Quick Answer: A hub on a long upstream cable loses 5 V before it loses data. USB 2 allows one 5 m (16 ft) passive cable per segment, and a 28 AWG cable at that length drops about 1 V at 500 mA, leaving 4 V at the hub against a 4.4 V minimum. Move to 20 or 22 AWG conductors, use a self-powered hub, or insert an active extension that repeats the signal. Expect random disconnects, then enumeration failures, in that order.
Data survives a long cable longer than power does. A 30 ft (9 m) passive lead will often enumerate a keyboard and then fail the moment a bus-powered drive spins up, because the current step pulls the rail below the threshold that the hub’s regulator needs.
Current fails before data
The 4.4 V threshold
A downstream port may supply 5 V at 500 mA, and a device must operate down to 4.4 V at its own connector. That 0.6 V allowance covers the cable, both connectors and any hub in the path. Spend more than 0.6 V in copper and a compliant device is entitled to drop out.
The symptoms to expect
First intermittent re-enumeration every few minutes, then a device that appears only on a cold boot, then a hub that reports an over-current condition and shuts all ports. A device that works alone on the long cable but fails behind a hub is showing a voltage margin problem, not a protocol problem.
The 5 m segment rule
USB 2 is specified for 5 m (16 ft) of passive cable per segment, and each active extension counts as a hub in the seven-tier limit. Five passive leads joined by couplers will not work however good the copper, because the signal rise time and the cable delay exceed the timing window before the voltage does. A coupler also adds a mating pair with roughly 0.03 ohm of contact resistance, which is trivial for data and not trivial at 2 A.
Gauge and voltage drop
| Power conductor | Resistance (ohm per 100 ft) | Drop at 500 mA, 16 ft | Drop at 900 mA, 16 ft | Verdict |
|---|
| 28 AWG | 6.5 | 1.04 V | 1.87 V | Fails |
| 26 AWG | 4.1 | 0.66 V | 1.18 V | Marginal |
| 24 AWG | 2.6 | 0.42 V | 0.75 V | Works to 500 mA |
| 22 AWG | 1.65 | 0.26 V | 0.48 V | Recommended |
| 20 AWG | 1.04 | 0.17 V | 0.30 V | Best for 2 A loads |
Figures assume the current travels out and back through two conductors, so the loop length is twice the cable length. A thick power pair in an otherwise thin usb cable is not a marketing detail; it is the difference between a stable port and a device that reconnects every ten minutes.
Options that actually extend the run
A self-powered usb hub placed at the far end is the cheapest fix, because the long cable then carries data and only the hub’s own 100 mA, and each port draws from the local 5 V supply. An active extension with a repeater chip in the far plug extends a segment to 10 m (33 ft) and is a better choice when no mains socket exists at the far end. Above 10 m, use a fibre or Cat 6 extender pair, which are transparent to USB 2 traffic and draw 1 to 2 W per end.
Practical scenarios
- Situation: A printer sits 16 ft (5 m) from a desktop. A single 22 AWG lead with a ferrite at each end works; adding a coupler and two 8 ft leads to reach the same distance does not.
- Situation: A KVM switch needs to reach a rack 13 ft (4 m) away. Use a usb b to usb b cable with 22 AWG power conductors, because KVM leads carry both keyboard data and the switch’s own current.
- Situation: A bus-powered 2.5 in drive fails on a 10 ft (3 m) extension. Move the drive to a self-powered hub at the far end and the spin-up current comes from the local adapter.
- Situation: A wall-mounted display needs a touch interface 33 ft (10 m) away. Use one active extension rather than three passive leads, and keep the total below five hubs so enumeration stays inside the seven-tier limit.
FAQ
How long can a USB cable be with a hub?
Each passive segment is limited to 5 m (16 ft) and the whole path to seven tiers with no more than five hubs. In practice, one active extension plus one passive lead gives a reliable 15 m (49 ft) at USB 2 rates.
Does a longer cable reduce data speed?
No, USB 2 runs at 480 Mbit/s or not at all. A marginal cable causes CRC errors and retries, which look like slowness, or it causes the device to fall back to the 12 Mbit/s full-speed mode after a failed high-speed handshake.
Why does the device work on one port and not another?
Ports on a hub share a current budget, so a port already loaded by another device has less headroom. Move the device to a port with nothing else attached, or use a hub with a 5 V 3 A adapter.
