I have a contemplated DC electricity system that has a battery room at a distance from the use. The battery room would be about 150 feet away from where the power is used (for lighting, computer systems, etc). The total wattage of the system will be around 5000 watts or on that order.

I am concerned about the power loss over the 150 foot distance. There would seem to be two basic approaches: using a very thick braided transfer cable, or to step up the voltage from 48 volts to 220V or something like that, transfer it, then step it down again when it gets to the point of use.

What should be my considerations in designing this part of the system?

  • \$\begingroup\$ Cost consideration: how much will the cheaper cable be carrying only 23 amps compared to the heavy duty cable carrying 104 amps? The price difference - does the saving justify building a conversion system? \$\endgroup\$ – Andy aka May 20 '20 at 15:13
  • \$\begingroup\$ If your loads (computer, light...) require mains voltage then you will need a UPS anyway to convert 48V to mains voltage. In this case it makes more sense to put the UPS right next to the batteries. Not the case if your loads run on 48V. So the answer will depend on what your loads are. Could you elaborate? \$\endgroup\$ – bobflux May 20 '20 at 15:35
  • \$\begingroup\$ @peufeu This is a pure DC load system. So, for example, I have a 1500 watt LED lighting system that operates at 24V. Then for computers, I plan to use DC step down power converters. \$\endgroup\$ – Tyler Durden May 20 '20 at 15:44

Alright, over 50 meters...

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The real cost of losses is the cost of batteries whose energy will be wasted to heat the wires.

At 52 W/m the 48V 35mm2 cable better not be buried in a conduit. Even the 50mm2 cable should get quite warm.

Going with 50mm2 however, losses are only 7.4%. Is it possible to step from 48V up to 230V then down to 48V again with losses much smaller than this to make it worth it, for less than 415€? Probably not. 90% efficiency is already pretty good for a UPS, and that's 10% losses already.

Also the power electronics are a point of failure.

I'd go with the 50mm2 48V cable.


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