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DC current over long distance

  1. Jul 22, 2015 #1
    Hello, I am an engineer working on a project and have several questions. I believe I have the answers I need, but this forum is such a great place to confirm my thoughts.

    I am installing USB charging ports on a luxury coach bus. There will be a charging module installed at every pair of seats for a total of 28 charging modules. Each charging module has two USB ports and the complete module can provide a maximum of 2.1 amps. The charging modules are powered by the 24Vdc system on the bus.

    The general wiring for this system will incorporate a main harness that runs the length of the bus (40 feet) on both the right and left sides of the bus. Branching off these main harnesses will be a secondary harness that runs to the middle of each set of seats. I've included a sketch to show what the installation would look like.

    My question involves the wiring size that I would need to select to create the main harnesses and the secondary harnesses. Here are my initial calculations for one side of the bus (the main harnesses will run independently as separate systems)

    Assuming only half of all available USB ports on one side of the bus are in use charging either a smartphone or tablet

    14 charging ports x 1/2 x 2.1 amps = 14.7 amps (call it 15 amps)

    So for wire size selection of the main harness I need to account for a maximum of 15 amps over a distance of 40 feet. The many tables available on the internet tell me in order to carry 15 amps over a distance of 40 feet with a voltage drop of not more than 3% I would need 6 AWG wire. Can anyone confirm this? Also, when computing the distance in this circuit do I need to account for only the general distance, or do I double the distance considering the flow of the electrons will go 40 feet down the positive leg and then return to ground through another 40 feet?

    For the secondary harnesses, can I assume that I need to select a wire size that would be large enough to handle only the current running to that specific charging module? To elaborate a little, in my mind the main harness will be carrying all the current that each charging module consumes at each set of seats. But....the secondary harnesses will only be carrying the current that it's specific charging module is consuming. Is my assumption correct?

    Thanks for any help!

    Attached Files:

  2. jcsd
  3. Jul 22, 2015 #2


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    what are you going to do when all the USB ports are in use, just accept that you might heat up wires too much or damage something? i would account for all the ports being in use. but that's just me. Its a risk, but you can define in your system specifications that it is only intended to be used with half of the ports active. make that clear
    yes, you do need to double the distance.

    yes, your assumption is correct
  4. Jul 22, 2015 #3


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    As long as the charging modules are using a switching supply you will need to calculate everything in watts and converter efficiency.

    2.1A at 5v is 10.5W. converting to amps at 24V you get 0.44A. Assume 80% efficiency and you get about 0.55A per station. or 7.7A per side.

    The voltage drop is probably not a big issue for the charging stations. Look at the specification and you can find the maximum and minimum voltages it will work at. They will probably run down to at least 80% of 24V if not 80% of 12V or lower.

    The maximum current and voltage drop will have to be determined by following the safety regulations for public transportation systems.

    Remember that the only part of the cable that will be subject to the full current is from the first seat (or last seat if the source is in back) to the power source.

  5. Jul 22, 2015 #4
    Good question regarding the number of ports in use. I'm still in the beginning stages, so I've only vaguely thought about that possibility. If I assume all ports are in use, my current load shoots up to 30 amps and then I'm using even larger gauge wire for the main harnesses. In order to meet budget constraints I may have to include a circuit breaker that will interrupt the circuit if the draw goes over 15 amps. I really don't want to have to use anything larger than 6 AWG wire for this project, so my decision is pretty much made for me in that case.

    Good to know about the distance.....I figured the distance would need to be doubled base don the fact that the current path is 80 feet even though the wiring length will on ly be 40 feet. Thanks for the input!
  6. Jul 22, 2015 #5


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    As long as you state that the system is not meant to be used with all the ports active, you should be fine! a circuit breaker is a good idea
  7. Jul 22, 2015 #6
    Very interesting Bob.....I forgot about the fact that the USB ports actually operate at 5V. So in this case even though they are providing 2.1 amps to the connected devices, it may not be pulling 2.1 amps from the main power source? According to your calcs it is only.44 amps? Is this an inherent characteristic of the switching mode supply? I should also add that the charging modules will be connected to 24Vdc, but will also operate at 12 Vdc, so the voltage drop could be more than 3% if needed. That will also help to downsize the wire size of the main harnesses. Please expound on your statement regarding the current draw to the charging module. How is it that the module can provide 2.1 amps to connected devices, but not draw 2.1 amps from the main 24Vdc power source?
  8. Jul 22, 2015 #7


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    Yes. A switching supply will provide a power out of a power in multiplied by the efficiency. so if you have a 100W load you will use 100W/efficiency on the input regardless of the voltages you are talking about.

    For example modern high performance PC processors (just the CPU not all of the other stuff) use more 100W in operation at just above 1V. There can be several inside a high end workstation. These types of computers can plug into a standard 15A socket even though the processors are drawing several hundred amps.

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