Why Does a 500 kW Charger Only Deliver 350 kW? How the Thermal De-Rating Curve of the Charging cable and Cable Actually Works

When a 500 kW charger only delivers 350 kW, it is almost never a false rating and rarely a defective car. It is the thermal limit of the charging cable and cable being reached, and the system actively limiting current to protect itself. Current through cable resistance and connector contact resistance generates I²R heat; NTC sensors at the terminals, inside the cable and in the handle feed temperature back to the charger, which then follows a pre-calibrated de-rating curve and smoothly reduces current. The hotter the ambient air, the longer and thicker the cable, the tighter it is coiled, and the poorer the connector cooling, the lower the delivered power. Understanding this curve is the only way to tell whether the bottleneck is the car, the charger, or the cable and cable.

DC fast-charging power is P = V × I. On an 800 V platform, 500 kW corresponds to roughly 625 A; on a 400 V platform, the same power requires over 1200 A. At these currents, even milliohms-per-metre of copper cable resistance add up as I²R heat.

The system does not wait for something to burn. NTC sensors are placed at the main contacts, inside the cable and in the handle, reporting temperature in real time. As temperature approaches material limits—the softening temperature of copper, the long-term rating of cable insulation, the glass-transition temperature of the connector housing—the charger control board follows a pre-calibrated curve and smoothly reduces current, holding temperature just below the threshold. This is thermal de-rating: it is not a broken part, it is the system running within its thermal envelope.

A common field mistake is blaming the charger for every "low power" complaint. The car-side BMS also limits current based on battery temperature, state of charge and cell imbalance. The distinction is practical: if power is high on a cold start and drops in the second half of the session, it is usually the car-side battery strategy. If battery temperature is normal, SOC is moderate, but the cable head or cable feels distinctly hot and the charger screen shows current being held back, the bottleneck is the connector and cable.

A simple field test helps: record delivered charger power and cable-head temperature over a 20-minute session. If power holds flat while temperature climbs steadily, you are watching the de-rating curve engage. If power drops sharply at a specific temperature threshold, that is the NTC trip point of that connector design—different suppliers set this threshold differently, and it is a number worth asking for directly.

The same cable and cable can deliver 30% more power on a cold winter morning than on a summer afternoon. Four variables dominate where the curve sits:

These variables compound: a long, thick, naturally cooled cable charged in summer while coiled can easily drop to 60–70% of nameplate. That is not a quality defect—it is the thermal limit of the system.

The difference between the two cable types is the table buyers should actually look at. A natural-cooling cable relies on copper cross-section and the housing for heat rejection; to carry high current the cable has to be thick and heavy—a 500 A-class natural-cooling cable with cable is often too heavy for one-handed use—and it still derates as ambient temperature climbs. A liquid-cooled cable circulates coolant through the connector and cable, removing heat right at the source (the terminals), which lets the cable stay slim and light while sustaining rated current across a wider temperature range.

A common misconception: liquid-cooled does not mean "never derates." Its curve is simply shifted to the right—it holds rated current over a wider ambient range, and still derates once the coolant loop hits its own limit. What to ask a supplier for is not a "supports 500 kW" sticker, but the actual data: at rated current, what current is allowed at 25°C, 35°C and 45°C ambient; at what coolant outlet temperature de-rating triggers; and how much the curve moves between fully extended and coiled states.

One point operators often miss: natural-cooling cables are friendly to the "thin client" use case—150–250 A is enough most days—but get thermally capped when ultra-fast-charge vehicles show up at peak. Liquid-cooled cables cost more upfront, but in high-ambient and high-utilisation scenarios they deliver a higher average output, shorter per-vehicle charging time and better bay turnover.

One engineering detail worth understanding: moving from 400 V to 800 V reduces current for the same power by half, which is why 800 V platforms make high-power charging physically easier. But it does not remove the thermal problem—at 500 kW the current is still around 625 A, and the connector contact resistance and cable resistance still generate I²R heat. The voltage platform changes how thick the cable needs to be; it does not eliminate the need for liquid cooling in sustained sessions above roughly 350–400 kW.

Translating the selection method above into product design: on its DC high-current series (GB/T, CCS1, CCS2, NACS), DEGSON designs around terminal temperature rise—silver-plated main contacts with spring geometry keep contact-resistance heat low, and NTC sensors monitor both terminal temperature and cable temperature, so that de-rating is triggered by two signals rather than a single reading.

Among these, the DEGSON CCS2 connector is rated 380 A / 1000 V with a peak power of 600 kW. Power terminals use ultrasonic welding, which delivers lower temperature rise and a more reliable joint than conventional welding. Both power and signal terminals are field-replaceable, so maintenance becomes a module swap rather than a full-gun replacement. The housing reaches IP67 protection, and the connector includes a four-wire precision metering loop for accurate energy billing and current monitoring at the station side.

This is not a claim that "DEGSON guns never derate"—every connector derates at its thermal limit. It is a selection method: when you face a "rated 500 kW but customers can only get 350 kW" complaint, what a supplier should provide is not a power label but the allowable-current curve at 25°C, 35°C and 45°C, in both extended and coiled states. The DEGSON DC high-current series is one option that can provide the full curve, after you have already screened suppliers on these numbers.

Scope note: this article covers gun- and cable-side thermal de-rating. It does not cover car-side BMS limits, the charger power module itself, grid voltage variation, or battery cold-weather heating. If the real bottleneck is one of those, changing the gun will not solve it.

For procurement teams, a practical sign-off checklist: ask each shortlisted supplier for (1) the allowable-current curve at 25°C, 35°C and 45°C ambient; (2) whether the test was run extended or coiled; (3) the NTC trip temperature; and (4) the combined weight of gun plus cable at rated current. Comparing these four numbers across suppliers predicts field performance far better than comparing nameplate kW.

Q1: Is a 500 kW rating just marketing?

A1: No. 500 kW is the maximum capability of the charger power module, constrained jointly by the connector, cable, car-side battery and ambient temperature. It is the same as a grid transformer rating not meaning every appliance on the line runs at full power at once. The gap between nameplate and delivered value is normal, by design.

Q2: If I make the cable thicker and use a heavier-duty cable, will de-rating go away?

A2: It helps on the natural-cooling route, but with physical limits—the cable becomes too heavy to handle, and the contact resistance heat at the terminal does not shrink just because the cable got thicker. The real solution for sustained current above 400 kW is liquid cooling, not endlessly thicker copper.