Why a 240 Watt Cable Can Charge at 60 Watts

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It does, and almost never for the reason people reach for. The instinct is that a 240 W cable has heavy copper in it and a slow one has thin copper, so slowness is a conductor problem. Run the numbers on the standard as it actually works and the copper turns out to be doing the same job at 240 W as it does at 100 W, down to the last milliwatt, because every one of those wattages is the same five amps.

A cable's wattage rating is a permission, not a delivery. What grants the permission is a chip, and a cable can have excellent copper and still be held to a quarter of its printed figure because the chip is absent, wrong, or not being believed.

The connector pins the current, so wattage is a voltage label

USB-C does not raise power by pushing more current. It cannot: the plug has four small power contacts and the standard holds them to 3 amps by default, or 5 amps on a cable that identifies itself as able to take it. Five amps is the end of the road. So the only remaining way to get more watts is to raise the voltage, and that is exactly what Power Delivery does, stepping up through fixed profiles.

Which means the headline wattages are voltages wearing a power label. At 5 amps, 20 volts is 100 W, 28 volts is 140 W, 36 volts is 180 W and 48 volts is 240 W. Those are the same four steps our Yiicumhh 90 degree Thunderbolt 4 cable rated 240 W at 40 Gbps describes when its record says it supports up to 240 W over PD 3.1 with Extended Power Range and stays backward compatible at 140 W or 180 W. Read as currents, that sentence says the cable carries 5 amps, 5 amps, or 5 amps.

So work out what the conductor actually suffers. Copper has a resistivity of 1.724 times ten to the minus eight ohm metres. One metre of 20 gauge wire has a cross section of 0.518 square millimetres, which is 0.0333 ohms per metre, and current has to go out and come back, so the loop is 0.0666 ohms. Loss is current squared times resistance, so 5 amps gives 25 times 0.0666, which is 1.67 watts.

That 1.67 watts is the answer at 100 W. It is also the answer at 140 W, at 180 W and at 240 W, because the current never changed. The cable is not working harder at the top of its range. As a fraction of the power being delivered it is working considerably less hard: 1.67 watts out of 240 is 0.69 percent, and 1.67 out of 100 is 1.67 percent. The highest wattage profile is the gentlest one the cable will ever see.

Where the real penalty is, which is at the bottom

Turn it around and the loss appears where nobody looks for it. When negotiation fails and a device falls back to 5 volts at 3 amps, it is drawing 15 W. The loss is 9 times 0.0666, which is 0.60 watts, and that is a smaller number in absolute terms. As a fraction it is 4.00 percent, which is 5.8 times worse than the 240 W case on the identical cable. On thinner wire it gets ugly: one metre of 24 gauge is 0.168 ohms out and back, so the same 3 amps loses 1.52 watts out of 15, which is 10.11 percent.

And the voltage drop that produces is the part that bites, because 5 volts has no margin. Three amps through 0.168 ohms drops 0.505 volts, so a device expecting 5 volts sees about 4.5. Charging circuits have minimum input voltages, and a phone that drops below one simply charges more slowly or stops. At 48 volts the same cable drops 0.84 volts and nothing notices. This is the general rule and it is worth keeping: the low voltage profiles are where cable quality shows, and they are precisely the profiles you end up on when something has gone wrong.

The chip that decides, and which carries no current

Here is the mechanism that actually answers the question. A charger will not put 28 volts down a cable on trust, because 28 volts into a device expecting 5 would destroy it. So before anything happens, the charger asks the cable what it is. A cable rated above 3 amps, or for any voltage above 20, has to contain a small chip, the electronic marker, which answers that question. The chip carries no power. It holds a few numbers.

If it is missing, the consequence is not a warning or a reduced speed. It is a different operating point. Without a marker the charger will not exceed 3 amps and will not go above 20 volts, so the ceiling becomes 20 times 3, which is 60 W. Against 240 W that is a factor of four, decided entirely by a component with no conductor in it. A cable with generous copper and no chip is a 60 W cable. A cable with mediocre copper and a correct chip is a 240 W cable that runs warm.

Which is why the hedge on that Yiicumhh record is doing real work and deserves quoting: Both the device and charger must support these levels to reach full 240W, a normal condition worth confirming. That is correct and it is the honest half of the listing. The cable is one of three parties, and the figure on it describes only its own willingness. Its record also lists as a drawback that the wattage ratings are maker stated and not verified, which given that the entire claim rests on what a chip reports about itself is a more pointed admission than it looks. Our Yiicumhh 90 degree Thunderbolt 4 cable review has the rest.

