and anchors the maximum conductor temperature to the ISO 6722 class you select.

Bundling: cables cook each other

A wire run in free air sheds heat easily; the same wire buried in a thick harness bundle is surrounded by other warm cables and can't. That mutual heating derates the allowable current, often substantially. The widely-published NEC 310.15(B)(3)(a) adjustment factors (e.g. 3 conductors → 1.00, 6 → 0.80, 30 → 0.45) are commonly reused as a disclosed approximation for this.

Worked example & sizing checklist

How the ampacity heat balance plays out for a typical HV cable, the numbers that anchor it, and a checklist for sizing EV powertrain cable without over- or under-building it.

The heat balance, concretely

Suppose an 80 °C conductor surface in 20 °C ambient air, 20 mm outer diameter. The Churchill–Chu horizontal-cylinder correlation gives a natural-convection coefficient of about h = 8.06 W/m²K — comfortably inside the textbook-typical 2–25 W/m²K range for natural convection in air. That coefficient, times the surface area and the surface-to-ambient temperature difference, is the convective heat shed; radiation adds to it. Set that total equal to I²R at the AC resistance and solve for the current that holds the conductor at the insulation class limit — that's the ampacity. Push the current higher and the conductor exceeds the class temperature; that's the failure the standard is protecting against.

Sizing checklist

  1. Pick the conductor size (mm² or AWG) and the ISO 6722 temperature class of the insulation you'll actually use.
  2. Use the true operating ambient — under-hood and near-motor ambients are far above 20 °C and eat directly into ampacity.
  3. Use AC resistance (skin effect) at the drive's fundamental frequency for phase cables, not DC resistance.
  4. Apply a bundling derate for the number of current-carrying conductors in the harness.
  5. Check voltage drop separately — a cable can be thermally fine but still drop too much volts over a long run.
  6. Treat the result as a screening estimate; real harnesses are qualified by OEM test, not calculation alone.

Common mistakes

  • Using a building-wiring ampacity table — wrong thermal environment, wrong assumptions for a powertrain harness.
  • Sizing at 20 °C ambient — a powertrain cable rarely lives at 20 °C; ampacity collapses as ambient rises toward the class limit.
  • Ignoring bundling — a cable rated fine in free air can overheat in the middle of a thick loom.
  • Confusing temperature class with operating temperature — the class is a ceiling, not the temperature the cable will run at.

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