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My company provided me a Lenovo USB-C dock when I received my Mac Book Pro 2017 model. That dock worked to a certain degree, but I happily replaced it with the Lenovo TB3 dock this week (which basically works much better, not having the quirks of the USB-C dock).

What caught my attention: the TB3 dock comes with a cable that has a "lightning" label (probably to indicate it can transmit power) and a "3" printed on it.

Thing is: the USB-C cable is a bit longer (like 40 cm) whereas the TB3 cable is a bit shorter. So I tried connecting the dock using the longer cable ... and interestingly enough it worked, but one of my two monitors connected to the dock would stay dark.

According to this here, USB-C cables should be "equivalent" (unless getting to long), but I just experienced something else.

Can anyone shed some light on this? Are there physical differences between these cables?

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  • Short answer is that a TB3 cable is a kind of USB-C cable. In the set of USB-C cables is the subset of TB3 cables, all TB3 cables are USB-C cables but not all USB-C cables are TB3 cables. The longer answer I posted as a an answer to this question. The CNET article linked to in the question does a fair job answering the question. I don’t see how you “experienced something else” because you saw a longer cable fail to do what a shorter cable could.
    – MacGuffin
    Oct 15, 2022 at 21:35

2 Answers 2

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The "lightning" does not indicate that it can transmit power - it is actually the Thunderbolt logo indicating that it is a Thunderbolt cable.

All Thunderbolt 3 cables have USB-C connectors. Note that this is just the connector itself - it doesn't mean that the rest of the cable is the same.

Ordinary Thunderbolt 3 cables (i.e. cobber-based) also work as USB-C cables. In this manner they are compatible. You can buy optical Thunderbolt 3 cables (eg. if you want a 50 meter cable) - these are normally not usable as a USB-C cable.

On the other hand any USB-C cable is not Thunderbolt 3 compatible. It might be (as ordinary Thunderbolt 3 cables also work as USB-C cables). In general they are not.

The differences are mainly the design and quality of the cable, and to which standard they have been tested before leaving the factory. For example Thunderbolt 3-cables are generally specced to allow much higher amounts of power delivery than USB-C cables.

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  • That was quick, basically confirming what I assumed. Thank you ;-)
    – GhostCat
    Jun 21, 2018 at 7:44
  • "You can buy optical Thunderbolt 3 cable" -- you can? Where?
    – chx
    Apr 16, 2019 at 0:13
  • Lots of places - just google it. For example you can buy Corning cables on Amazon.
    – jksoegaard
    Apr 16, 2019 at 3:30
  • Optical Thunderbolt 3 (and 4) cables are available in various lengths, they are much more expensive than their copper equivalents by the foot. Here’s one example: bhphotovideo.com/c/product/1577010-REG/… I’ve seen them longer and shorter, some will have backward compatibility with USB 3.x and/or DisplayPort while others are Thunderbolt only. Anything less than 1 meter is likely passive copper, anything copper and longer may still work but you will see degraded performance, such as a display going dark.
    – MacGuffin
    Oct 15, 2022 at 18:31
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Nearly every USB-C to USB-C cable on the market is a passive copper cable. With a passive copper cable we can achieve only so much data throughput before the signal gets mangled by electric resistance or something. The longer the cable the less data bandwidth.

This means USB 2.0 cables can be as long as 2 or 3 meters. (I believe 3 meter passive USB cables technically violate the spec but they work well enough that people buy and use them without too much complaint.) USB 3.x cables can be as long as 1 meter, and Thunderbolt cables as long something like 3/4 of a meter. If the cable is just right then a 1 meter USB 3.x cable will pass the self test for Thunderbolt when plugged in but almost certainly at a lower data rate. With the lower data rate you’ll see effects from limited bandwidth, like a display going dark.

There are active Thunderbolt cables that can be longer than 3/4 meter, much longer. There’s 30 meter (thirty meter, I spell that out so people don’t think it is a typo) Thunderbolt cables but these will be active, usually optical, and quite more expensive than passive cables.

One note is that USB-C is a type of connector, not a protocol. Calling something a “USB-C cable” isn’t always helpful because this can mean USB 3.x, Thunderbolt, USB 2.0, DisplayPort, and even some rare cables wired for HDMI or whatever that protocol is called for 3D glasses. Because passive cables are cheap to make the length is almost always in indication of the bandwidth. That is again 2 meters == USB 2.0, 1 meter == USB 3.x, 3/4 meter == Thunderbolt.

The USB spec requires USB-C cables to be labeled with the protocol it supports. If the cable is a Thunderbolt cable then it should have a little thunderbolt symbol on it. If the cable is a USB cable then it should have the USB trademarked trident icon, and the icon varies based on the version/speed. USB 1.1/2.0 cables will have the original trident. USB 3.x cables will have the trident with the “SS” integrated into the symbol. USB4 cables will have the trident with the number 20 or 40 integrated into it, with that number indicating max supported bandwidth in Gbps. USB 3.x cables sometimes have a small 5, 10, or 20 on them for maximum tested bandwidth but the way the spec is written that number really doesn’t mean much in practice, it works or it doesn’t. Then there’s icons for other protocols as well, and those are less common. Basically the symbol on the cable should match one of the symbols on the port for it to work, sometimes the symbols don’t match and it still works which is perhaps quite common.

What is a bit frustrating is not all hardware manufacturers label the ports as they should, so you are left digging through spec sheets or using trial-n-error to find what works. There’s a lot of cheap USB-C cables out there that don’t bother with labeling the cables, but most of the time just the length indicates the speed.

There’s more to discuss about the power ratings of USB-C cables but I’ll leave that alone as this is already long and that part is perhaps off-topic.

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