The same condition appears from the other end on a 140 W smart auto stop USB-C charger, whose record lists among its drawbacks that full speed depends on device and cable, and repeats in its performance text that you should check your device supports the wattage and cable you are using. Note what 140 W means in the arithmetic above: 28 volts at 5 amps. This charger cannot reach its own headline figure through a cable without a marker chip, because 28 volts is above the unmarked ceiling. The two conditions in that one-line drawback are not a disclaimer, they are the specification. Our 140 W auto stop charger review covers the disconnect feature, which is its real selling point.

Contact resistance, the term with no published figure anywhere

One more resistance sits in series with the copper and no listing in this category publishes it. Every plug and socket pair meets through contacts that press against each other, and that junction has resistance. It starts small, and it rises as the spring force relaxes, as the plating wears through, and as oxide and pocket lint accumulate.

At 5 amps a little goes a long way, because loss squares the current. Ten milliohms in the power path dissipates 0.25 watts. Forty milliohms dissipates 1 watt. A tired connector at a hundred milliohms dissipates 2.5 watts, inside a shell a few millimetres across with almost no surface to lose it from, which is the entire explanation for the plug that gets hot while the cable stays cool. It is also self worsening in the way these things usually are: heat accelerates oxidation and relaxes the contact springs, both of which raise the resistance that made the heat.

The practical consequence is the one piece of advice in this whole area that costs nothing and works: when a cable that used to charge quickly stops doing so, try the other end of it first. Reverse it, or move it to another port. A connector fault is on one specific pair of mating surfaces, so swapping which end goes where changes the junction without changing anything else, and it takes two seconds. If reversing it fixes the problem you have found a connector, not a cable.

The dock case, where the power is spent on purpose

There is a third reason a port underdelivers and it is neither loss nor permission. An Omars 11 in 1 USB-C docking station with triple display takes 100 W of passthrough charging, and its record is unusually straight about what happens next: while it accepts 100W in, some power runs the dock, so the laptop charges at a little less than the full figure. That is a series load, not a loss. The dock is running two HDMI outputs, a VGA output, gigabit ethernet, four USB ports and two card readers, and all of that is drawn from the same 100 W before the laptop sees any of it.

Worth being clear that this is the honest design. A dock that did not consume anything would not be a dock. But it changes how to size the charger: if your laptop wants 65 W and the dock wants some, a 100 W supply is closer to the edge than the arithmetic suggests, and the dock's record also notes the laptop needs a full featured USB-C port supporting video and Power Delivery rather than a data-only one, which is the same point our page on USB-C being a plug and not a promise makes about ports. That page is right, and the only thing this page adds is that the cable is a fourth party with its own promises to keep. Our Omars 11 in 1 USB-C docking station review covers the triple-display catch, which matters more to most buyers than any of this.

Credit where it is due on the other mechanism too. Our USB-C charger wattage explainer already sets out the shared pool on a multi-port block, and already says the thing people most need to hear, which is that your device sets its own ceiling and a bigger charger will not beat it. Both are correct and neither is repeated here.

The honest limit, and the free technique

The limit is almost comic. This entire page is about conductor resistance, and resistance is set by three numbers: resistivity, length and cross section. Not one cable listing in our catalogue publishes a gauge. The Yiicumhh record does not publish a length either, which is remarkable for a cable, and it is the figure resistance is directly proportional to. The 20 and 24 gauge figures in the drawing are worked examples chosen to bracket what cables of this class typically use, and they are labelled as such, because there is nothing to read.

What listings publish instead is the marker's claim. The number 240 is a report by a chip about a cable, repeated by a maker, and our own record is right to flag that it is maker stated. That is not nothing, because a cable that falsely claims 5 amp capability is a safety matter rather than a performance one, and the thing stopping that is certification rather than any figure you can read.

So the free technique, and there are two, neither of which involves buying a cable. The first is the reversal test above. The second is better, and it is to stop guessing about profiles and let the device tell you. Both major phone and laptop platforms will show you the negotiated wattage, voltage or charging state in their own battery or power settings, with no app and no meter: if the figure sits at 15 W you are on the 5 volt fallback and the problem is negotiation, and if it sits at 60 W you have found an unmarked cable, because 60 W is 20 volts at 3 amps and that is exactly the unmarked ceiling. Those two numbers are diagnostic. Seeing 60 W where you expected 140 W identifies the fault precisely, and no amount of reading the box would have told you.

And one myth to put down, since it belongs to the same family. A charger rated far above what your device wants does not force power into it and does not wear the battery faster, because the device requests the profile and the charger complies. Our page on whether a charger can overcharge a phone battery works that one through properly.