USBefuddled: Untangling the Rat’s Nest of USB-C Standards and Cables

It feels like they should be moving into a state of less confusion by consolidation, but instead of made it more confusing by adopting multiple labeling standards over time. However, the USB 4/Thunderbolt 4 consolidation should be the ultimate prize, and drive cable prices lower as those become the only ones people want to buy (as another commenter noted).

Since the article published, we’ve done a little tweaking based on feedback. Originally, I said USB-C cables could support less than 60 watts, but only USB host and peripheral ports can do so! The USB-C cable spec notes very obscurely that a cable has to be able to “tolerate, or be protected from, a VBUS voltage of 21 V,” which translated means, “all USB-C cables must pass 3A at 20V.” The 21V is a margin of error.

You can find some cable/adapter pairs labeled 15W, but it is possible the cable is rated at 60W, while only the adapter is rated at 15W. Some cables also claim 15W by themselves, but those appear to be out of date or mislabeled—or just out of compliance.

On the Thunderbolt 3 side, I did note earlier that you could have 15W Thunderbolt cables, but that’s a misreading on my part: a Thunderbolt 3 cable that handles 100W of power may only allow 15W of bus power for a connected drive! So the 15W isn’t the cable’s capability, but it’s charging capability off a host adapter.

And to make things more confusing, there is still the legacy standard for devices/chargers that don’t implement the USB PD spec:

  • For USB 1.x and 2.x, a device can request up to 5 “units” of 100mA each, at +5v. Which means up to 2.5W.

  • For USB 3.x, a device can request up to 6 units of 150mA each, at +5v. Which means up to 4.5W.

  • A multi-lane SuperSpeed (3.2 gen 2) device or a device/charger that supports Battery Charging (BC) mode can request up to 6 units of 250mA each, at +5v. Which means up to 7.5W.

It’s all quite messy, IMO.

See also:

As good as that is, I challenge anyone to find the clear statements:

  • USB-C cables must have 3A and 20V support at a minimum.
  • USB 4 controllers have the option but not requirement to implement Thunderbolt 3.

The first is found in Wikipedia, but requires assembling data from USB technical specifications to reach that conclusion (everyone just quotes Wikipedia, which in fact does not link to the appropriate spec, just a press release that doesn’t include that information); the second is more readily available, but not quite clear, because all USB 4 hubs must support Thunderbolt 3, while cables and controllers don’t have to, which is a weird requirement that will ensure more problems.

Great educative article. A good case study of technology industries/companies creating an absolute mess.

Time to bring in the EU to sort out this mess.

You forgot to mention OPTICAL Thunderbolt cables, which are a niche of their own.

None carry power (except maybe a weird 10m TB3 one from Cable Matters; described as having “4 fiber+7 copper cores” – unsure if those copper were for power though?).

Each have controllers in both ends of the cable’s connectors to convert electric to optical signals and back again (hence generally longer connectors than non-optical cables).

I had a massive 60m TB1/2 one that meant my loud storage was away from my home work/sleep area and could get super fast connection, well beating even 10GbE. Also used several 10m TB1/2 ones at work.

The TB1/2 spec failed to deal with overheating issues, that the TB3 ones have supposedly fixed. And of course the TB3 (USB-C connector vs miniDisplayPort) took over FOUR YEARS(!) to arrive (~early 2021) from the release of Apple’s first 2016 TB3 MBPs. Which has made them already slightly obsolete given copper TB4 spec cables had more or less arrived at the same time.

Typically, if you needed one of the alt modes, AFAIR, you’re best to connect via a TB hub, as some modes aren’t supported, and official spec info remains impossible to find.

Oh, then there’s the big issue: price. The TB1/2 ones were a bit cheaper for lower lengths but v.expensive at long lengths (60m one cost well over a grand, and the 10m $300), but the TB3 ones start at pushing ~$400 but then 50m longest ones can be gotten for ‘only’ ~$500.

Given it took Corning over four years to get an optical TB3 one out, I wonder if we’ll ever get to see optical TB4 cables at all, to be frank?

Even if one presumes to understand all of this, i.e., what goes with what and what will work/won’t work with what, how is one supposed to remember it all?? I know I wouldn’t. Which is why the only thing to do is make a .pdf of Glenn’s article to use for reference!

P.S. I still miss FireWire.


USB-C you, C me, we’re all over the place.
Reminds me of RS232C - it actually was a standard, but everybody had their own version.

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As so often, xkcd has a pertinent comment:


I would expect the existing TB3 cables to work as TB4. TB3 and TB4 have the same data rate and (as far as I know) it is encoded identically.

The primary differences between TB3 and TB4 are different use-cases - that is, the way the data is used by the devices. And most of that is not actually introducing new features, but is making formerly-optional usages mandatory.

