>
>
> USB power is not guaranteed to be current-limited.  On my computer, it
> is tied directly to the +5V rail.  If you draw more than about 30
> amperes, the computer will reset.


Well, right you are!
http://www.ce-mag.com/archive/03/ARG/mikolajczak.html  (and other sources)

The USB specification states that current limiting, power switching, or both
may be required in a USB product, as shown in Table I. Where current
limiting is required, the UL 60950 specifications must be met. This means
that in the event of a short circuit or other fault condition, current
output must be limited to below 5 A within 60 seconds. The USB specification
also defines acceptable voltage output levels and limits total voltage drop
in the system.
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