Quick answer: A bus powered USB-C hub with four USB-A ports takes all its current from the host port, typically 900 mA at 5 V. That is enough for a keyboard, a mouse, a card reader and a phone charging slowly. It is not enough for two external hard drives, and the result is a drive that disconnects when the second one spins up.
Bus powered hubs are small, silent and cheap, and they are sold as if port count were the only specification. The number that matters is the current the host will give you, and what each downstream device expects.
Where the current comes from
A USB-C host port advertises its capability through the configuration channel. A hub with no external supply typically declares itself as a legacy device and draws 500 mA, or negotiates 900 mA if the host allows. Either figure is the total for all four downstream ports plus the hub controller.
Unit load accounting
A mouse or keyboard draws under 100 mA. A card reader draws about 200 mA when active. A 2.5 inch hard drive draws 500 mA idle and up to 1 A at spin up. A phone can request 1.5 A or more and will simply charge slowly when refused.
What happens when the budget is exceeded
The hub does not usually fail loudly. The 5 V rail sags, a device resets, and the operating system logs a disconnect. On a bus powered hub this appears as a drive that vanishes during a large copy and reappears afterwards.
Port budgets in practice
| Load | Typical draw | Bus powered hub | Powered hub |
|---|
| Keyboard and mouse | 100 mA combined | Fine | Fine |
| USB flash drive | 200 mA | Fine | Fine |
| 2.5 inch HDD | 500 mA to 1 A | One only | Two or three |
| SSD enclosure | 400 mA | One, marginal | Three |
| Phone fast charge | 1.5 A and up | Slow charge only | Depends on supply |
Why the shape of bus powered hubs varies so much
Connector and cable strain
A four port hub with a short captive cable puts all the mechanical load on the laptop port. Look for a hub with a cable long enough to lie flat on the desk, and a strain relief where the cable enters the shell.
Heat
The controller and the regulator dissipate a couple of watts in a small plastic box. Under sustained load the case reaches 45 to 55 C, which is normal but uncomfortable on a lap. An aluminium shell runs cooler at the same load.
Data rate claims
A hub advertised at 5 Gbps shares that link across all four ports. Two SSDs copying to each other will not each see 5 Gbps. This is a bandwidth fact, not a fault.
Choosing between bus and mains power
Bus power is the right answer when the hub stays with the laptop. It travels easily, needs no socket, and the loads are keyboards, mice, headsets and flash drives. Mains power is the right answer the moment a spinning disk or a second display adapter joins the list, because those two devices alone can exceed the entire host budget.
A middle option exists: a hub with a power input that works either way. Run it bus powered on the road and plug the supply in at the desk, and the same ports stop dropping drives. Check that the hub does not backfeed power up the upstream cable when the supply is connected, because some laptops object to being charged from a port that is supposed to be a host.
Practical scenarios
- Laptop with two USB-C ports: bus powered hub for keyboard, mouse, headset and a card reader, with the charger on the other port.
- Photographer on location: one SSD on the hub, camera tethered directly to the laptop, because two storage devices exceed the budget.
- Meeting room: hub fixed under the table for a presentation clicker and a USB stick, powered by the room laptop.
- Desktop replacement: powered hub with a 12 V supply for two drives and a phone, because bus power cannot carry that load.
FAQ
Can a bus powered hub charge a laptop?
No. It draws from the laptop, so there is nothing to send back. Charging needs a hub with its own supply and a power delivery input.
Why does my hard drive disconnect?
Spin up current exceeds the hub budget. Move the drive to a host port or use a powered hub.
Is a bus powered hub slower?
The data rate is the same 5 Gbps. Power limits, not speed, are what usually go wrong.
Next step
Count the current before counting the ports. The USB hub guide covers controller generations, the Type-C hub page compares bus powered and powered designs, and the USB-C charging port guide explains what the host will actually negotiate.
