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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[–] Nougat@fedia.io 16 points 3 weeks ago (2 children)

This is also why you should be very careful with what you plug into power strips. Your 15A circuit may be able to supply all the power being drawn from all the things plugged in, but the power strip may not. Do you want an electrical fire? Because that's how you get an electrical fire.

[–] scarabic@lemmy.world 4 points 3 weeks ago

I’m sure they exist but I’ve never seen one that didn’t have a breaker for this very reason.

[–] mic_check_one_two@lemmy.dbzer0.com 3 points 3 weeks ago

But also, the 15A breaker is only capable of supplying 12A continuously. The breaker is rated based on heat, (it trips when it gets too hot), and the 15A rating is an “instant” trip rating. Meaning 15A creates enough heat to “instantly” (it takes a few seconds to heat up, but the same is true for the wires in your walls that would cause house fires) trip.

It’s only capable of dissipating enough heat for ~12A of continuous usage, but many devices take more power when they’re first turned on. Like a box fan may only need 1.5A to run, but may take 2A to initially get the blades moving.

All of this is to say that if you truly need 15A of continuous usage, install a 20A breaker (and wiring, and outlets) instead. Because a 20A breaker is capable of handling 15A continuously.