USB-c connectors are tiny. How is it possible for them to carry 100W at 20V without melting? I understand that power = VA and with V=20V then A only needs to be 5. But 5A through most tiny wires makes them melt. So how does USB-c pull off this trick?
Wikipedia has several articles on USB-c power delivery, including https://en.wikipedia.org/wiki/USB_hardware#PD, but I still don’t understand how the standard avoids this problem. It’s not like the cable is made from room temperature super-conductors…
Best Answer
There are two sides in this question.
One side is the Type-C connector itself, and the other is about wires along the cable.
First, the Type-C connector dedicates 4 contact groups for VBUS and 4 for return GND. Therefore each contact carries only 1.25A, which is reasonable, and this is the solution on connector side.
The second concern is about the cable. As specified (Section 3.3.3 Wire Gauges and Cable Diameters (Informative)), the power and ground cables can have AWG from 20 to 28. Standard Ampacity of a AWG20 copper wire is 5 A. So the standard allows multiple gauges to be used, resulting in cables that have different ampacity rating.
It is obvious that two AWG20 wires will make the C-C cable quite thick and stiff, so a thicker-looking cable is more likely to be capable of 5A. However, it is impossible to determine what kind of wires are used inside a cable from external look, even if something is imprinted on the cable jacket. To solve the problem of insufficient wire gauges and possibility of melting, the Type-C standard defines and mandates the use of "electronic markers", see Section 4.9 "Electronically Marked Cables" of Type-C Standard.
Electronic marker is a tiny special microprocessor that should be embedded into the cable's overmold and be connected to one of CC lines. The marker is supposed to communicate with Type-C port electronics via a defined set of serial messages, so the power provider and power consumer know the connected cable capability, and negotiate their "power contract" accordingly.
The markers are designated as " SOP' " in drawings. If the cable doesn't report that it is 5A capable during "Discover Identity" inquiry, the Type-C PD system won't engage the 5-A mode, and power consumer won't consume this current.
Every full-featured Type-C-Type-C cable must have the electronic marker to be USB Type-C/PD compliant. This is how the standard avoids the melting problem.