# 22 DEC 2025 · HARDWARE

USB-C is not one thing: cables, power and data

Why two USB-C cables that look the same can charge at 240 W or 15 W, copy at 480 Mbit/s or 40 Gbit/s, and drive a monitor or not. Connector versus protocol, explained.

6 MIN READHUMANLY
>_[ FIG. 00 · HARDWARE ]×
Four USB-C cables with the same plug, each shown with a cross-section of its wires and a row of capabilities for data, power and video, from a basic USB 2.0 charging cable to a USB4 or Thunderbolt cable SAME PLUG, FOUR DIFFERENT CABLES CABLE DATA POWER VIDEO USB 2.0, 3 A 480 MBIT/S 60 W NO USB 2.0, 5 A 480 MBIT/S 100 TO 240 W NO CHARGES A LAPTOP, COPIES LIKE 2001 USB 5 TO 20 GBPS 5 TO 20 GBIT/S 60 TO 240 W DP ALT MODE USB4 / THUNDERBOLT 40 TO 80 GBIT/S 60 TO 240 W YES 1 2 3 1 SAME 24-PIN PLUG 2 E-MARKER CHIP 3 HIGH-SPEED PAIRS SIMPLIFIED · WIRE LAYOUT VARIES BY MAKER

Most of us now have a drawer of USB-C cables that all look the same. One charges a laptop at full speed, one copies files at a crawl, one runs an external monitor and one does none of these very well. It is not your imagination and it is not bad luck. USB-C is a connector, not a promise. This post explains what USB-C cables can differ in, why a cable that charges a laptop may still only do USB 2.0 data, and how to buy the right one.

We are a software studio, but we spend a lot of time around cables and drives. dskcopy, one of our desktop apps, writes disk images to USB drives, and the first question when a write is slow is almost always which cable and which port.

Connector versus protocol

USB Type-C describes the plug and the socket: a small, reversible, 24-pin connector. It says nothing about what travels through those pins. The things people actually care about are separate specifications that happen to use that connector:

  • Data: USB 2.0, USB 3.x (5, 10 or 20 Gbit/s), USB4 (20, 40 or 80 Gbit/s), and Thunderbolt 3, 4 and 5.
  • Power: basic USB-C current (up to 3 A at 5 V) and USB Power Delivery, which negotiates higher voltages up to 240 W.
  • Video: DisplayPort Alternate Mode, and DisplayPort tunnelled over USB4 and Thunderbolt.

A cable, a port and a charger each support some subset of these. What you get is roughly the lowest common level of all three. A 40 Gbit/s cable between two USB 2.0 ports runs at USB 2.0 speed. A 240 W charger through a 60 W cable delivers at most 60 W.

What is inside the cable

The cross-sections in the drawing at the top are simplified, but they show the main idea. Every USB-C cable has wires for power and ground, a configuration channel (CC) that lets the two ends talk about what they support, and one twisted pair for USB 2.0 data. That is all a basic USB 2.0 Type-C cable needs, so that is all it has.

Faster cables add more high-speed pairs, usually four shielded pairs for USB 3.x, USB4 and Thunderbolt. These are the pairs that carry 10 or 40 Gbit/s, and they are also the pairs that DisplayPort borrows for video. A cable without them cannot carry fast data or DisplayPort Alt Mode, however thick it is and whatever it cost.

Longer high-speed cables are harder to make. Signal quality drops with length at these speeds, so passive cables above 10 Gbit/s are usually a metre or less, and many long 40 Gbit/s cables are active cables with small chips inside that condition the signal. A long, cheap cable that claims full speed deserves some suspicion.

USB Power Delivery and the e-marker

Without any negotiation, a USB-C port can supply 5 V at up to 3 A, which is 15 W. That is enough for a phone at a gentle pace and nowhere near enough for a laptop.

USB Power Delivery (PD) lets the charger and the device agree on a higher voltage. The standard range tops out at 20 V. At 3 A that gives 60 W, which any properly made USB-C cable is expected to handle. Going past 60 W needs 5 A, and 20 V at 5 A is 100 W. USB PD 3.1 added Extended Power Range (EPR), with fixed voltages of 28, 36 and 48 V. At 48 V and 5 A that is 240 W, enough for large laptops and some monitors.

