USB charging cables: what they are and why data moves slowly over them
A cable charges your laptop reliably, yet copying files through it turns out to be unexpectedly slow. Another looks the same but will not enable the fast charging mode you need. The labels on the packaging do not always explain these differences: power, data speed, and charging quality depend on different cable characteristics.
How a charging cable is built
A USB-C cable consists of several groups of wires, and each has its own job:
VBUS and GND - power wires. The charging current flows through them, and it is their resistance that determines the losses.
CC - the line over which the charger and the smartphone negotiate power using the USB PD protocol, and over which the cable's e-marker is read.
D+ and D- - the USB 2.0 pair. Data travels over it at up to 480 Mbps. In USB-A - USB-C cables, these wires are also used to detect the type of charging port and to start fast charging protocols, for example, Quick Charge.
Additional pairs are present only in cables for fast data transfer: USB 3.0-3.2, USB4, Thunderbolt. Cables that support only USB 2.0 do not have them.
Besides the wires, the USB-C connector may also contain an e-marker, a chip. When the charger asks, it reports the cable's characteristics: what current it can handle, what data speeds it supports, who made it, and whether it is active or passive. According to the specification, the e-marker is mandatory for all full-featured USB-C cables and for any cable rated above 3A.
Without an e-marker, the charger cannot tell that the cable handles 5A and stays at 60W.
Power classes: 60W, 100W and 240W
USB-C - USB-C cables are divided into three classes by rated power:
60W - up to 20V3A. The simplest cables. A cable that supports only USB 2.0 and is rated for 3A does not need an e-marker chip.
100W - up to 20V5A, PD 3.0 protocol. A 5A cable must have an e-marker chip.
240W - up to 48V5A, PD 3.1 EPR protocol. The current is the same as for 100W, but the allowed voltage is higher. The e-marker is mandatory and reports that the cable supports the extended EPR range.
A 240W cable also charges at lower power, so it suits any charger. In the Tinker Vault catalog, the cables are split into three categories with test results: 240W, 100W, and 60W. Separately, there are 2 in 1 Y-cables.
The specification itself already looks beyond this split. In revision 2.5 (March 2026), the USB Type-C specification calls the former 5A cables with a range up to 20V obsolete and replaces them with EPR cables, and USB-IF power logos come in only two kinds: 60W and 240W. 100W cables are still sold and work, but a new 5A cable under the standard is a 240W cable.
Why data moves slowly over charging cables
Most charging cables support only USB 2.0 - that is 480 Mbps, 60 MB/s in theory and noticeably less in practice. This is deliberate: a charging cable needs only two thick power wires, the CC line and one D+ and D- pair. Without additional shielded pairs, the cable is thinner, more flexible, and cheaper.
USB-IF explicitly allows this.
For USB 2.0, a long cable is not a problem. The specification's reference table allows up to 4 m for it, whereas for cables supporting 5 Gbps the practical length is 2 m, for 10 Gbps 1 m, and for USB4 at 40 Gbps only 0.8 m. That is why cables of 2-3 m are mostly USB 2.0.
What this means in practice:
Copying large files, backing up a smartphone, and using an external SSD over such a cable will run at USB 2.0 speed.
Video output to a monitor over USB-C requires DisplayPort Alternate Mode, which cables support starting from USB 3.1. A USB 2.0 cable does not carry video.
If you need both high power and high data speed, look for USB4 or Thunderbolt cables rated 240W. In the catalog, the USB protocol and data speed are listed in the cable parameters next to the power.
A cable that charges but does not carry data
Most often these are cheap USB-A - USB-C cables bundled with small consumer electronics. The manufacturer does not connect the D+ and D- wires or does not fit them at all. The standard requires these wires: in the USB-A - USB-C cable wiring table given by the USB Type-C specification, D+ and D- are present. So such a cable does not comply with the USB standard. However, in Tinker Vault's experience, it is safe to use, although it has limitations.
Without D+ and D-, charging from a USB-A port stays at the basic level. Fast protocols such as Quick Charge are negotiated over exactly these wires, so they will not start. The USB specification also describes the smartphone's behavior: if the device checks the charging port type over D+ and D- and finds none, it must limit the current to the standard USB levels. For USB 2.0, that is about 0.5A.
In practice, the current can be higher. The specification itself notes that there are many non-standard chargers that give more than the basic level, and that is why a USB-A - USB-C cable is rated for 3A. According to Tinker Vault's observations, the current over a cable without D+ and D- usually reaches 2A and is sometimes even higher. You cannot rely on this: the final charging speed depends on the specific device and charging adapter.
It is also worth noting that if a device ships with such a cable, that is an important signal that the device most likely cannot charge from PD chargers either, because it lacks the CC line needed to start charging. This practical observation holds in most cases for small electronics that come with such cables.
