WhatCable Blog

Hello, world

2026-05-20T00:00:00Z

Welcome to the WhatCable blog.

This is where I'll write about the messy, fun world underneath USB-C: why your fast cable charges slowly, what a Thunderbolt 5 cable actually negotiates with your Mac, which e-marker fields are honest and which are theatre, and the odd counterfeit or quirk that turns up in cable reports.

What you'll find here

A few rough buckets:

Releases: what shipped in the latest WhatCable version, and why.
Deep dives: long-form pieces on the spec, the IOKit data, and how the app reads it.
Investigations: single-cable mysteries, mostly from community reports.
Guides: practical "how do I know if X" walkthroughs.
Notes: short observations, dev diary entries, posts like this one.

How to follow along

Drop the Atom feed into your RSS reader and you're set. No newsletter, no tracking, no comments. If you want to chime in, that's what GitHub issues are for.

Why is my MacBook charging so slow? A real diagnosis.

2026-05-21T00:00:00Z

In macOS Tahoe 26.4, Apple added a "Slow Charger" label. Plug in a charger that can't deliver full power and the menu bar tells you so.

What it doesn't tell you is why.

That's the gap. The OS confirms what you already suspected (yes, it's charging slowly) and then leaves you to guess whether the problem is the adapter, the cable, the port, the battery, or the laptop itself. Most of the time it's one of two things, and both are easy to confirm if you know where to look.

The actual causes, in order of how often they happen

1. The adapter doesn't have enough watts for your Mac

This is the most common cause by a long way. Apple publishes minimum wattage figures for each MacBook and they're not suggestions, they're the floor for full-speed charging. Anything below the minimum and the Mac throttles down to whatever it can pull.

The rough numbers, from Apple's adapter guide:

MacBook Air (M-series): 30W minimum
MacBook Pro 14": 70W for the base chip, 96W for Pro/Max
MacBook Pro 16": 140W

A 30W adapter charging a 16" MBP will work, but it'll feel like it's not charging at all under load. Sometimes the battery still drains because the Mac is using more than the adapter can supply.

How to confirm: check the wattage printed on the adapter itself. If it's below the minimum, that's your answer.

2. The cable is rated below the adapter

This one trips up a lot of people because the adapter and the Mac are both capable, but the cable in the middle is the bottleneck.

USB-C cables carry an e-marker chip that declares what they can handle. A cable rated for 60W (3A at 20V) physically cannot pass 96W (4.7A at 20V) no matter what the adapter or the laptop ask for. The PD negotiation drops to whatever the weakest link supports.

The giveaway: you bought a 96W or 140W charger, the Mac is the right model, and charging is still slow. Nine times out of ten it's the cable. Especially if it's the cable that came in the box with something else, or a generic spare from a drawer.

How to confirm: this is where it gets fiddly without help. The cable's rating is usually not printed on the cable. You can't see the e-marker contents from the Finder or from System Information. We'll come back to this.

3. Optimised Battery Charging is holding at 80%

macOS learns your routine and parks the battery at 80% if it thinks you're about to leave it plugged in for hours. From its perspective this is a feature, since holding at 80% is much kinder to long-term battery health than sitting at 100% all day.

From your perspective it looks like the charger isn't doing its job.

How to confirm: System Settings → Battery → Battery Health → check the charging schedule. If Optimised Charging is on and the battery is sitting at exactly 80%, that's the cause. Click "Charge to Full Now" if you actually need it now.

4. The workload is outpacing the supply

If you're rendering video, compiling a large project, or running a sustained GPU load on a 14" or 16" MBP, you can genuinely draw more than the adapter delivers. The battery makes up the difference and the percentage creeps down despite being plugged in.

How to confirm: open Activity Monitor → Energy tab → look at Energy Impact. If you're maxing out CPU or GPU, you're not going to charge while doing it on a 30W or 70W adapter.

5. Dirty port, damaged cable, hardware fault

The boring causes, but they happen. A USB-C port full of pocket lint won't seat the connector properly and the contacts won't make. A cable that's been kinked too many times near the connector can lose one of its conductors and drop from 96W to 60W (or worse) without looking obviously broken.

