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.(adsbygoogle = window.adsbygoogle || []).push({});

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!

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# DC current over long distance

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