Above a certain current, a charging cable stops being a wire and becomes a thermal problem. The copper needed to carry the current without overheating grows faster than the current does, and so does its weight. At some point the cable is technically capable and practically unusable, because a driver cannot lift it, coil it or push it into a connector.
Liquid cooling is the engineering answer to that wall. It is not a marketing feature bolted onto a high number; it is what makes the number possible without a cable that weighs as much as a fire hose. Understanding where the wall sits explains why some terminals are liquid cooled and others on the same site are not.
Heat Scales With the Square of the Current
The heat generated in a conductor follows the square of the current passing through it, multiplied by its resistance. Double the current and the heat quadruples. That relationship is the whole story of high-power cable design, and it is why the jump from a moderate charger to an ultra-fast one is not a linear engineering exercise.
The heat has to go somewhere. It leaves through the insulation, through the cable jacket, and into the surrounding air. The rate at which it can leave depends on the surface area of the cable and on the temperature difference between the cable and its surroundings. Both are limited: surface area grows with diameter, and ambient temperature is whatever the site offers, often with sun loading on top.
So the design question becomes circular. More current means more heat, which means more surface area, which means a thicker cable, which means more insulation and more weight, which makes the cable harder to handle. Every step of that loop makes the next one worse.
What Happens to Copper Weight
Keeping resistance low at high current requires cross-sectional area, and copper is heavy. The cable also carries a protective conductor, shielding, sensing wires and the cooling path if there is one, all inside a jacket that has to survive being dragged across a forecourt.
The result is a cable whose stiffness increases faster than its diameter suggests. A cable that is merely thick is awkward. A cable that is thick and stiff cannot be coiled by a driver in winter gloves, and a cable that cannot be coiled gets dropped, dragged and run over. Handling damage then becomes the dominant failure mode, which no amount of current capacity compensates for.
This is the practical reason the industry did not simply scale up air-cooled cables. Beyond a certain point the product would meet its electrical specification and fail as a product.
Where Air Cooling Stops Being Practical
Air cooling in this context is not a fan blowing on the cable. It means the cable relies on natural or forced air movement over its surface to remove heat, which is what every ordinary cable does. It works well up to a point, and that point moves with conditions.
Cold ambient air and short sessions favour it. Hot climates, direct sun, cables lying on hot asphalt and continuous back-to-back sessions all work against it. A cable rated for a current in a laboratory may need to be derated in a forecourt in summer, and that derating is applied by the charger, not by the driver, so the visible effect is a slower session with no explanation.
The transition is therefore not a sharp threshold but a region. Around the middle of the high-power range, designs diverge: some use larger conductors and accept the weight, some accept derating in heat, some move to liquid. Which choice a supplier made is visible in the cable's diameter and in the derating curve published for the terminal.
How Liquid Cooling Works Inside a Cable
A liquid-cooled cable carries a coolant circuit alongside or around the conductors. Heat moves from the copper into the coolant, the coolant travels back through the cable to a heat exchanger in the terminal or cabinet, and the cooled liquid returns. The cable is no longer limited by how fast heat can escape through its own surface, because heat is being carried away from the inside.
Two consequences follow directly. The conductor cross-section can be smaller for the same current, because the temperature rise is controlled by the coolant rather than by the copper alone. And the cable can be made slimmer and lighter, which restores the handling qualities that a purely air-cooled design loses at high current.
This is why liquid cooling is concentrated on the connector rather than on the whole run. Most of the heat and nearly all of the handling problem sit in the last few metres: the cable the driver touches, the connector body and the contact pins. Cooling that section delivers most of the benefit for a fraction of the complexity of cooling the entire DC run.
A single hub can therefore contain both kinds of terminal. The HPC terminal is rated at 500 A with a short boost to 600 A and uses a liquid-cooled cable, while the booster terminal on the same host is air cooled. The pairing is deliberate: the cooled terminal serves the driver who needs the fastest possible stop, and the air-cooled one serves the driver who does not, at lower cost and with simpler maintenance.
What Changes for Maintenance
A liquid-cooled cable introduces a new subsystem and with it a new set of failure modes. The cable is no longer a passive part that either conducts or does not. It has a circuit that can leak, a pump that can fail, a reservoir that can drop below level and connectors that can develop a slow weep where the coolant path is joined.
The consequence for an operator is that the inspection list changes. Cable condition still matters, but it is joined by coolant level, visible residue at joints, pump operation and the temperature readings the terminal reports during a session. A terminal that derates earlier than it used to may have a partially blocked coolant path rather than a failing power stage.
There is also a practical point about repairs. A damaged liquid-cooled cable is not a field splice. It is a unit that is replaced and returned, which means the site needs a spare or accepts downtime. That belongs in the maintenance plan and in the spare-parts list, and it is rarely discussed at the procurement stage.
When 500 A Is the Wrong Answer
The case for liquid cooling is strong, which makes it easy to over-specify. Four situations argue against it.
When the site cannot support the power behind the cable, the cable's rating is decorative. A 500 A terminal on a supply that permits a fraction of that delivers nothing extra and adds a maintenance subsystem for no benefit.
When the vehicles cannot accept it, the same applies. A fleet of vans with modest DC acceptance rates spends most of a session below what an air-cooled terminal provides.
When dwell time is long, throughput is not the constraint. A retail car park with two-hour stays gains little from halving a charging time that is already shorter than the shopping trip.
And when the operator cannot maintain the cooling circuit, an under-maintained liquid system is worse than an air-cooled one, because it fails in ways that are harder to see. The realistic comparison is not peak current but delivered energy per week, and that figure depends on uptime as much as on amperes. The ultra-fast charging range pairs both terminal types for this reason, so a corridor site can mix them rather than cooling every bay.
Liquid cooling exists to solve a handling problem disguised as an electrical one. Where handling is not the constraint, it buys nothing that more bays would not buy more cheaply.
The useful specification question is therefore not whether a terminal reaches a headline current, but under what conditions it sustains it. A derating curve, a coolant maintenance interval and a stated replacement policy for the cable answer that. A single peak figure does not.
Deciding between cooled and air-cooled terminals?
Send the vehicle mix, the expected dwell time and the supply available. Whether the extra current converts into extra throughput follows from those three.
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