Dear Glenn
I appreciated the effort to help “un geeks” out there [myself included] get a better grip on the web of cables sitting in our drawers. And I do not mean to be un grateful for the tons of information provided in your article, but I found the piece almost as confusing as the standards it tries to untangle. In the spirit of "help from the perspective to the un-informed’ I would like to offer some suggestion on to make this rich material more accessible to a user level audience.
Please make more transparent the difference between references to physical objects and “standards”. Perhaps a table might be useful to first clarify what USB standard exist, and following this material, which USB of these standards are (can be?) accessed with which physical connectors (and wires?)
Next, it would be helpful if there was a clear explanation of the relation between FireWire/Thunderbird as “standards” (in various flavours) and as physical objects sitting in drawers. Also the relationship among these standards & physical objects. (It seems from reading the article that at least some of the Fire/Thunder standards can be associated with USB-C physical connectors, but I think that I have some of these [older] wires with different endings). As a user with hardwares from anno 1980 to 2021, I could use some help…
I know (have an M1 mini) that some USB-C connectors can link to screens through different end-pieces. Would be nice to have this topic covered from the user’s perspective.
I suggest that the topic of “hubs” either deserves a separate article, or if you choose to include it, do so after the above topics are taken care of, again keeping in mind that from the users perspective the differences among hubs supporting these different standards and physical formats (and which standards support various ‘chains’) may not be obvious or clear.
Thanks again for all the information provided!
Adam (38+ Macs staring with IIe)

Thanks for the article.

I recently got bitten purchasing a Thunderbolt Envoy Express SSD from OWC for my iMac, which has Thunderbolt ports. Unfortunately, it is an iMac (late 2013) and the Thunderbolt ports are Thunderbolt 1. The new drive requires Thunderbolt 3. According to OWC, there is no way I can use the new drive on my 2013 iMac. I guess I’ll have to wait until the new 27" iMacs come out next year.

I think the issue is that the Envoy Express is a bus-powered device.

Apple’s TB-TB adapter (to connect a TB3 device to a TB1 computer) is a data-only adapter. It can not supply power to the downstream device.

I suspect (but can’t say for sure) that you might be able to work around this problem using a TB3 hub/dock that has an external power brick. Use a TB-TB adapter to connect it to your Mac and connect the Envoy Express to the hub.

The idea being that the hub can supply the power that the Apple adapter can’t.

But I don’t know if this will actually work, and TB3 hubs can be expensive devices. You’re probably better off looking for a USB 3.0 enclosure for your SSD (if you want to buy from OWC, the Envoy Pro EX looks like it will do the job), which will work with your Mac (but a bit slower than Thunderbolt).

In the future, when you get a computer with TB3, you can transplant the SSD into the Evoy Express enclosure.

Yeah sounds about right.

The thing is that TB4 cables offer these extra things – well at least potentially as optical ones:

  1. Backwards compatibility with all USB standards (optical TB3 don’t do USB at all).

  2. DP 1.4 (TB3 was only TB1.2).
    TB4 offers 8K & better 6K support (Apple’s XDR Display uses a special TB3 type of DP1.4 DSC to work).

  3. TB ‘Alternate Mode’ USB hubs (aka Multi-port Accessory Architecture).
    Means hubs/docks can now have >two TB ports (eg. CalDigit Thunderbolt 4 Element Hub & OWC’s Thunderbolt 4 Hub), enabling ‘hubbing’ rather than just daisy chaining. TB4 cables will be of benefit running between M1/Pro/Max Macs and that hub. (although there is something I can’t exactly remember about PCIe lanes reducing on 4-port hubs, thus data throughput being lower compared to TB3?)

  4. TB4 mandates wake from sleep for hosts and peripherals (eg. shake mouse; tap trackpad; press keyboard).

  5. TB4 mandates Intel VT-d-based DMA (direct memory access) protection (aka DMA remapping), for security.

  6. TB4 mandates PCIe at 32Gb/s, ie. speeds up to 3GB/s (only optional in TB3; which mandated 16Gb/s).
    Although recent MacBooks had this already; some early TB3 MBPs had less bandwidth on right-hand-side TB3 ports. TB4 assures all four lanes of PCIe available, so PCIe can consume up to 32Gb/s of the total 40Gbps TB bandwidth.

So an optical TB4 cable would potentially be able to work with all previous USB ports, along with the other extras above.

(incidentally, copper TB4 cables are all passive and can be up to 2m long and still handle the 40Gb/s bandwidth; previously passive T3 cables had to be <0.8m to do full 40GB/s bandwidth. Strangely, 2m TB ones are still being sold as “Active” and shorter ones “Passive”, which contradicts the specs!)

That would make optical TB4 cables significantly more appealing to purchase right now if they existed, given the high prices optical TB3 optical commands, with the extra functionalities missing.

To quote Office Space, “Well, I wouldn’t exactly say I’ve been missing it…”

Optical Thunderbolt is its own expensive, compatibility “diverse,” and specialized thing as you well document!


Thanks, David. I’ll investigate the Envoy Pro Ex.

Honestly, the tl;dr is “buy a couple USB 4/Thunderbolt 4 cables,” which should at this point all be rated for 100 watts. They’re expensive, but if you have them on hand, you would probably never have to figure out what USB-C cable goes with what. It’s not a cheap solution.


Yep, likely best idea. There are no 240W ones yet are there, so best option. :wink:

Great article!

To get ahead of much of the problems I had heard about, I invested in the latest OWC TB4 docks and hubs and some very expensive TB4 cables from Apple.
My current main machine is an M1 MBP 13" (my 2016 MBP 15" now secondary) with a 34" LG TB-monitor. I have three external 12TB WD drives attached, one for TM backups, one for media and one as a mirror for that. For those external drives I had to go look for USB-C to USB 3.0 Micro-B cables and they were readily available at a specialist cable store. The MBP communicates all power and data via one 2m Apple TB4 cable. The monitor has the second TB-port on the MBP.
The result so far is: no connection problems at all. A while ago there was a scare about some cheaper TB hubs and docks mishandling the power part, but my OWC ones didn’t have that problem.
Happy camper here :slight_smile: