how many USB drives can be powered from a PC


i searched on the net, but found no figures for the question above
i know the max power on USB = 5V at 0.5A max

So, my question may also be : if i have 6 usb ports on a PC;,  can i connect 6 usb disk drives, each using 2.5W ?  
what is the limiting factor?
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Power consumption from the hub itself. If you have it externally powered, you can put as much as you want as long as the wattage lets you. I wouldn't recommend "daisy chaining" hubs off of 1 usb port coming from the computer.
nobusAuthor Commented:
that was not my question - but thanks for posting
that would be maxing the pull on each port 2.5watts = .5amp . It really all depends on how much your powersupply is putting out. I would say you would be safe to not go over 3. If they are all drawing the max at the sametime it may cause issues with reading and writing. I have seen this happen with external DVD burners.
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You can hook up USB disk drives to all 6 USB ports. If they are indeed externally powered USB drives then you should have no issues. If you are using all 6 USB ports as USB powered drives, you will indeed experience issues during a simultaneous read/write. I had 4 drives hooked up to a off the shelf standard fast ship dell optiplex (230w PSU) and it could not write to all 4 simultaneously without problems or delayed write.
I should have rephrased. Power consumption from the 6 ports themselves = total wattage < total power output (PSU). Drawing more current will probably not give you the simultaneous results you are looking for. Although, external powered HUB's will compensate for that.
I suppose I misunderstood you when you asked for 6 USB ports. I was quick to assume you were using a HUB since a mouse and keyboard would typically use 2 of them and wouldn't see any PS2 style connectors sold today.
The limit is 0.5A on each port, so the limiting factor will be your power supply (which should have been engineered in the design to supply 0.5A on each port).

You can see how much each port is using in
orb/start, type devmgmt.msc and click the one that appears at the top under Programs (1)
(enter an admin password if prompted)
expand Universal Serial Bus controllers
right-click each USB Root Hub and choose Properties
Ports that are used will tell you how much they're drawing on the Power tab.
USB Power Used
nobusAuthor Commented:
to all :
i know the 0.5 A limit - as you can see from my initial post, and i did not ask the question for EXTERNALLY powered ones as you can read also; i know you can hookup to 127 different ones
the question is about the POWER from USB
Several say the power supply is the limiting factor - that can be so, but can anybody back this up from a trustworthy source?
Did anybody test it?
AFAIK the power comes in a single copper line from the PS to the USB block, meaning if i hook up 6 drives (as above) the current draw over that line would be 3A, which looks way too much for me - but again, - i may be wrong on this

even in mobo manuals there is nothing to be found regarding this

looking forward to exact data and figures
Looking at the power section on, you can see that a bus-powered hub has much lower power per port than a self-powered hub.  What a laptop can provide on each port depends on how the motherboard manufacturer configured the USB ports - this can vary, as well as the actual load capability.  If the ports are configured as part of a self-powered hub, then the hub takes 100mA and each port gets 100mA, up to 500mA total - a lot less than 500mA per port.

There's a good article on USB power delivery problems on motherboards here:
nobusAuthor Commented:
Callandor, tx for posting that article
It seems to confirm what i was thinking - but strangely no mobo  manufacturer gives out any info on it; that's the reason for posting this Q
According to this trace calculator

To reach 4A ampacity (that gives 133% oversize) in 140 degree F ambient temp allowing for a maximum 10 degree F rise in temp (of the conductor, which keeps voltage drop below 5%), a 12'' long copper trace that's 0.013'' (13 mils) thick (about 3x the thickness of a $1 bill) would need to be about 0.032'' (32 mils) wide if internal, or only about 0.012'' (12 mils) wide if a surface trace.

That internal trace has about the same cross-sectional area (0.000416 sq inches) as 24AWG wire (the size typically used in Cat5e wire), which coincidentally is rated @ 3.5A ( ), by the way.
nobusAuthor Commented:
ok Darr for the calculation method.
so it seems there is no standard for it, as to how much current is supplied to the USB blocks.
i mean : some boards come with 2 USB ports, some wit a 6 port block.
or can we say that the construction is sufficient to mpower all ports with 0.5A ?

it reinforces the use of external powered devices imo
Even with motherboards with the same number of ports, there is no uniform construction - you have to depend on the manufacturer's quality assurance.  There will be some who cut corners to increase profit margins, and some that want to maintain their reputation.  They probably only respond when products stop selling, due to returns and shoddy workmanship.
nobusAuthor Commented:
may i attach another side to this Question in order to complete the picture?
on some mobo's, you have 1 or up to 8 usb Host controllers

are these separate chips -  and do they provide the power - or only the usb signals ?

anyone has info on this?
Traditionally, USB is included in the SouthBridge chipset, but for USB3.0, Intel has not yet done this  Separate chipsets are used to do this currently, and they vary,3024-2.html

USB power on the motherboard is typically handled by a separate circuit - the current is too high for chipsets to do this directly; they only handle signals.  Gigabyte advertises extra USB power on one of their models:
nobusAuthor Commented:
>>   Gigabyte advertises extra USB power   <<  i did not see it in the link - can you explain a bit?
This is on the Overview tab:

"3x USB Power Boost

GIGABYTE motherboards feature a 3x USB power boost, delivering greater compatibility and extra power for USB devices. GIGABYTE’s unique USB power design is also able to efficiently regulate output over the full voltage range, which greatly enhances USB device compatibility. In addition, dedicated lower resistance fuses ensure lower voltage drops, and provide more stable and plentiful power delivery."

