I've had problems with one long (4 meter) USB Mini-B to USB Type-A cable not being able to boot a 2.5'' external hard disc due to not supplying enough current. On top of that, the cable used a Type-A to Mini-B adapter for the Mini-B part, which probably made things worse.

Three different shorter cables I got around made the hard disk work without extra current, so it was definitively the cable's fault. However, if I plugged the hard disk to the power, and used the long cable just for data it worked.

Here is some related information on powering through USB cables:

I have not any long cables that don't have an intermediary Type-A to Mini-B adapter to try them out.

My question is: is there a way to guess if a cable will provide enough power for charge/disk drive power? Is it related to the length of the cable, to the build quality of the cable, or the fact that uses intermediary adapters?


Thanks for the answers guys. The computer is a Macbook Pro 13' (2010), so the USB connector is directly soldered to the motherboard (no front panel USB).

Up so far the most convincing explanations seem the ones from @user12889 (the USB device being over the spec and causing problems because of that) and @Tog (the adapter in the cable causing the power loss), although everybody else provided useful information too.

  • How do you know that cable is unable to supply current? Does it overheat and are there any traces of melting?
    – AndrejaKo
    Jan 10, 2011 at 18:53
  • 2
    If the cable needs adapters, then it is not standards compliant. Replace it with a true USB2.0 certified cable, and all will be a lot better. It may not be perfect, however, because the drive itself may be stretching the standard as well.
    – RBerteig
    Jan 11, 2011 at 9:15
  • AndrejaKo: When I say 'unable to supply current' I mean that the HD won't boot with the long/adapted cable unless I connect the additional HD power supply. With the short/unadapted cables the HD works fine without connecting the additional HD power supply. Jan 12, 2011 at 11:47

5 Answers 5


Unless it is a really low quality cable, your voltage drops are most likely to be caused by the connections. Using adapters could make it worse.

If this is a PC with a front panel USB connector, it would be worth checking to make sure that you have a good connection at the mother board end of the front panel lead. As the current drawn through the cable increases, so the voltage drop across any poor connections increases, so your cable may not be able to provide the correct voltage for the HDD.

Signal lines, however, don't draw a lot of current, so the voltage drop could be insufficient to affect the logic levels involved.


I find it impossible to believe that USB cable can't supply enough current for a device and that for power supply use length of SUB cable can matter.

Here's some reasoning why:

I'll cite USB specification 2.0. It says that the minimum cable which may be used for USB is AWG 24 cable. If we say that the device has 10% voltage tolerance, it'll need at least 4.5 V to function. Let's calculate how much resistance we need for such voltage drop. Maximum current consumption for USB 2.0 is 500 mA. So 4.5 V/0.5 A = 9 Ω. That resistance is quite big for simple cable. Let's take a look at typical resistances of AWG wires. It is said here that AWG 28 resistance is 0.0764 Ω/′.

So 9 Ω/0.0764 = 117,80 ′ which is around 35.9 m.

There are several other reasons why it would work with external power and other cables but not that cable. First thing which comes to mind is timing. Maximum length of USB 1.1 cable was 3 m and for USB 2.0 it's 5 m. The lengths are limited by signal propagation. But you said that if you connect power source to the device, it works. So while we are close to maximum length for signal problems, we aren't in the critical region yet.

This is the point where the problem becomes really complicated as there are lots of variables which could be causing problems. First thing that now comes to my mind would be bad USB chips at drive or motherboard side or both. Also, it is interesting to know that drive has a power source connector. It could happen for example that chip used at the drive can't handle delays caused by the cable if the load is high enough. When the adapter is used, there could be another power supply inside the drive which powers the chip.

On the other hand, it could happen that the motherboard itself isn't providing clean enough power. In that case voltage could be carrying some signal which could interfere with data lines. Length of the cable could make interference worse. It could also happen that signal source shows only when the load is high enough.

Basically, form this point on, it's mainly guesswork, because there could be combinations of bad chips at both sides, bad shielding in the cable, bad filtering and so on. To sole such problems, large amount of knowledge would be required about internal construction of the motherboard, hard disk and of course, there's the test equipment.