How to confirm: try a different port on the same Mac. Try the same charger and cable on a different Mac if you can. If you get fast charging on a different port but not the original, the port's the problem. If you can't see anything obvious in the port, a wooden toothpick (never metal) is the safe tool for clearing lint.

If you're on an Intel Mac and you've exhausted everything else, an SMC reset is the next step. On Apple Silicon there's nothing to reset, it's all handled differently.

How to actually check what's happening

The diagnostic question that matters: what is the cable, adapter, and Mac actually negotiating right now?

USB Power Delivery is a conversation. The adapter says "I can offer 5V/3A, 9V/3A, 15V/3A, 20V/4.7A." The Mac picks the highest its battery can accept. The cable's e-marker sets the ceiling on current. They agree on a contract, and that contract is what determines your charging speed.

macOS knows all of this. It reads the e-marker, it tracks the active PD contract, it knows what each port can do. It just doesn't surface any of it.

This is what WhatCable was built for. It sits in the menu bar and reads what macOS already has, then tells you in English:

The cable's e-marker rating (max watts, max current, max data speed)
The active PD contract (volts and amps being negotiated right now)
What the port itself can do
Where the bottleneck is, if there is one

A healthy reading on a 16" MBP with a 140W adapter and a 240W USB4 cable: 28V at 5A, cable rated for 240W, port rated for 140W. Everything matches.

A throttled reading on the same setup but with a wrong cable: 20V at 3A, cable rated for 60W, port still rated for 140W. The cable is the bottleneck and you can see it.

That's the answer to "why is it slow." You can stop guessing.

If you want to see exactly what your own setup is negotiating, WhatCable reads the PD contract and the cable e-marker and shows it in the menu bar. The Slow Charger Indicator tells you there's a problem. WhatCable tells you which link in the chain caused it.

Thunderbolt vs USB-C: what the connector hides

2026-05-21T00:00:00Z

USB-C is the shape of the connector. Thunderbolt is one of several high-speed protocols that uses that shape.

That sentence is the entire answer to the headline question, and every page that ranks for this query opens with some version of it. The reason it keeps getting asked is that the visual is identical. A Thunderbolt 4 port and a basic USB-C 2.0 port look the same. The cables look the same. The plugs go in the same way. What changes is what's happening behind the connector.

Here's the breakdown.

The comparison at a glance

Standard	Max data rate	Video	Power Delivery	Daisy chain	Cable needed
USB 2.0 (USB-C)	480 Mbps	None	Up to 240W (PD 3.1)	No	Basic USB-C
USB 3.2 Gen 2x2	20 Gbps	DisplayPort Alt Mode	Up to 240W	No	USB 3.2 cable
Thunderbolt 3	40 Gbps	2x 4K @ 60Hz or 1x 5K	Up to 100W	Yes (up to 6)	TB3-certified
USB4	20 or 40 Gbps	DisplayPort 1.4	Up to 240W	Limited	USB4 cable
Thunderbolt 4	40 Gbps	2x 4K or 1x 8K	Min 15W, up to 100W	Yes (up to 6)	TB4-certified
Thunderbolt 5	80 Gbps (120 Gbps boost)	3x 4K @ 144Hz	Up to 240W	Yes	TB5-certified active

The table is the artefact most people are looking for. The rest of this post is the why.

Thunderbolt 3 vs USB-C

TB3 was the first generation to share the USB-C connector, which is when the confusion started. Before TB3, Thunderbolt used Mini DisplayPort. After TB3, you couldn't tell a Thunderbolt port from a USB-C port without checking the lightning bolt icon next to it.

Underneath, TB3 is doing a lot more than basic USB-C. It tunnels PCIe and DisplayPort over the same wire, which is what makes external GPUs and high-bandwidth docks possible. It runs at 40 Gbps where basic USB-C 3.2 caps out at 20 Gbps. It supports daisy-chaining up to six devices off a single port.

