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How long can a USB cable be?

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Vladimir Ya/Shutterstock.com

USB cables come in different lengths. Some are only a few inches, while others can be several meters long. But is there a maximum length for a USB cable? Here’s everything you need to know.

Why does USB cable length matter?

Before we talk about how long a USB cable can be, it’s important to understand the role that cable length plays in its performance.

USB is an interface that is used by computing devices to transfer data or power from one device to another. When two devices are connected via a USB cable, they use data packets to communicate with each other. But to make sure the receiver receives each data packet, the source device waits for an acknowledgment signal from the receiver before sending the next data packet.

However, the source device can only wait a fixed amount of time (PDF), typically in nanoseconds, before deeming the packet as lost and retrying. Thus, time is a critical factor for the efficient flow of data from one device to another. And how long a data packet must travel before it reaches its destination affects this time.

As a result, the length of the USB cable becomes very important. If it is short, the data travel time from source to destination, also known as signal propagation delay, will be shorter. But if the USB cable is too long, the propagation delay will be large, and this may drastically reduce the data transfer rate.

Another factor that affects the maximum length of a USB cable is power loss or signal attenuation. When a data signal travels down a USB cable, it encounters resistance and loses some of its power as a result. The further the data travels, the more the signal deteriorates. After a certain point, the signal level becomes too low for the receiver to understand.

So USB cables can only be a certain length before they have too much propagation delay or too much attenuation.

Maximum USB cable length

USB-C cable with fast charging capability
Chris Wouk

The maximum length of a USB cable depends on the USB generation it supports. The USB-IF or USB Developers Forum, the organization responsible for maintaining the USB standard, sets the specifications for each generation of USB.

The organization recommended maximum cable lengths for the first two generations of USB—USB 1.0 (3 meters) and USB 2.0 (5 meters)—but since then, USB-IF no longer offers a maximum cable length for any USB generation. Instead, it requires USB cables to meet certain performance requirements, including propagation delay and attenuation.

For given performance requirements, the maximum practical cable length for USB 3.2 Gen 1 (also known as USB 3.0, USB 3.1 Gen 1) is 2-3 meters. Similarly, USB 3.2 Gen 2 (aka USB 3.1, USB 3.1 Gen 2) and USB 3.2 Gen 2×2 cables can be no longer than one meter. And finally, USB 4 cables will be limited to a maximum length of 0.8 meters or 31 inches.

StarTech USB-C to USB-C cable

This StarTech USB-C to USB-C cable is three feet long and USB-IF certified. The cable also supports data rates up to 10 Gbps.

Notably, USB-IF recommends slightly different practical lengths for USB 2.0 and USB 3.2 Gen 1 cables with a USB-C connector. This is most likely because the USB-C cable specification includes higher power delivery requirements. For example, all USB-C cables must be rated at 3A or 5A, allowing them to deliver up to 240W of power.

USB 3.2 Gen 1 Type-C cables can be up to two meters long, and USB 2.0 Type-C cables can be up to four meters long.

One of the key reasons why the maximum length of a USB cable decreases with each new generation of USB is data transfer speed. This is due to the fact that with each new generation we get a higher data transfer rate. As a result, USB cables must have even lower signal propagation delay, which can only be achieved by reducing the length of the cable.

It’s important to note here that these cable lengths are only valid for passive copper USB cables, the most common cable type you’ll find on the market. Other types of cables may be longer for the same speed.

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Active and optical USB cables

Pluggable active USB cable
pluggable

Active or optical (or fiber optic) USB cables can be longer than passive copper cables. Active USB cables use electronic circuitry to amplify the data signal. As a result, they can compensate for attenuation and help transmit data over longer distances. But they still have to deal with signal propagation delay. Therefore, longer than standard, active USB cables cannot provide all USB features such as 4K video output.

In such cases, optical USB cables are a much better option. Unlike conventional USB cables, optical cables use light to transfer data, which is significantly faster and less prone to fading.

In addition to the active and optical cable, you can also use a self-powered USB hub, USB over Ethernet extenders, or USB over fiber extenders if you need a really long USB cable.


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