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USB-C extensions are not allowed for a reason (www.youtube.com)
submitted 7 months 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 6 points 7 months 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.

[-] Nougat@fedia.io 2 points 7 months ago

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 3 points 7 months ago

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

[-] vane@lemmy.world 1 points 7 months ago

New mission to doom the world is to give away usb-c cables that will activate order 66 after random time.

[-] paraphrand@lemmy.world 0 points 7 months ago

Weird, I extended lightning cables fine.

[-] stoly@lemmy.world 1 points 7 months ago

But lightning isn't usb-c.

[-] Dvoid@lemmy.world 0 points 7 months ago

Why is it designed like that? Why cant the devices on either side talk to each other about power requirements etc instead of going through an extra chip?

[-] Ciderpunk@lemmy.world 2 points 7 months ago

Because the cable itself is a factor. Just because the supplier says “I can give you 200 watts”, and the secondary device says “I can take 200 watts” doesn’t mean the cable between is able to carry that.

It’s not even just power but other things like DisplayPort over USB, Ethernet over USB, Thunderbolt, and whole slew of other things a theoretical USB-C cable can do, but not necessarily every USB-C cable.

[-] stoly@lemmy.world 1 points 7 months ago

It's important to note that not all cables connect all the wires. You don't need them all if your cable is only for charging. You need more if your cable transfers data.

[-] akilou@sh.itjust.works 0 points 7 months ago

Why can't the extenders pass along the same data the original cord is sending?

[-] ChaoticNeutralCzech@feddit.org 2 points 7 months ago

They do (if they carry all wires), it's just that they don't add their own current rating into account.

[-] akilou@sh.itjust.works 2 points 7 months ago

So then do that

[-] ChaoticNeutralCzech@feddit.org 1 points 7 months ago

I guess there is a way to implement this true to USB-C standard, either with a simple comparison of reported capabilities or a 1-port hub. Somebody would have to make a cost-effective chip that does that though, and manufacturers are happy just printing "max 3 A" on the packaging (if even that).

[-] stoly@lemmy.world 2 points 7 months ago

The USB consortium has responded to this sort of need over time, but they have to do it in a way that is completely backwards compatible. It might be possible to do this in a new release of the standard, but it may also be that millions of cheap cables would be rendered useless in the process.

this post was submitted on 18 Aug 2025
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