The catch: TB3 cables are not the same as USB-C cables. A TB3-certified cable contains active electronics that maintain signal integrity over longer runs, which is why a 2m TB3 cable costs significantly more than a 2m USB-C cable. Use a generic USB-C cable in a TB3 port and you'll get USB speeds, not Thunderbolt speeds.

Thunderbolt 4 vs USB-C

TB4 didn't push the headline speed up. It's still 40 Gbps, same as TB3. What TB4 did was tighten the minimum requirements.

Where TB3 said "up to 40 Gbps", TB4 says "must be 40 Gbps". Where TB3 video support varied by host, TB4 requires support for two 4K displays. Where TB3 had no minimum charging spec, TB4 requires at least 15W for accessory charging and 100W host charging on at least one port. TB4 also requires support for PCIe data tunneling at higher minimum rates than TB3.

For the user, TB4 means fewer surprises. A TB4-certified port and a TB4-certified cable will hit the spec sheet every time. You don't have to read the small print.

Thunderbolt 5 vs USB-C

TB5 is the current top of the pile, on Macs with M4 Pro and M4 Max chips and later. The headline number is 80 Gbps symmetric, double what TB3 and TB4 offered. In "Bandwidth Boost" mode it goes to 120 Gbps in one direction and 40 Gbps in the other, designed for driving very high-refresh-rate displays.

TB5 also bumps the power spec. Up to 240W of Power Delivery, matching USB PD 3.1's ceiling.

For a basic USB-C port, none of this applies. A USB-C device in a TB5 port still runs at USB speeds. A TB5 device in a basic USB-C port either drops to USB mode or doesn't work at all, depending on the device.

TB5 cables are required to be active. The bandwidth is too high for passive copper at any meaningful length.

USB4 vs Thunderbolt 4

This is the comparison that confuses people the most, because USB4 and Thunderbolt 4 are essentially the same thing under different names.

USB4 was developed in collaboration with Intel and licensed from the Thunderbolt 3 specification. The result is that USB4 and TB4 share most of their underlying mechanics. Both can run at 40 Gbps. Both tunnel DisplayPort and PCIe over USB-C. Both support up to 240W via USB PD 3.1.

The difference is in what's mandatory.

USB4 has two tiers, 20 Gbps and 40 Gbps. Many features are optional. A USB4 port might or might not support PCIe tunneling, might or might not hit the full 40 Gbps, might or might not charge external devices.
Thunderbolt 4 is strict. All features are mandatory at the full spec. Buy something labelled TB4 and you know what you're getting.

In practice, if you've got a Mac with a TB4 or TB5 port and you plug in a USB4 device, it should work. The reverse (USB4 host, Thunderbolt device) is also fine for Thunderbolt 3 and later devices, because USB4 hosts are required to be backward compatible with TB3.

How to tell what you actually have

The visual cue is the lightning bolt icon next to the port. If you see one, the port supports Thunderbolt. If you don't, it's basic USB-C.

The icon tells you what the port can do. It does not tell you what your cable can do.

This is the part nobody talks about. A USB-C cable in a Thunderbolt port is still a USB-C cable. The port will negotiate down to whatever the cable supports. You can have a TB5 port and a 40 Gbps device and still get USB 3.2 speeds because the cable in the middle is a 20 Gbps cable that came with a hard drive five years ago.

Every USB-C cable rated above 60W, and every cable faster than USB 2.0, contains an e-marker chip. Either one on its own is enough to require it. The chip declares what the cable can carry: max current, max voltage, max data rate. macOS reads this chip when the connection calls for it, above 3A of current or on a Thunderbolt link. It just doesn't show you what it reads.

WhatCable reads the e-marker and shows you what the cable is. Not what you hoped it was, not what the box claimed, what the cable itself is telling the Mac. If you've ever wondered whether the "Thunderbolt cable" you bought online is actually Thunderbolt, this is how you check. You can also see how your cable rates against known references in the cables database.