To answer your question directly, you can't know in advance if a cable and adapters will work correctly with a particular combination of devices, so the answer is no.

  • 5
    I think much more likely: The device (at least sometimes) needs more than 500mA. This USB port in question is providing more than 500mA (thus exceeding the specs); but probably only barely enough for your device. In this case cable length (and quality) can matter. I have an external USB drive which shows about the same behavior: Works fine on some USB ports with any cable, on some only with a short cable, on some not at all (or only with a split cable which draws additional power from a second USB port)
    – user12889
    Jan 10, 2011 at 23:42
  • @user12889 Interesting! If the USB device assumes that the voltage is stabilized and doesn't have its own stabilizer, that could cause problems if it exceeds maximum current.
    – AndrejaKo
    Jan 11, 2011 at 0:37

You can't guess, you have to test. Either just try it out or measure the resistance. Don't believe any labelling on anything not known good brand. Not all USB cables meet specs or labels. I just bought a pair of 3m usb extension cables, labelled as 28+24AWG. They are actually 30+30AWG and useless for my device as they have 4x the resistance ands voltage drop that they should have. I worked this out before putting them in a mission critical installation. Caveat emptor!


The cable pictured in this review of an iomega external hard drive may be of use. It has two USB-A male plugs to draw additional power. I am not sure this is standard, but it does seem to work - I have another drive that came with this cable, and depending on where I plug it in I don't always need the second plug.



In the case of USB, there are length limits of 3M for lo speed devices, 5M for high speed devices. http://www.usb.org/about/faq/ans5

Beyond the USB cable length limits, you can get USB Boosters.. Each one goes a certain length(5M maybe) and is then plugged into a power source. Or you can get USB-Cat5 adaptors, it's like 2 bases. plug them in(either one or both plug into the mains, I don't recall if both) and effectively extend it really far like 30M.

That's the case for USB.

Ps2, I think the limit might be 5M. I have a PS2 extension thing, it involves 2 bases and plugs into the mains. I think in between it uses what looks like and probably is, a ps2 cable. KVM extenders commonly use cat5 and extend ps2 and VGA. (nowadays of course KVM Extenders would do USB)

VGA cables of course don't send any power.. people often find the signal gets worse over long distance, but i've bought a really thick one that worked over very long distance, 50M. So for VGA certainly the bulk of the VGA cable makes a difference for length.

So different cables and situations can differ.. I did once attach a wire to an electronic switch, the wire was long, it ended up not working 'cos it acted like a big aerial! So I think I had to get shielded cable.

So sometimes each situation can seem to have its own weird solution.. but generally whatever you want to extend, ways have been devised! And solutions can be similar or the same kind of thing. But different things like usb,ps2, just naturally, have different limits.. as you can see. But these can be worked around.

  • Those USB extenders are not for providing power. While some do, they primarily re-modulate the signal being sent so it can be carried farther distances.
    – Brad
    Jan 10, 2011 at 20:20
  • @Brad Which USB Extenders? The ones i'm talking about that need to be plugged in every 5M, the plugging in isn't for providing power? Why do they need to be plugged in every 5M and not just once? Anyhow, they provide the ability for the signal to go further. You just wrote -some- provide power. So even if you are right, what I said wasn't wrong according to you.
    – barlop
    Jan 10, 2011 at 22:04
  • @Brad Can you quote exactly what I said where you took issue and downvoted me for?
    – barlop
    Jan 10, 2011 at 22:05
  • @Brad I wrote about a "power source" meaning THE MAINS. That is a source of power. Try sticking your finger in.
    – barlop
    Jan 10, 2011 at 22:07
  • @Brad I agree the USB Extender doesn't provide power.. Where did I say it does? It does have a place to plug into to the mains and that is a power source, so I might presume that it gets power from there -the wall, though I don't think I wrote that. and I don't know if you even disagree with that. Obviously you could also say that the Mains/wall socket isn't a power source, it just carries the power from elsewhere. The electricity generator is not even near the house! But in terms of electricity travelling, from source to destination, the power source is at the wall socket.
    – barlop
    Jan 10, 2011 at 22:19

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