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The 3.3V pin on the Joule is pin 30 and its signal name is +VDD3. If you look at section 3.4 of the Joule datasheet you can see the maximum current in +VDD3 is 300mA.
You can see this information in the Joule datasheet here: http://www.intel.com/content/www/us/en/support/boards-and-kits/intel-joule-compute-modules/000022327.html
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