this post was submitted on 18 Aug 2025
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USB-C extensions are not allowed for a reason (www.youtube.com)
submitted 3 weeks ago by Chozo@fedia.io to c/videos@lemmy.world
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This popped up randomly in my feed today, and I found it to be pretty interesting and informative.

tl;dw: All USB-C cables have a microchip inside them which runs a small bit of software that tells the devices its plugged into exactly what they're capable of, such as their power rating and transfer speeds. When you plug the cable into your device, it reads the data from this chip, which then dictates how much data/power it is allowed to transmit along the cable.

The problem is that when you use a USB-C extension cable, the device you're plugging into can only see the chip data from the first cable; the cables beyond that first one are completely invisible to your device. And if your first cable is rated for 200 watts, and your extension is only rated for 100 watts, your device will still send 200 watts down the line, without ever realizing that it's overloading the extension cable and creating a possible fire hazard.

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[–] billwashere@lemmy.world 38 points 3 weeks ago

No, all USB-C cables do not have hardware to negotiate power (voltage and current). USB-C is simply a connector type like USB-A and USB-B. What you’re referring to is called USB-PD. This is when the charger and device “handshake” to agree on the optimal voltage and current, then adjust dynamically as needed. Yes extensions can be a problem for things like laptops or anything that can draw large current over high voltage. It’s really not any different than trying to run a table saw over a cheap extension cord meant for a lamp or a set of Christmas lights. If you’re using one of the usb extension cables to charge your phone, it’s fine.