In addition to my “Demystifying USB-C and Thunderbolt 3” thread, I wanted to write one specifically dedicated to cables, primarily focused on USB-C but also touching on Thunderbolt 3 where appropriate, since they use the same physical connector and to some extent, the cables are interchangeable, but even that isn’t simple. The reality is that both USB-C and Thunderbolt 3 are so capable, so flexible, and can carry so many different types of signals that it’s created a bit of a mess in the cable space. USB-C may well deliver on its promise of a future that only has one connector to do basically everything, but that doesn’t mean that every cable that features that connector will be capable of carrying everything the port can offer. So for people looking for cables, I wanted to call out the key specs in which various cables on the market can differ so that you purchase the appropriate product for your needs. Here goes:
#1: Data Rate (and DisplayPort Alt Mode support)
I’ll cover Thunderbolt 3 first because it’s so easy: the only options here are 20 Gbps and 40 Gbps, with the latter currently being achievable only on a 0.5m / 18in cable unless you use an active cable (more on that later). All Thunderbolt 3 cables can support all types of traffic that Thunderbolt 3 can carry: USB 3.1, PCIe, and DisplayPort.
For USB-C, you may be aware that USB-C can carry up to USB 3.1 Gen 2 (10 Gbps), but did you know that very few USB-C cables on the market will actually do that? There are actually 3 classes of USB-C cables, just looking at data rate, before even getting into anything else. The cheapest USB-C cables actually only carry USB 2.0 data! Yes, you read that right. The reason is that USB-C connectors have a dedicated set of pins for USB 2.0, totally separate from the pins that carry USB 3.1, and therefore cables can be made thinner, more flexible, and at lower cost if they only have the necessary wires to connect those USB 2.0 pins. These cables do NOT support DisplayPort Alt Mode, so don't expect to use them for displays. The market for this cable type is devices that don’t require high speed data or might require no data at all, such as cables intended solely for charging devices. Additionally, not having to support higher data rates or DisplayPort allows the cables to be longer. Note that all USB-C cables are required to have wiring for the USB 2.0 pins, but some cables have only that wiring (plus power, which is also required, but more details on that later).
Next up from there are cables that can carry up to USB 3.1 Gen 1 (5 Gbps), which also means they support DisplayPort Alt Mode. All else being equal, these cables will be thicker, less flexible (due to the extra wires), and more expensive than the cables that only support USB 2.0. They are also currently limited to 2m / 6ft in length. And then of course there are the cables that can carry USB 3.1 Gen 2 (10 Gbps), but those are currently limited to just 1m / 3ft in length, at least until cable technology improves to support those higher data rates across longer distances. Also note that USB-C cables capable of carrying either generation of USB 3.1 are required to have an “e-Marker” chip embedded in them that advertises this capability to the devices that it will be connecting.
UPDATE: At least one vendor (Cable Matters) has now developed an active USB-C cable that can carry USB 3.1 Gen 2 up to 3 meters / 10 feet.
#2: Power Delivery
All USB-C and Thunderbolt 3 cables are required to be able to carry 3A (amps) of current. Since the current USB Power Delivery spec, which TB3 also uses, currently allows for 20V (volts), and volts * amps = watts, every USB-C/TB3 cable on the market is rated to carry 60W of total power. Some USB-C cables are instead rated for 5A, or 100W, which is as high as the formal USB Power Delivery spec goes (currently). These cables currently seem limited to 1m / 6ft lengths, and note that this increased Power Delivery capacity also triggers the requirement for such cables to include the aforementioned “e-Marker” chip to inform connected devices that they can safely carry the extra power. Finally, note that cable design isn’t affected by the amount of voltage it needs to carry, only by the amount of amperage, with higher amperage requiring thicker wires to carry safely. This is why some manufacturers rate their cables in amps (technically correct) while others just rate the resulting total power under the current spec (easier for consumers). However, if the USB Power Delivery spec were expanded to allow, say, 30V delivery, all existing 3A-rated cables would technically become rated for 90W, and existing 5A cables would technically become 150W cables – barring any additional requirement that cables include updated e-Marker chips to use that higher voltage. But for now, just know that for USB-C/TB3 cables, 3A = 60W and 5A = 100W, and those are the only types today.
#3: Active/Passive (or: “Better cables for Thunderbolt 3 can be WORSE for USB-C”)
Unless a cable is specifically indicated as active, it is overwhelmingly likely to be a passive cable. Thunderbolt 3 cables can be used to connect regular USB-C devices, but this active/passive distinction for TB3 cables has ramifications for that use case. You may have seen “active cables” in the HDMI world, but if not, the designation refers to cables that have electronics built into them to compensate for signal degradation along the length of the cable. This design can make more sense than better electronics in the devices themselves because a major source of degradation is crossing the connector between device and cable. Having the electronics inside the cable allows the signal to be “boosted” once as soon as that gap is crossed on the transmitting side (and that penalty is incurred) and then again immediately before crossing that gap on the receiving side. This allows higher data rates to be sustained across longer cable lengths. I mentioned earlier that currently, Thunderbolt 3 passive cables can only sustain 40 Gbps across a 0.5m / 18in cable – well, an active cable can sustain 40 Gbps across 2m / 6ft. I also mentioned that this active/passive distinction has ramifications when using TB3 cables for regular USB-C, and here it is: the TB3 data signal boosted by these active cables uses the same pins as USB 3.x traffic in USB-C cables, and the chips used to improve TB3 signaling are not compatible with regular USB 3.1 signaling, so active TB3 cables will only support USB 2.0 data rates when used as USB-C cables. Going back to the passive realm, any passive TB3 cable should carry up to USB 3.1 Gen 2 when used as a USB-C cable, but this does not mean that any USB-C cable that supports USB 3.1 Gen 2 will work as a TB3 cable. It may work, but it is definitely not supported, so there are no guarantees.
Phew! Well for those of you who read this far, I hope you found this illuminating or at least interesting. If you have any questions, please don’t hesitate to ask and I will do my best to answer. Enjoy!