USB-C and Thunderbolt 3 are no longer new technology. But the similarities and differences can still wind up confusing even long-time users of the Mac. Where this tends to manifest itself these days isn’t so much with connectors and adapters, but with power.

  Some cables with USB-C connectors can carry data, video, and networking traffic while also supplying low-wattage power to USB devices if they have a USB Type A connector on the other end. They’re capable of charging iPhones and iPads, but not maintaining or recharging an Apple laptop.

  Others cables with the same connectors can pass up up 29W (watts), 30W, or 100W of power—but may be limited to USB 2.0 data transfer speeds…or might handle anything built into the Thunderbolt 3 specification including 100W of juice!

  Let me break this down as to why:

  USB-C is a broadly defined connection and cabling format, rather than a protocol for defining specific kinds of data that passes over it. Hardware devices—like the 2015 to 2018 12-inch MacBook and some non-Apple smartphones, tablets, and laptops—use USB-C to carry USB 2.0, 3.0, and 3.1, as well as ethernet networking and DisplayPort/HDMI video signals (and even DVI and VGA!) via adapters.

  Thunderbolt 3 is a data-transfer standard that can carry up to 40Gbps of information. It was built to work only with the USB-C connector style and specification. It can also carry all the kinds of traffic noted above for “plain” USB-C devices.

  The trouble intrudes when we talk about power delivery, a separate part of the USB specification and incorporated into USB-C and Thunderbolt 3. Before USB-C, even though it was possible to carry laptop levels of power over a USB cable designed for that purpose, manufacturers generally avoided this—probably to avoid cable confusion.

  USB-C allows up to 100W of power by incorporating the Power Delivery 2.0 specification from the USB trade group. (The spec has been updated to 3.0 with additional smarts about multiple connected devices, but it’s fundamentally the same.)

  The trick is that you have to have the right cable for the right voltage. You won’t harm a device by plugging in a cable that’s rated for lower wattage—unlike an AC cord, which can overheat—because USB-C negotiates power usage and prevents passing voltages above the level the cable “says” it can carry.

  But you will find many different cables with seemingly identical USB-C connectors.

  Low-wattage USB-C: Some cables with USB-C connectors at one end and a different kind of USB connector (like Type A or Micro Type B) can’t pass a lot of power. They mostly manage USB 2.0 data (480 Mbps) plus no more than 15W (5 volts at 3 amps), which is enough to charge smartphones and tablets. These cables may have USB 3.0 SuperSpeed (SS + a USB logo) or similar branding or logos on one tip. Or not! (See figure above.)

  High-wattage USB-C: Higher-wattage USB-C cables come in many varieties. Even Apple has three: 29W (for the original 12-inch MacBook), 30W (2018 MacBook Air), and 100W (all other USB-C models); the 29W and 30W cables are effectively interchangeable. But if you use a 61W laptop with a 30W cable, it won’t charge quickly or may slowly lose charge while you use it. Apple doesn’t put icons on these cables connectors, but there is information printed in very very tiny characters on the cable itself. See Apple’s power-cable identification guide for details about figuring out which cable is which.

  Despite Apple’s listing of varieties, all cables that have a USB-C connector on both ends are supposed to carry up to 60 watts (20 volts at 3 amps), according to the specification. At Apple and other manufacturers’ sites, I’ve seen cables listed that are rated for a lower maximum. Check the details on anything you order to be sure they’re promising

  Some non-Apple cables may sport the USB SuperSpeed logo on their plug ends, but with the logo reversed out of a stylized battery to indicate they carry more power than an old-style USB cables. However, that doesn’t reveal the maximum wattage. You will have to consult the manufacturer’s website and Google if you didn’t retain packaging material.

  And some of these cables will only carry USB 2.0 data rates—but a few are designed to handle USB 3.0 and 3.1 speeds, like the Anker PowerLine II.

  Thunderbolt 3 cables: From the specification, all Thunderbolt 3 cables carry either 20 Gbps or 40 Gbps of Thunderbolt 3 data rates and pass along power as high as either 60W or 100W, depending on the cable design. Apple sells such a cable. These cables will always have the Thunderbolt lightning-bolt logo on each USB-C tipped cable end.

  You can use a Thunderbolt 3 cable with a non-Thunderbolt 3 USB-C device, and it should correctly carry power and no more than the maximum either side can handle. It won’t hurt anything, but it’s overkill given the higher cost of Thunderbolt 3 cables compared to USB-C cables. If it’s a more-expensive active Thunderbolt 3 cable, which allows for 40 Gbps of throughput among Thunderbolt 3 devices up to about six feet, it also can’t handle more than USB 2.0 rates.

  This Mac 911 article is in response to a question submitted by Macworld reader Moisés.

  Updates: This article has been revised to better clarify how low-wattage cables mix USB-C and other USB connection styles and the maximum power for a USB-C to USB-C cable, and to note that Thunderbolt 3 cables may have a maximum of 60W or 100W.

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