USB-C - USB-C cables without D+ and D- exist only as special solutions that do not comply with the USB standard. For example, there is a USB-C - USB-C cable with a switch where D+ and D- are not connected, but PD and PPS are supported: the PD protocol works over the CC line and does not depend on D+ and D-. Another example is data blockers for charging in public places: the port charges but does not carry data. The USB-C - USB-C blocker tested in the catalog does not restrict fast charging protocols up to PD 3.1 240W.
Proprietary 6A-10A cables
Smartphone makers Xiaomi, OPPO/OnePlus, vivo, and Infinix build their own fast charging protocols. They need cables with reinforced contacts and thick wires, rated for 6A-10A. Such cables come both as USB-A - USB-C and as USB-C - USB-C.
They deliver full power only with their own charger and their own smartphone. The official page of the OnePlus SUPERVOOC USB-A - USB-C cable says outright: up to 10A it works with a smartphone that supports SUPERVOOC and only with a SUPERVOOC adapter, and it charges other devices at 3A. Data runs over it at 480 Mbps. Xiaomi states 6A and support for PD and Quick Charge for its USB-A - USB-C cable.
Proprietary >5A USB-C - USB-C cables with non-native devices often work like ordinary standard cables for PD 3.0 at 100W or PD 3.1 at 240W. For example, the CUKTECH 6A cable from the Xiaomi ecosystem is detected by a USB tester and works like an ordinary 240W (5A) cable with USB 2.0.
Cable resistance and voltage drop
For charging, cable resistance matters more than data speed. Part of the energy is lost in the wires and connectors and turns into heat, and the device receives fewer volts than the charger delivers. The higher the current, the more noticeable the losses.
Example: a laptop requests 100W, that is 20V5A. If the cable resistance is 100 mOhm, the laptop receives 19.5V instead of 20V, and 2.5W is lost in the cable. At 200 mOhm the voltage drops to 19V and the losses grow to 5W. According to Tinker Vault's observations and tests, with low-quality cables with high internal resistance and high current draw, a significant voltage drop occurs, which leads both to overheating of the cable and to unstable charging, since the voltage can fall to 18.5V and even lower, and the device being charged may interrupt charging because of an out-of-range charging voltage.
The standard limits the voltage drop directly: no more than 0.5V on the VBUS wire and no more than 0.25V on the GND wire at the cable's maximum current, including the connector contacts. Dividing the sum of 0.75V by the current gives roughly 250 mOhm for 3A cables and 150 mOhm for 5A cables. That is why a 5A cable has a lower allowed resistance than a 3A cable. But this is only a limit: a particular 3A cable can turn out better than a poor 5A cable.
Below are some examples of internal resistance measurements of cables from the Tinker Vault catalog:
Cable Matters TBT5 240W (0.3 m) - 64.6 mOhm.
Amazon Basics USB-IF Certified TBT4 240W (1 m) - 90 mOhm.
Baseus 100W (1.2 m) - 118 mOhm.
Club 3D CAC-1573 USB-IF Certified 240W (2 m) - 140.7 mOhm.
Without equipment, you cannot determine a cable's internal resistance. The precise method is the four-terminal Kelvin method with a milliohmmeter. USB testers give only a relative estimate: the BitTradeOne USB Cable Checker2 deviates from the Kelvin method by 25 mOhm on average, and the FNIRSI FNB58 shows results that fluctuate noticeably. So in practice two options remain: buy a cable from a proven brand or look up the measured resistance in the catalog.
What USB-IF certification and logos give you
USB-IF certification is a laboratory check of a cable sample against the specification. According to the logo license terms, a USB-IF logo may be placed only on a product that has passed testing and is listed in the Integrators List. Every USB-C - USB-C cable in this program must carry a logo with the power 60W or 240W, and cables with fast data must also carry the speed.
What this gives the buyer:
The claim can be checked in the USB-IF certified products list, instead of trusting the text on the packaging.
The power and speed on the logo are confirmed by laboratory tests, not by the manufacturer's word.
Two certified 240W cables from the catalog: Amazon Basics USB-IF Certified TBT4 240W (1 meter) with a resistance of 90 mOhm and Club 3D 240W USB 2.0 (2 meters) with a resistance of 140.7 mOhm. Both cables went through the internal resistance measurement on the site.
How to choose a charging cable
Take a 100W or 240W cable even if the charger is weaker: by the standard, it has a lower allowed resistance, so the losses are smaller and charging is more stable.
A 5A cable must have an e-marker: without it, charging is limited to 60W.
If you need to transfer data or video over the cable, choose a cable with USB 3.2, USB4, or Thunderbolt.
For long cables, look at the measured resistance, because it inevitably grows with length.
Buy cables for proprietary charging for your own smartphone. With other devices, they work like ordinary cables.
🔍 Choose a charger for your cable from the catalog of tested chargers.