There is one more trick. On a Thunderbolt or USB4 connection, WhatCable does not just trust the chip. It reads the speed the Mac's controller actually negotiated with the cable and shows it next to the e-marker's claim, so a cable that performs better than its chip admits gets caught. That only works on a live Thunderbolt or USB4 link, and the measured figure is a floor: at least this fast, sometimes more if the device at the far end was the limit.

Compatibility, both directions

USB-C device into a Thunderbolt port: works, at USB speeds. The TB port has full USB-C compatibility built in. Plug in a phone, a basic USB hub, or a regular external drive and it'll run at whatever the device supports.

Thunderbolt device into a USB-C port: often doesn't work. Thunderbolt requires an explicit handshake between host and device that USB-C ports don't perform. Some Thunderbolt docks have a USB fallback mode and will partially work, with reduced features. Most TB-only accessories (external GPUs, high-end audio interfaces, fast NVMe enclosures) will simply not appear.

This is why "is this port USB-C or Thunderbolt" matters before you spend money on a TB accessory.

What about charging

Both USB-C and Thunderbolt use the same USB Power Delivery spec for charging. The difference is the minimums, not the maximums.

A TB4 host port has to deliver at least 15W. A TB4 PC host port has to deliver at least 100W on at least one port. Basic USB-C has no such minimum.

For charging specifically, the protocol matters less than the wattage. A 140W basic USB-C charger will charge a 16" MacBook Pro just as fast as a 140W Thunderbolt cable would, because they're using the same PD spec underneath. We've written about why your MacBook might still charge slowly even with the right adapter, and it almost always comes down to the cable.

Cost and why TB cables are expensive

A passive USB-C cable is cheap because it's just wires. It works because the signal at USB 2.0 speeds is forgiving over a metre or two of copper.

A Thunderbolt cable at 40 Gbps or 80 Gbps cannot be passive at any useful length. The signal degrades too fast. TB cables contain active electronics that reshape the signal at the connector, which is why a 2m TB4 cable costs five times what a 2m USB-C cable does. TB5 cables go further, requiring active electronics in the connectors even at short lengths.

If you see a "Thunderbolt 5 cable" for £8 on a marketplace, it probably isn't one.

If you want to see whether your cable is genuinely Thunderbolt or just USB-C in a Thunderbolt port, WhatCable reads the e-marker and tells you straight.

WhatCable doesn't just take the cable's word for it

2026-07-17T00:00:00Z

Every review of WhatCable lands on the same caveat. Some cables lie about what they can do, so an app that reads a cable can be fooled. That's true. It's also half the story, and the other half is the part that makes WhatCable worth having.

Here's the distinction.

What a cable claims vs what the link actually does

A USB-C cable with an e-marker chip carries a small block of self-description. Rated speed, current, voltage, vendor ID, active or passive. When you plug it in, your Mac asks the chip "what are you?" and the chip answers. That answer is a claim. Nothing more. A cheap cable can claim 240W and 40 Gbps and be wired for neither.

If all WhatCable did was print that claim back to you, the worry would be fair. A liar with a nice font is still a liar.

But the claim is one input, not the answer.

It reads the whole chain, then checks it against itself

Every connection has more than a cable in it. There's the port on your Mac, the cable, and the thing on the other end: a charger, a dock, a drive, a display. Each part reports what it can do, and the two ends negotiate a result they can both live with.

WhatCable reads all of it. The Mac port's own capability. The cable's e-marker. The device or charger's identity. Then it does the part that matters. It compares what each part claims against what the link actually negotiated, and it names the weakest link.

So when a cable claims 40 Gbps but the connection came up at 10, you don't just see the cable's boast. You see the boast, the reality, and where they diverge. And WhatCable is careful about who it blames. It ranks the suspects, device first, then the Mac port, then the cable, so it won't tell you to buy a new cable when the thing actually holding you back is the drive you plugged in. Replacing the cable there would change nothing, and saying so would be worse than saying nothing.

That's a diagnosis of the connection. Not a recital of what the cable says about itself.

The Thunderbolt cross-check: a second, independent witness

This is the part the "cables can lie" caveat doesn't account for.

