Quick Answer: A USB 2 hub splits one 480 Mbit/s upstream link across every port and every device, which yields about 40 MB/s in total rather than 40 MB/s each. Every port may draw 500 mA (2.5 W) and the hub takes 100 mA from the upstream bus for itself, so a bus-powered four-port unit has roughly 400 mA to divide. That covers keyboards, mice, serial adapters and audio interfaces. It does not cover bus-powered drives, phones or anything that charges.
480 Mbit/s is a shared figure, and that single fact decides most USB 2 hub purchases. Two webcams on one hub compete for the same pipe, while four keyboards on the same hub consume almost none of it. Counting ports is the least useful way to specify one.
The shared bandwidth budget
The hub multiplexes transactions from all downstream ports onto one upstream pair. Overheads and polling mean about 40 MB/s of payload from a 480 Mbit/s link, and there is no arbitration that guarantees a slice to a busy port.
What shares the pipe well
Keyboards, mice, printers, label printers, serial and RS-485 adapters, MIDI interfaces and measurement instruments use a few hundred kbit/s each. Eight of those on one hub are unremarkable.
What does not
Uncompressed webcam video at 720p30 needs roughly 100 to 150 Mbit/s, so two on one hub already compete. External drives are worse in a different way: a spinning drive needs peak current at spin-up, which the port budget cannot supply.
Power: 500 mA per port and where it comes from
A usb hub declares itself as bus-powered or self-powered. A bus-powered hub may draw 500 mA total from the host, of which it keeps 100 mA, leaving 400 mA to share across four ports, which is 100 mA each before charging inrush. A self-powered hub with a 5 V 2 A adapter gives every port its full 500 mA and keeps the upstream link stable during hot-plug events. Current limits are enforced by the hub, so an over-drawing device simply fails to enumerate rather than damaging the host.
When USB 2 is the better choice
USB 3.x hubs inject 2.4 GHz noise and add a second signalling pair, and a USB 3 hub plugged into a USB 2 host still runs every device at 480 Mbit/s. Where the workload is serial, audio or HID, a USB 2 hub is cheaper, draws 60 percent less current and produces no measurable interference on 2.4 GHz receivers. A modern type c hub is worth its cost only when displays, high-speed storage or power delivery are part of the job.
| Hub class | Upstream rate | Shared payload | Current per port | Best for |
|---|
| USB 2 bus-powered | 480 Mbit/s | 40 MB/s | 100 mA | Keyboards, mice, flash drives |
| USB 2 self-powered | 480 Mbit/s | 40 MB/s | 500 mA | Instruments, serial, audio |
| USB 2 industrial, DIN rail | 480 Mbit/s | 40 MB/s | 500 mA | Control cabinets, 24 V plant |
| USB 3.2 Gen 1 | 5 Gbit/s | 400 MB/s | 900 mA | SSD, gigabit Ethernet |
| USB 3.2 Gen 2 | 10 Gbit/s | 900 MB/s | 900 mA | NVMe enclosures |
Topology and the five metre rule
A USB 2 cable is limited to 5 m (16 ft) per segment and the standard allows seven tiers, five of which may be hubs. Exceed that and enumeration becomes intermittent before it fails outright. A USB hub is also not a network device; it has no addressing and no collision domain of its own, which is why a question about sharing one port between computers belongs with hub vs switch vs router rather than with cable choice.
Practical scenarios
- Situation: A test bench needs six serial adapters at 115200 bit/s. One self-powered USB 2 hub handles all six at under 1 Mbit/s total, with 500 mA available per port for the adapters that draw power from the bus.
- Situation: A conference table has four cable cubbies feeding one laptop. Add a self-powered hub with a 5 V 2 A adapter, because four bus-powered ports on a laptop are usually limited to 900 mA in total.
- Situation: A machine vision camera at 60 frames per second needs 300 Mbit/s on its own. Give it a dedicated host controller, not a hub shared with three other cameras.
- Situation: A USB 2 hub on a 16 ft (5 m) extension drops a keyboard intermittently. The cable is at the segment limit and the cheap extension uses 28 AWG conductors, so the 5 V rail sags below 4.4 V under load.
FAQ
Does a USB 2 hub slow down a USB 3 device?
Yes. A USB 3 device on a USB 2 hub negotiates at 480 Mbit/s, so an SSD that manages 400 MB/s directly will deliver about 40 MB/s. The hub cannot pass a rate it does not support upstream.
How many devices can one USB 2 hub support?
Up to 127 addresses in theory across seven tiers, but current is the real limit. Four to seven low-power devices on a self-powered hub is a sound working rule, and each device that charges reduces the number of ports you can actually use.
Why do devices disconnect when a hard drive starts?
Spin-up inrush current pulls the 5 V rail down and the hub trips its per-port limit. Use a self-powered hub or a drive with its own supply, and check that the upstream cable uses 20 or 22 AWG power conductors rather than 28 AWG.
