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We install tablets on walls that require a constant supply of charging power. We use different cables with various lengths and gauges. Most of our installs use Cat5E so we usually use 2 conductors of that cat5E for the positive and 2 conductors for the negative. I do know that USB chargers all have a voltage of around 5.4 volts and the usual chargers put out between 1 amp and 2.1 amps. We have used 2 amp chargers with charger cables as short as 15 feet and they do not charge.

Q. Is there a formula for determining the proper amperage for a charger output with the variables with the gauge and length? Q. If not, how do I find a charger that will charge my devices with varying lengths and gauge of wire?

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  • \$\begingroup\$ Give up on USB chargers. Do Kelvin (voltage) sensing at the end of the cable. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 12 '14 at 0:24
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    \$\begingroup\$ Adfitionally, most high current devices won't charge (well) without the USB data lines connected for charger sensing. Can you have the mains wiring brought closer to the tablet location? \$\endgroup\$ – Chris Stratton Aug 12 '14 at 4:23
  • \$\begingroup\$ related thread: How can USB extension cords affect of USB charging? \$\endgroup\$ – Nick Alexeev Sep 12 '14 at 5:36
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Consider "power-over-ethernet-to-usb" e.g http://www.ipadenclosures.com/ipad_kiosk_enclosure/accessories/poe-to-usb-adapter-for-ipad

That uses 48V along the Cat5 cable, and a converter at the recieving end. So power dissipated in the cable is minimised and you're guaranteed a nice stable 5V over the charger.

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    \$\begingroup\$ Thanks for the link. In case anyone is interested, I just received a return email with a quote for 2 at 99 dollars each and I would still need a POE injector. At that price I would just install 110 outlet behind each tablet. \$\endgroup\$ – Jamr Aug 12 '14 at 22:43
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You don't necessarily have to run the data lines from USB charger to iPad, but you do need to have pullup & pulldown resistors on those lines, or the iPad won't charge. The resistors could be at the iPad end of the cable rather than the USB end, if your cable doesn't include data lines. Reference: Adafruit, where D+ & D- each get 75kohm pullup to 5V & 49.9kohm pulldown to ground.

For wire gauge, Cat5E is specified for wire gauge between 22-24AWG. 22AWG is ~0.053ohms/m, 24AWG is ~0.084ohms/m. Reference: Wikipedia. Worst case with 24AWG would be 2.1A x 0.084 ohms/m = 0.176 Volts per meter drop in the cable. So anything much over 2 meters and the 5.4V you start with at the USB adapter will fall below 5.0V at the device. At 5 meters you'd have 0.88V drop in the cable or 5.4-0.88=4.62V at the device, which may be too low. It is within 10% of 5.0V but not within 5%, depends on what the device expects. The longest USB-to-Lightning cable Apple sells is 2 meters.

You could try using a thicker wire gauge, but by the time you get to 16AWG you basically have an extension cord rather than a data cable. Probably better to wire AC to near the device (as you mention), or if not practical, run an extension cord to near the device and plug the USB adapter into the extension cord. Maybe wrap where the cord, adapter, and cable meet together in electrical tape so they don't pull apart easily.

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  • \$\begingroup\$ Thank you for your detailed answer. I did double up some 22/4 stranded and that seemed to do the trick. The tablet is staying charged and not turning off. \$\endgroup\$ – Jamr Sep 12 '14 at 5:31

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