On a Thunderbolt or USB4 link, WhatCable doesn't only ask the cable how fast it is. It also reads Apple's own Thunderbolt controller, which measures the speed the link actually trained to, in hardware, with no reference to the e-marker at all. Two separate sources: what the cable says, and what the controller measured.

When they agree, fine. When they disagree, that's the interesting case, and it usually runs the opposite way to what you'd expect.

Some active Thunderbolt 4 cables under-sell themselves in their e-marker. The chip reports "passive" or a low speed, while the cable is quietly carrying a full 40 Gbps. Read the e-marker alone and you'd under-rate a perfectly good cable. WhatCable sees the controller measuring more than the chip claims, trusts the hardware measurement over the self-report, and tells you the cable is doing 40. The cable lied, and WhatCable corrected it upward from an independent reading. That's the exact opposite of being fooled.

It also refuses to make the mistake in reverse. A genuine 80 Gbps cable run between two 40 Gbps ports will negotiate 40, because 40 is all the endpoints can do. The controller's measurement is a floor on what the cable can carry, never a ceiling. So WhatCable treats "the link ran at 40" as proof the cable does at least 40, not as evidence the cable's 80 Gbps claim is a lie. A cable rated above what its endpoints can use is completely normal, and calling it suspicious would be a false alarm.

Charging: which of the three parts is the reason

Same idea, different signal. When you ask "why is my MacBook charging slowly", there are three numbers in play: what the charger can output, what the cable is rated to carry, and what the connection actually negotiated. WhatCable reads all three and tells you which one is the bottleneck.

If the cable is rated below the charger, it says so plainly, replace the cable. But it's just as careful about the times the cable is fine. Charging at 30W off a 96W charger is usually not a fault at all. It's your Mac asking for less because the battery is nearly full, or because a charge limit is active, or because macOS draws from one charger at a time and this port is on standby while another does the work. WhatCable names each of those states rather than leaving a healthy cable under suspicion. The goal is a correct answer, not an alarming one.

Displays: it clears the cable, it won't frame it

Video is the one dimension where WhatCable is deliberately more cautious, and the caution is the point.

It reads the display's EDID, the block the monitor uses to describe its resolutions and refresh rates, works out the bandwidth the monitor's best mode needs, and compares that against the bandwidth the link is currently carrying. If the link already covers the top mode, it says so with confidence.

If it doesn't, WhatCable will not automatically blame the cable, because from this kind of passive reading it genuinely can't tell a cable that's maxed out from a resolution you simply haven't selected yet. A DisplayPort link trains itself down to whatever's on screen to save power, so a "slow" link is often just an unused one. What WhatCable can do is clear the cable when the evidence allows, for instance when the video is tunnelled over Thunderbolt (so the cable is carrying far more than any picture needs) or when every DisplayPort lane the Mac provides is already in use. It can also spot when your display is using compression (DSC) to fit a big mode like 4K120 through the link, and say the picture is fine rather than cry shortfall. It exonerates the cable on evidence. It never convicts it on a guess.

Where the caveat is still fair

Software can't see inside the jacket. If a cable's e-marker claims 240W and the copper can't carry it, and you never draw enough power to expose the gap, WhatCable will show you the claim with nothing to contradict it. At that point the chip is lying and there's nothing in the data to catch it out.

No pretending otherwise. WhatCable does flag a handful of internal tells that tend to travel with dodgy cables: reserved bit patterns the spec doesn't allow, a vendor ID that resolves to no known maker, an e-marker that claims high-voltage charging while reporting a voltage that can't support it. Ten checks in all. But it's deliberately restrained about them. A blank vendor ID on its own doesn't trip a warning, because plenty of genuine cables ship without one, Apple's included. WhatCable only leans on that signal when the rest of the e-marker is inconsistent too. The flags are a reason to look closer, not a verdict. The only thing that proves what's physically inside a cable is a hardware tester, and WhatCable has never claimed to be one.

Trust is earned by delivering, not by claiming

That restraint is the whole design of WhatCable's trust rating, and it's worth spelling out because it's the direct answer to "an app that reads a cable can be fooled".