The cable matters here because current is what heats wires. So a cable rated for 5 A has to say so, electronically. It carries a small chip in the plug called an e-marker, which the charger reads over the CC line before offering anything above 3 A. No e-marker, no 5 A, and the charger falls back to 60 W or less. For 240 W the e-marker also has to report that the cable is rated for EPR voltages. The specification also requires an e-marker in every full-featured cable with the high-speed pairs, so that devices can learn the cable's data speed, not only its current rating.

The charger is the third party in this. A 240 W cable does nothing for you with a 65 W charger. The device takes what it can safely get, and the laptop's own limits apply too.

Why a charging cable can be slow for data

This is the part that surprises people, and it follows directly from the two sections above. Power rating and data speed are separate properties of a cable. Nothing requires a 5 A cable to have high-speed pairs.

So there are plenty of cables built for charging: thick power conductors, a 5 A e-marker, sometimes a 240 W label, and inside, a single USB 2.0 data pair. They charge a big laptop perfectly. Plug in an external SSD, and you get 480 Mbit/s, about 40 MB/s in practice. A copy that should take a minute takes fifteen. The second row in the drawing is that cable. It is not faulty. It is doing exactly what it was built for.

Many cables sold with phones and laptops are exactly this kind, because most people use the cable in the box only for charging. Phones add to the confusion, since some phones run their own USB-C port at USB 2.0 speed regardless of the cable. A fast cable cannot make a USB 2.0 port faster.

The same logic applies to monitors. DisplayPort Alt Mode uses the high-speed pairs, so a USB 2.0 cable cannot carry it. If a monitor lights up on one cable and shows nothing on another, a charging cable is the usual suspect.

Logos, names and what to trust

Naming has not helped. USB 3.0 became USB 3.1 Gen 1, then USB 3.2 Gen 1, all for the same 5 Gbit/s. The USB Implementers Forum (USB-IF), the body that manages the standards, has since moved its logos toward plain numbers. Certified cables and packaging now show the data speed (USB 5Gbps, 10Gbps, 20Gbps, 40Gbps or 80Gbps) and the power rating (60W or 240W) directly. That is the most useful change in years, but it only applies to cables that went through certification, and plenty on sale did not.

Thunderbolt is Intel's branding on top of this. Thunderbolt 4 and 5 cables are tested for their full data rate and for video, and Thunderbolt 5 cables are specified for up to 80 Gbit/s. A good Thunderbolt cable generally works as a very good USB4 cable too. A certified USB4 cable usually works with Thunderbolt devices as well, but check the device's documentation when it matters.

There is a lot we would like to be simpler here. Rules differ between USB4 versions and Thunderbolt generations, and cable makers print whatever they like on the box. When a spec sheet and a product listing disagree, we believe the spec sheet.

Practical buying tips

This is roughly what we do:

  • Decide what the cable is for. Charging only, fast data, or a monitor. A charging-only cable can be long, cheap and 240 W. A data or display cable should be short and full-featured.
  • Read the packaging for both numbers. Look for a data speed and a wattage, ideally in the USB-IF format. If only one is listed, assume the other is the minimum: USB 2.0 data, or 60 W.
  • Keep one short, known good fast cable. A certified 40 Gbit/s or Thunderbolt cable of about a metre or less covers drives, docks and monitors, and it is the first thing to try when something is slow.
  • Label your cables. A strip of tape or a coloured tie saying "240 W, USB 2.0" or "40 Gbps" saves a lot of guessing later.
  • Consider a USB-C cable tester. Small, inexpensive testers can read the e-marker and show the cable's rated current and whether the high-speed pairs are present. They do not measure real throughput, but they settle most arguments.

For a quick check without any extra hardware, copy a large file to a fast external SSD. Around 40 MB/s means USB 2.0 somewhere in the chain. Several hundred MB/s or more means the high-speed pairs are doing their job. After that you only need to work out whether the slow part is the cable, the port or the drive, and quite often it is the cable.