I don't know how they do this, nor if it actually performs as advertised.
nobusAuthor Commented:
ah - found it in the overview...but is that not standard with USB3 ? (not clear imo), but it would help to know
It is not standard with USB - that's why Gigabyte puts it in their advertising (if everyone had it, how can you make a case for you being different?).
Mohammed RahmanCommented:
ID: 37841280
the power comes in a single copper line from the PS to the USB block, meaning if i hook up 6 drives (as above) the current draw over that line would be 3A, which looks way too much for me - but again, - i may be wrong on this

PIN 4,6,19,20 of the ATX connector carry +5V to the motherboard. I do not know which of the four lines supply power to the USB circuit.

connect the negative (ground) of Multimeter to any black line wire on the atx connector (as all blacks are returning lines) and connect the red of the multimeter to pin 4 and measure the current. Repeat same for pin 6,19 and 20. Make a note of current values for all the 4 lines (without load - no USB devices connected)

You can try connect all 6 USB devices to the computer and then follow the same aboce procedure with multimeter and make a note of current values for all the 5V rails.

You may be able to come to conclusin, whether all are drawing current simultanously and making a total of 3A or not.

Try use the multimeter probe as in the pic below. It will help you to easily make contact with the ATX Pins without stripping the wires.

MM Probe
nobusAuthor Commented:
tx for the post, but i know how to measure something mody, but what are you suggesting?
the question is general ; but no manufacturer carries the info

i think i got enough info to close this tomorrow
Mohammed RahmanCommented:
Hi Nobus,

I think the current will pass from the power supply to the motherboard through one of the 4 5V lines when we apply load to the USB port (i.e., connect device to USB port)

I was thinking you can measure the current (Amps) on all 5V rails without connecting USBs. You will get some values in Amps. Then apply load (connect all 6 USB devices) and push them to work so that they start drawing current (dont let them be idle).

Now, measure the current (amps) on all the 4 5V rails. The current flowing on these rails may be more this time (compared to no load on USBs)

Full load current drawn - No load current drawn will give you the total current drawn by USBs.

If that comes close to 3A, it is an indication that all the USBs are drawing current at same time. If not, they are toggling.
When you measure current, it should be in series with the actual load, so I don't see how this method does anything except short out the line - an ammeter is a low resistance in series with the device to be measured.  By itself without a load, it's like a straight wire across the source and ground.
nobusAuthor Commented:
it does not answer the question - i'll rephrase it for clarity:
where can i find the number of USB devices - or loads -  that you can apply on the different PC's USB ports

the question is not for knowing how much i can have on one - though it is a bit of help, but more general, and as adressed yet, no info seems availavable now from the manufacturers, which make the boards just as they like
Mohammed RahmanCommented:

Thanks, I should have said connect the ammeter in series on the 5V rail.


Will keep looking into this and post information if I come come across any.
nobusAuthor Commented:
ok we have good faith the manufacturers made their boards well - but why not post the info ?
then we have something to go on; now it's only guessing (or measuring if you like)
Mohammed RahmanCommented:
@nobus: The question is quite clear. I still have something to ask you.
You originally asked "if i have 6 usb ports on a PC;,  can i connect 6 usb disk drives, each using 2.5W ?"
Does this mean, if you connect 6 devices to 6 available USB ports on a PC, does each drive consume 2.5W CONTINUOUSLY ? (continuously is the word here).

Also, please let us know what exactly you meant by asking
"what is the limiting factor?"

We all will try to get the right answer or the closest one and try not give up or keep assuming stuff or rely on manufacturers info :)
nobusAuthor Commented:
ha mody i'll expand a bit for you
>>  Does this mean, if you connect 6 devices to 6 available USB ports on a PC, does each drive consume 2.5W CONTINUOUSLY ? (continuously is the word here).    <<  i could not care less i fit's continuesly or not, i take the USER's position - every device you put in should work, without hassles

  >>  "what is the limiting factor?"   <<   obviously, if the above is not true , then this comes into play ?

i agree that there can be alot more being said about the questions -and the answers

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nobusAuthor Commented:
i got a fairly informative answer, but no 100% proofed one, as a manufacturer's spec
all in all i'm happy with the result, though in essence it was not fully answered
i hope the points are correctly distributed?
if not post your argument, please
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