WhatCable does not hand a cable a clean bill of health for having a tidy e-marker. We tried scoring cables on their e-marker bits and it failed against real hardware: the "suspicious" flags fired on genuine cables, Apple's own among them. So a cable earns a green rating one way only, by being watched to deliver its claim. The link actually carried its rated speed, or a charge actually pulled its full rated power. A registered vendor ID isn't enough. A confident-looking e-marker isn't enough. Pedigree is not proof. Delivery is.

Until then a cable sits at "unverified", which is not an accusation, just an honest "nothing demanding has been connected yet, so there's nothing to confirm". And a cable can lose trust too: if the connection repeatedly drops or degrades under load over a session, that observed failure outranks any earlier claim.

The point

When something's slower than it should be, "is this cable any good" is really "why is this slow, and is the cable the reason". Answering that means reading the whole chain, not the cable's boast, and on Thunderbolt, checking that boast against what the hardware actually measured.

WhatCable does that. The cable's own claim is one witness, and not always an honest one. It's never the only one in the room.

How to tell if a USB-C cable is Thunderbolt

2026-07-30T00:00:00Z

How to tell if a USB-C cable is Thunderbolt

You can't tell by looking. Not reliably.

The cable knows what it is. There's a chip inside the connector called an e-marker that declares its capabilities, and your Mac can read that declaration. The trick is getting it back out in a form you can use.

Three ways to do it, worst to best.

Method 1: look at the connector (mostly useless)

Every guide on the internet tells you to look for a lightning bolt symbol. It's the first answer on every page, and it's the weakest one.

Intel does require certification for the Thunderbolt mark. Computers, accessories and cables all have to pass their test suites before they can carry it. So when the symbol is there and genuine, it means something.

The problem is everything else:

Most cables in your drawer have no markings at all
The bolt often appears on the port rather than the cable, which tells you what your Mac can do, not what the cable can do
Counterfeit marks exist, and marketplaces are full of them
A cable can be genuinely capable and completely unmarked, because plenty of manufacturers don't bother certifying

If your cable has a bolt and a number stamped on the housing, you have your answer. If it doesn't, you've learned nothing. Which describes most cables.

Method 2: System Information

macOS does expose this. It's buried, but it's there.

Apple menu > About This Mac > More Info > System Report. Then look under Hardware for Thunderbolt or Thunderbolt/USB4, depending on your Mac.

This works, with one significant catch: you need a Thunderbolt device plugged in at the other end. The pane reports on the link, and there's no Thunderbolt link without a Thunderbolt device. Plug in a phone or a basic hub and it tells you nothing useful about the cable.

The second catch is subtler. What you're looking at is the negotiated result of three things: your Mac's port, the cable, and the device. If it comes back slower than expected, System Information won't tell you which of the three is responsible. You're left swapping components one at a time until the number changes.

It's the free answer and it's better than guessing. It's just not a cable test.

Method 3: read the e-marker

This is the actual answer.

An e-marker is a small chip in the cable's connector. It holds a description of what the cable can do: maximum current, data rate, whether the cable is active or passive, who made it. The USB-IF publishes the test methodology labs use to verify them.

The specification requires an e-marker under two conditions:

Any cable carrying more than 3A. That's the 100W and 240W cables.
Any full-featured cable. Meaning anything doing USB 3.x signalling or faster, regardless of how much power it carries.

Both conditions matter, and the second one is the useful one. Every Thunderbolt cable has an e-marker. So does every 10Gb/s cable, every USB4 cable, and every cable that charges above 60W.

Which means a cable with no e-marker at all is telling you something quite specific. It's a basic charging cable: USB 2.0 data at 480Mb/s, 60W or less. Not Thunderbolt, and not close.

And a cable with an e-marker is telling you exactly what it is, in its own words, regardless of what the packaging claimed or what the seller wrote in the listing.

One practical note before you go looking. The e-marker isn't read the moment the plug goes in. It's read during power negotiation, which means something has to be connected at the other end to trigger it. A cable hanging out of a port with nothing on the far end tells you nothing.

One more condition. macOS only reads the e-marker when the connection negotiates above 3A, or when the link is Thunderbolt. In practice: a charger above 60W will do it, and so will any Thunderbolt device. A low-power phone charger won't. Still an easier ask than the Thunderbolt pane in System Information, which wants an actual Thunderbolt device before it shows you anything at all.

WhatCable reads the e-marker and puts it in plain English. Plug the cable into a charger above 60W, look at the menu bar. Speed, power rating, active or passive, and whether the thing is genuinely Thunderbolt or a USB-C cable sitting in a Thunderbolt port. No menu diving, and no need to dig out a Thunderbolt dock just to identify a lead.

Download WhatCable free, open source, Apple Silicon.

What the answer actually means

Once you've read the e-marker you'll want to know what it's telling you. The generations differ more than the marketing suggests.

Cable	Max data	Passive length at full speed	Power
Plain USB-C (no e-marker)	480Mb/s	n/a	60W
USB-C 5Gb/s	5Gb/s	2m	60-100W
USB-C 10/20Gb/s	10-20Gb/s	1m	60-100W
Thunderbolt 3	40Gb/s	0.5m	100W
Thunderbolt 4	40Gb/s	2m	100W, 240W on EPR models
Thunderbolt 5	80Gb/s, 120Gb/s boost	around 1m	240W

A few things worth pulling out of that table.

Thunderbolt 3 passive cables fall off a cliff. A passive TB3 cable holds full 40Gb/s at half a metre. At two metres, the same passive cable drops to 20Gb/s. Half the bandwidth, no warning, no error message. If you want 40Gb/s over two metres on TB3 you need an active cable, which is why the long ones cost so much more.

Thunderbolt 4 fixed that. The TB4 specification requires certified passive cables to hold 40Gb/s all the way to two metres. If you're buying a long cable and you have the choice, this is the generation where the certification earns its money.

Thunderbolt 5 tightens it up again. TB5 runs at 80Gb/s bidirectional as standard, with a 120Gb/s Bandwidth Boost mode that reallocates lanes to push more in one direction. Higher frequencies mean less tolerance for signal loss, so the passive limit comes back down to roughly a metre. Beyond that you need retimer chips in the connectors.

Active cables aren't automatically better. An active cable regenerates the signal, which buys you length. It doesn't buy you speed the cable wasn't already rated for. A long active TB3 cable is still a 40Gb/s cable.

For reference, Apple's own Thunderbolt 4 Pro Cable handles 40Gb/s and 100W. The Thunderbolt 5 Pro Cable handles 120Gb/s and 240W. Both are USB-C at both ends and look near enough identical in a drawer.

The mistakes worth avoiding

A 240W cable is not necessarily a fast cable. EPR charging and high-speed data are separate capabilities. Plenty of 240W cables carry USB 2.0 data and nothing more. They'll charge a MacBook Pro at full tilt and make an external SSD crawl. If a cable is advertised only by wattage, assume the data side is basic.

An eight-pound "Thunderbolt 5" cable isn't one. TB5 signalling is unforgiving, so the cable has to be built to much tighter tolerances than a general-purpose USB-C lead. Anything past about a metre needs retimer silicon on top of that, and Intel certification costs money to obtain. Cheap plus TB5 means one of the two claims is false.

USB4 40Gb/s is not the same as Thunderbolt certified. They frequently behave identically. Thunderbolt 4 is built on top of USB4, but it makes mandatory a set of things USB4 leaves optional. An uncertified USB4 cable may do everything you need. It just hasn't been tested to prove it.

In a daisy chain, the weakest cable sets the ceiling. Chain a dock, a display and a drive together and every device downstream of your worst cable is limited by it. This is the failure mode that produces the most confused support requests, because the problem cable is often the one nowhere near the thing that's misbehaving.

Still not sure what's in your drawer? WhatCable reads the cable, the port, the charger and the connected device, then tells you which one is the bottleneck. Free on Apple Silicon.